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How to Make a Scientific Calculator Android App using Android Studio?

  • Difficulty Level : Expert
  • Last Updated : 11 Jun, 2021

The calculator is the app that is present on every android device. This app comes pre-installed or we can also install another application from Play Store. It is one of the most used applications for college students for making any calculations. In this article, we will take a look at building a simple scientific calculator app in Android using Kotlin

How-to-Build-a-Scientific-Calculator-Android-App-using-Android-Studio

Note: To build a simple calculator app please refer to this article How to build a simple Calculator app using Android Studio?

What we are going to build in this article? 

We will be building a scientific calculator in which we will be performing several mathematical operations such as addition, subtraction, square root, factorial, and many more. A sample video is given below to get an idea about what we are going to do in this article.



Step by Step Implementation

Step 1: Create a New Project

To create a new project in Android Studio please refer to How to Create/Start a New Project in Android Studio. Note that select Java as the programming language.

Step 2: Adding new colors to the colors.xml file 

Navigate to the app > res > values > colors.xml file and add the below code to it for different colors. Comments are added in the code to get to know in more detail. 

XML




<?xml version="1.0" encoding="utf-8"?>
<resources>
    
    <color name="purple_200">#0F9D58</color>
    <color name="purple_500">#0F9D58</color>
    <color name="purple_700">#0F9D58</color>
    <color name="teal_200">#FF03DAC5</color>
    <color name="teal_700">#FF018786</color>
    <color name="black">#FF000000</color>
    <color name="white">#FFFFFFFF</color>
    
    <!--three different shades of black color-->
    <color name="blac_shade_1">#292D36</color>
    <color name="black_shade_2">#272B33</color>
    <color name="black_shade_3">#22252D</color>
    <color name="yellow">#ffa500</color>
    
</resources>

Step 3: Working with the activity_main.xml file

Navigate to the app > res > layout > activity_main.xml and add the below code to that file. Below is the code for the activity_main.xml file. 

XML




<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/black_shade_3"
    tools:context=".MainActivity">
  
    <TextView
        android:id="@+id/idTVSecondary"
        android:layout_width="match_parent"
        android:layout_height="70dp"
        android:background="@color/black_shade_3"
        android:gravity="bottom"
        android:maxLines="1"
        android:padding="10dp"
        android:paddingTop="30dp"
        android:text=""
        android:textAlignment="viewEnd"
        android:textColor="@color/white"
        android:textSize="15sp"
        tools:ignore="RtlCompat" />
      
    <TextView
        android:id="@+id/idTVprimary"
        android:layout_width="match_parent"
        android:layout_height="100dp"
        android:layout_below="@id/idTVSecondary"
        android:background="@color/black_shade_3"
        android:gravity="bottom"
        android:maxLines="1"
        android:padding="10dp"
        android:text=""
        android:textAlignment="viewEnd"
        android:textColor="#fff"
        android:textSize="50sp"
        tools:ignore="RtlCompat" />
  
    <androidx.cardview.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@id/idTVprimary"
        android:background="@color/blac_shade_1"
        app:cardCornerRadius="4dp"
        app:cardElevation="2dp">
  
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_weight="7"
            android:background="@color/blac_shade_1"
            android:orientation="vertical">
  
            <LinearLayout
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:orientation="vertical"
                android:weightSum="7">
  
                <LinearLayout
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:layout_weight="1"
                    android:orientation="horizontal">
  
                    <LinearLayout
                        android:layout_width="match_parent"
                        android:layout_height="match_parent"
                        android:orientation="horizontal"
                        android:weightSum="4">
  
                        <Button
                            android:id="@+id/bac"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="AC"
                            android:textColor="@color/yellow"
                            android:textSize="15sp"
                            tools:targetApi="lollipop" />
  
                        <Button
                            android:id="@+id/bc"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="C"
                            android:textColor="@color/yellow"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/bbrac1"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="("
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
  
                        <Button
                            android:id="@+id/bbrac2"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text=")"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
  
                    </LinearLayout>
  
                </LinearLayout>
  
                <LinearLayout
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:layout_weight="1"
                    android:orientation="horizontal">
  
