How to Convert String to Number

Given a string representation of a numerical value, convert it into an actual numerical value. In this article, we will provide a detailed overview about different ways to convert string to number in different languages.

Convert String to Number in C:

1. Using Built-in Functions:

• Use the built-in function to convert the string to a number.

C

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char str[] = "123";

    // Step 1
    int num = atoi(str);

    printf("%d\n", num);
    return 0;
}

Output

123

2. String Concatenation:

• Check if the string represents a negative number and set a flag.
• Iterate over each character of the string and convert it to its corresponding numerical value.
• Calculate the numerical value using the digits.

C

#include <stdio.h>

int main()
{
    char str[] = "-123";
    int i = 0, sign = 1, num = 0;

    // Step 1
    if (str[0] == '-') {
        sign = -1;
        i++;
    }

    // Step 2
    while (str[i] != '\0') {
        num = num * 10 + (str[i] - '0');
        i++;
    }

    // Step 3
    num *= sign;

    printf("%d\n", num);
    return 0;
}

Output

-123

Convert String to Number in C++:

1. Using Built-in Functions:

• Use the built-in function to convert the string to a number.

C++

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string str = "123";

    // Step 1
    int num = stoi(str);

    cout << num << endl;
    return 0;
}

Output

123

2. String Concatenation:

• Check if the string represents a negative number and set a flag.
• Iterate over each character of the string and convert it to its corresponding numerical value.
• Calculate the numerical value using the digits.

C++

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string str = "-123";
    int sign = 1, num = 0;

    // Step 1
    if (str[0] == '-') {
        sign = -1;
        str = str.substr(1);
    }

    // Step 2
    for (char c : str) {
        num = num * 10 + (c - '0');
    }

    // Step 3
    num *= sign;

    cout << num << endl;
    return 0;
}

Output

-123

Convert String to Number in Java:

1. Using Built-in Functions:

• Use the built-in function to convert the string to a number.

Java

public class Main {
    public static void main(String[] args)
    {
        String str = "123";

        // Step 1
        int num = Integer.parseInt(str);

        System.out.println(num);
    }
}

Output

123

2. String Concatenation:

• Check if the string represents a negative number and set a flag.
• Iterate over each character of the string and convert it to its corresponding numerical value.
• Calculate the numerical value using the digits.

Java

public class Main {
    public static void main(String[] args)
    {
        String str = "-123";
        int sign = 1, num = 0;

        // Step 1
        if (str.charAt(0) == '-') {
            sign = -1;
            str = str.substring(1);
        }

        // Step 2
        for (char c : str.toCharArray()) {
            num = num * 10 + (c - '0');
        }

        // Step 3
        num *= sign;

        System.out.println(num);
    }
}

Output

-123

Convert String to Number in Python:

1. Using Built-in Functions:

• Use the built-in function to convert the string to a number.

Python

str_num = "123"

# Step 1
num = int(str_num)

print(num)

Output

123

2. String Concatenation:

• Check if the string represents a negative number and set a flag.
• Iterate over each character of the string and convert it to its corresponding numerical value.
• Calculate the numerical value using the digits.

Python

str_num = "-123"
sign = 1
num = 0

# Step 1
if str_num[0] == '-':
    sign = -1
    str_num = str_num[1:]

# Step 2
for c in str_num:
    num = num * 10 + int(c)

# Step 3
num *= sign

print(num)

Output

-123

Convert String to Number in C#:

1. Using Built-in Functions:

• Use the built-in function to convert the string to a number.

C#

using System;

class Program
{
    static void Main()
    {
        string str = "123";

        // Step 1
        int num = int.Parse(str);

        Console.WriteLine(num);
    }
}

Output

123

2. String Concatenation:

• Check if the string represents a negative number and set a flag.
• Iterate over each character of the string and convert it to its corresponding numerical value.
• Calculate the numerical value using the digits.

C#

using System;

class Program
{
    static void Main()
    {
        string str = "-123";
        int sign = 1, num = 0;

        // Step 1
        if (str[0] == '-')
        {
            sign = -1;
            str = str.Substring(1);
        }

        // Step 2
        foreach (char c in str)
        {
            num = num * 10 + (c - '0');
        }

        // Step 3
        num *= sign;

        Console.WriteLine(num);
    }
}

Output

-123

Convert String to Number in JavaScript:

1. Using Built-in Functions:

• Use the built-in function to convert the string to a number.

JavaScript

let str_num = "123";

// Step 1
let num = parseInt(str_num);

console.log(num);

Output

123

2. String Concatenation:

• Check if the string represents a negative number and set a flag.
• Iterate over each character of the string and convert it to its corresponding numerical value.
• Calculate the numerical value using the digits.

JavaScript

let str_num = "-123";
let sign = 1;
let num = 0;

// Step 1
if (str_num[0] === '-') {
    sign = -1;
    str_num = str_num.slice(1);
}

// Step 2
for (let c of str_num) {
    num = num * 10 + parseInt(c);
}

// Step 3
num *= sign;

console.log(num);

Output

-123

Conclusion:

Converting a string to a number involves interpreting the characters in the string as numerical values. This typically requires iterating through each character, converting it to its numeric equivalent, and considering any sign indicators. By following this process, strings can be transformed into their corresponding numeric representations, facilitating numerical operations and calculations in programming.