                    <LinearLayout
                        android:layout_width="match_parent"
                        android:layout_height="match_parent"
                        android:orientation="horizontal"
                        android:weightSum="5">
  
                        <Button
                            android:id="@+id/bsin"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="sin"
                            android:textAllCaps="false"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/bcos"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="cos"
                            android:textAllCaps="false"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/btan"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="tan"
                            android:textAllCaps="false"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/blog"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="log"
                            android:textAllCaps="false"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
  
                        <Button
                            android:id="@+id/bln"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="ln"
                            android:textAllCaps="false"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
  
  
                    </LinearLayout>
  
                </LinearLayout>
  
                <LinearLayout
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:layout_weight="1"
                    android:orientation="horizontal">
  
                    <LinearLayout
                        android:layout_width="match_parent"
                        android:layout_height="match_parent"
                        android:orientation="horizontal"
                        android:weightSum="5">
  
                        <Button
                            android:id="@+id/bfact"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="x!"
                            android:textAllCaps="false"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/bsquare"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="x²"
                            android:textAllCaps="false"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
  
                        <Button
                            android:id="@+id/bsqrt"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="√"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/binv"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="1/x"
                            android:textAllCaps="false"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
  
                        <Button
                            android:id="@+id/bdiv"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="÷"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
                          
                    </LinearLayout>
  
                </LinearLayout>
  
                <LinearLayout
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:layout_weight="1"
                    android:orientation="horizontal">
  
                    <LinearLayout
                        android:layout_width="match_parent"
                        android:layout_height="match_parent"
                        android:orientation="horizontal"
                        android:weightSum="4">
  
                        <Button
                            android:id="@+id/b7"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="7"
                            android:textColor="#fff"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/b8"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="8"
                            android:textColor="#fff"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/b9"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="9"
                            android:textColor="#fff"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/bmul"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="×"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
                          
                    </LinearLayout>
  
                </LinearLayout>
  
                <LinearLayout
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:layout_weight="1"
                    android:orientation="horizontal">
  
                    <LinearLayout
                        android:layout_width="match_parent"
                        android:layout_height="match_parent"
                        android:orientation="horizontal"
                        android:weightSum="4">
  
                        <Button
                            android:id="@+id/b4"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="4"
                            android:textColor="#fff"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/b5"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="5"
                            android:textColor="#fff"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/b6"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="6"
                            android:textColor="#fff"
                            android:textSize="15sp" />
  
                        <Button
                            android:id="@+id/bminus"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="-"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
                          
                    </LinearLayout>
  
                </LinearLayout>
  
                <LinearLayout
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:layout_weight="1"
                    android:orientation="horizontal">
  
                    <LinearLayout
                        android:layout_width="match_parent"
                        android:layout_height="match_parent"
                        android:orientation="horizontal"
                        android:weightSum="4">
  
                        <Button
                            android:id="@+id/b1"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="1"
                            android:textColor="#fff"
                            android:textSize="15sp" />
  
                        <Button
                            android:id="@+id/b2"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="2"
                            android:textColor="#fff"
                            android:textSize="15sp" />
  
                        <Button
                            android:id="@+id/b3"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="3"
                            android:textColor="#fff"
                            android:textSize="15sp" />
  
                        <Button
                            android:id="@+id/bplus"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="+"
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
  
                    </LinearLayout>
  
                </LinearLayout>
  
                <LinearLayout
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:layout_weight="1"
                    android:orientation="horizontal">
  
                    <LinearLayout
                        android:layout_width="match_parent"
                        android:layout_height="match_parent"
                        android:orientation="horizontal"
                        android:weightSum="4">
  
                        <Button
                            android:id="@+id/bpi"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="π"
                            android:textColor="#fff"
                            android:textSize="15sp" />
  
                        <Button
                            android:id="@+id/b0"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="0"
                            android:textColor="#fff"
                            android:textSize="15sp" />
  
                        <Button
                            android:id="@+id/bdot"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="."
                            android:textColor="#fff"
                            android:textSize="15sp" />
                          
                        <Button
                            android:id="@+id/bequal"
                            android:layout_width="match_parent"
                            android:layout_height="match_parent"
                            android:layout_margin="3dp"
                            android:layout_weight="1"
                            android:backgroundTint="@color/black_shade_2"
                            android:padding="6dp"
                            android:text="="
                            android:textColor="#ffa500"
                            android:textSize="15sp" />
  
                    </LinearLayout>
  
                </LinearLayout>
  
            </LinearLayout>
  
        </LinearLayout>
  
    </androidx.cardview.widget.CardView>
  
</RelativeLayout>

Step 4: Working with the MainActivity.kt file

Go to the MainActivity.kt file and refer to the following code. Below is the code for the MainActivity.kt file. Comments are added inside the code to understand the code in more detail.

Kotlin




import android.os.Bundle
import android.widget.Button
import android.widget.TextView
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
  
class MainActivity : AppCompatActivity() {
  
    // creating variables for our text view and button
    lateinit var tvsec: TextView
    lateinit var tvMain: TextView
    lateinit var bac: Button
    lateinit var bc: Button
    lateinit var bbrac1: Button
    lateinit var bbrac2: Button
    lateinit var bsin: Button
    lateinit var bcos: Button
    lateinit var btan: Button
    lateinit var blog: Button
    lateinit var bln: Button
    lateinit var bfact: Button
    lateinit var bsquare: Button
    lateinit var bsqrt: Button
    lateinit var binv: Button
    lateinit var b0: Button
    lateinit var b9: Button
    lateinit var b8: Button
    lateinit var b7: Button
    lateinit var b6: Button
    lateinit var b5: Button
    lateinit var b4: Button
    lateinit var b3: Button
    lateinit var b2: Button
    lateinit var b1: Button
    lateinit var bpi: Button
    lateinit var bmul: Button
    lateinit var bminus: Button
    lateinit var bplus: Button
    lateinit var bequal: Button
    lateinit var bdot: Button
    lateinit var bdiv: Button
  
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
         
        // initializing all our variables.
        tvsec = findViewById(R.id.idTVSecondary)
        tvMain = findViewById(R.id.idTVprimary)
        bac = findViewById(R.id.bac)
        bc = findViewById(R.id.bc)
        bbrac1 = findViewById(R.id.bbrac1)
        bbrac2 = findViewById(R.id.bbrac2)
        bsin = findViewById(R.id.bsin)
        bcos = findViewById(R.id.bcos)
        btan = findViewById(R.id.btan)
        blog = findViewById(R.id.blog)
        bln = findViewById(R.id.bln)
        bfact = findViewById(R.id.bfact)
        bsquare = findViewById(R.id.bsquare)
        bsqrt = findViewById(R.id.bsqrt)
        binv = findViewById(R.id.binv)
        b0 = findViewById(R.id.b0)
        b9 = findViewById(R.id.b9)
        b8 = findViewById(R.id.b8)
        b7 = findViewById(R.id.b7)
        b6 = findViewById(R.id.b6)
        b5 = findViewById(R.id.b5)
        b4 = findViewById(R.id.b4)
        b3 = findViewById(R.id.b3)
        b2 = findViewById(R.id.b2)
        b1 = findViewById(R.id.b1)
        bpi = findViewById(R.id.bpi)
        bmul = findViewById(R.id.bmul)
        bminus = findViewById(R.id.bminus)
        bplus = findViewById(R.id.bplus)
        bequal = findViewById(R.id.bequal)
        bdot = findViewById(R.id.bdot)
        bdiv = findViewById(R.id.bdiv)
  
        // adding on click listener to our all buttons.
        b1.setOnClickListener {
            // on below line we are appending 
            // the expression to our text view.
            tvMain.text = (tvMain.text.toString() + "1")
        }
        b2.setOnClickListener {
            // on below line we are appending 
            // the expression to our text view.
            tvMain.text = (tvMain.text.toString() + "2")
        }
        b3.setOnClickListener {
            // on below line we are appending 
            // the expression to our text view.
            tvMain.text = (tvMain.text.toString() + "3")
        }
        b4.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "4")
        }
        b5.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "5")
        }
        b6.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "6")
        }
        b7.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "7")
        }
        b8.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "8")
        }
        b9.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "9")
        }
        b0.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "0")
        }
        bdot.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + ".")
        }
        bplus.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "+")
        }
        bdiv.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "/")
        }
        bbrac1.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "(")
        }
        bbrac2.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + ")")
        }
        bpi.setOnClickListener {
            // on clicking on pi button we are adding 
            // pi value as 3.142 to our current value.
            tvMain.text = (tvMain.text.toString() + "3.142")
            tvsec.text = (bpi.text.toString())
        }
        bsin.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "sin")
        }
        bcos.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "cos")
        }
        btan.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "tan")
        }
        binv.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "^" + "(-1)")
        }
        bln.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "ln")
        }
        blog.setOnClickListener {
            tvMain.text = (tvMain.text.toString() + "log")
        }
  
        bminus.setOnClickListener {
            // on clicking on minus we are checking if
            // the user has already a minus operation on screen.
            // if minus operation is already present 
            // then we will not do anything.
            val str: String = tvMain.text.toString()
            if (!str.get(index = str.length - 1).equals("-")) {
                tvMain.text = (tvMain.text.toString() + "-")
            }
        }
        bmul.setOnClickListener {
            // if mul sign is not present in our 
            // text view then only we are adding
            // the multiplication operator to it.
            val str: String = tvMain.text.toString()
            if (!str.get(index = str.length - 1).equals("*")) {
                tvMain.text = (tvMain.text.toString() + "*")
            }
        }
        bsqrt.setOnClickListener {
            if (tvMain.text.toString().isEmpty()) {
                // if the entered number is empty we are displaying an error message.
                Toast.makeText(this, "Please enter a valid number..", Toast.LENGTH_SHORT).show()
            } else {
                val str: String = tvMain.text.toString()
                // on below line we are calculation
                // square root of the given number.
                val r = Math.sqrt(str.toDouble())
                // on below line we are converting our double
                // to string and then setting it to text view.
                val result = r.toString()
                tvMain.setText(result)
            }
        }
        bequal.setOnClickListener {
            val str: String = tvMain.text.toString()
            // on below line we are calling an evaluate
            // method to calculate the value of expressions.
            val result: Double = evaluate(str)
            // on below line we are getting result
            // and setting it to text view.
            val r = result.toString()
            tvMain.setText(r)
            tvsec.text = str
        }
        bac.setOnClickListener {
            // on clicking on ac button we are clearing 
            // our primary and secondary text view.
            tvMain.setText("")
            tvsec.setText("")
        }
        bc.setOnClickListener {
            // on clicking on c button we are clearing 
            // the last character by checking the length.
            var str: String = tvMain.text.toString()
            if (!str.equals("")) {
                str = str.substring(0, str.length - 1)
                tvMain.text = str
            }
        }
        bsquare.setOnClickListener {
            if (tvMain.text.toString().isEmpty()) {
                // if the entered number is empty we are displaying an error message.
                Toast.makeText(this, "Please enter a valid number..", Toast.LENGTH_SHORT).show()
            } else {
                // on below line we are getting the expression and then calculating the square of the number
                val d: Double = tvMain.getText().toString().toDouble()
                // on below line we are calculating the square.
                val square = d * d
                // after calculating the square we
                // are setting it to text view.
                tvMain.setText(square.toString())
                // on below line we are setting 
                // the d to secondary text view.
                tvsec.text = "$d²"
            }
        }
        bfact.setOnClickListener {
            if (tvMain.text.toString().isEmpty()) {
                // if the entered number is empty we are displaying an error message.
                Toast.makeText(this, "Please enter a valid number..", Toast.LENGTH_SHORT).show()
            } else {
                // on below line we are getting int value
                // and calculating the factorial value of the entered number.
                val value: Int = tvMain.text.toString().toInt()
                val fact: Int = factorial(value)
                tvMain.setText(fact.toString())
                tvsec.text = "$value`!"
            }
  
        }
          
    }
  
    fun factorial(n: Int): Int {
        // this method is use to find factorial
        return if (n == 1 || n == 0) 1 else n * factorial(n - 1)
    }
  
    fun evaluate(str: String): Double {
        return object : Any() {
            // on below line we ar creating variable 
            // for tracking the position and char pos.
            var pos = -1
            var ch = 0
  
            // below method is for moving to next character.
            fun nextChar() {
                // on below line we are incrementing our position
                // and moving it to next position.
                ch = if (++pos < str.length) str[pos].toInt() else -1
            }
  
            // this method is use to check the extra space 
            // present int the expression and removing it.
            fun eat(charToEat: Int): Boolean {
                while (ch == ' '.toInt()) nextChar()
                // on below line we are checking the char pos 
                // if both is equal then we are returning it to true.
                if (ch == charToEat) {
                    nextChar()
                    return true
                }
                return false
            }
  
            // below method is to parse our 
            // expression and to get the ans
            // in this we are calling a parse 
            // expression method to calculate the value.
            fun parse(): Double {
                nextChar()
                val x = parseExpression()
                if (pos < str.length) throw RuntimeException("Unexpected: " + ch.toChar())
                return x
            }
  
            // in this method we will only perform addition and 
            // subtraction operation on the expression.
            fun parseExpression(): Double {
                var x = parseTerm()
                while (true) {
                    if (eat('+'.toInt())) x += parseTerm() // addition
                    else if (eat('-'.toInt())) x -= parseTerm() // subtraction
                    else return x
                }
            }
  
            // in below method we will perform 
            // only multiplication and division operation.
            fun parseTerm(): Double {
                var x = parseFactor()
                while (true) {
                    if (eat('*'.toInt())) x *= parseFactor() // multiplication
                    else if (eat('/'.toInt())) x /= parseFactor() // division
                    else return x
                }
            }
  
            // below method is use to parse the factor
            fun parseFactor(): Double {
                //on below line we are checking for addition 
                // and subtraction and performing unary operations.
                if (eat('+'.toInt())) return parseFactor() // unary plus
                if (eat('-'.toInt())) return -parseFactor() // unary minus
                // creating a double variable for ans.
                var x: Double
                // on below line we are creating 
                // a variable for position.
                val startPos = pos
                // on below line we are checking 
                // for opening and closing parenthesis.
                if (eat('('.toInt())) { // parentheses
                    x = parseExpression()
                    eat(')'.toInt())
                } else if (ch >= '0'.toInt() && ch <= '9'.toInt() || ch == '.'.toInt()) {
                    // numbers
                    while (ch >= '0'.toInt() && ch <= '9'.toInt() || ch == '.'.toInt()) nextChar()
                    // on below line we are getting sub string from our string using start and pos.
                    x = str.substring(startPos, pos).toDouble()
                } else if (ch >= 'a'.toInt() && ch <= 'z'.toInt()) {
                    // on below function we are checking for the operator in our expression.
                    while (ch >= 'a'.toInt() && ch <= 'z'.toInt()) nextChar()
                    val func = str.substring(startPos, pos)
                    // calling a method to parse our factor.
                    x = parseFactor()
                    // on below line we are checking for square root.
                    x =
                        if (func == "sqrt") Math.sqrt(x)
                        // on below line we are checking for sin function 
                        // and calculating sin function using Math class.
                        else if (func == "sin") Math.sin(
                            Math.toRadians(x)
                            // on below line we are calculating the cos value
                        ) else if (func == "cos") Math.cos(
                            Math.toRadians(x)
                            // on below line we are calculating
                            // the tan value of our expression.
                        ) else if (func == "tan")
                            Math.tan(Math.toRadians(x))
                        // on below line we are calculating 
                        // log value of the expression.
                        else if (func == "log")
                            Math.log10(x)
                        // on below line we are calculating
                        // ln value of expression.
                        else if (func == "ln") Math.log(x)
                        // f we get any error then
                        // we simply return the exception.
                        else throw RuntimeException(
                            "Unknown function: $func"
                        )
                } else {
                    // if the condition not satisfy then we are returning the exception
                    throw RuntimeException("Unexpected: " + ch.toChar())
                }
                // on below line we are calculating the power of the expression.
                if (eat('^'.toInt())) x = Math.pow(x, parseFactor()) // exponentiation
                return x
            }
            // at last calling a parse for our expression.
        }.parse()
    }
}

Now run your app to see the output of the app. 

Output:

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