Write your own atoi()

Difficulty Level : Easy
Last Updated : 11 Aug, 2021

The atoi() function in C takes a string (which represents an integer) as an argument and returns its value of type int. So basically the function is used to convert a string argument to an integer.

Syntax:

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int atoi(const char strn)

Parameters: The function accepts one parameter strn which refers to the string argument that is needed to be converted into its integer equivalent.

Return Value: If strn is a valid input, then the function returns the equivalent integer number for the passed string number. If no valid conversion takes place, then the function returns zero.

Example:

C++

#include <bits/stdc++.h>
using namespace std;
 
int main()
{
    int val;
    char strn1[] = "12546";
 
    val = atoi(strn1);
    cout <<"String value = " << strn1 << endl;
    cout <<"Integer value = " << val << endl;
 
    char strn2[] = "GeeksforGeeks";
    val = atoi(strn2);
    cout <<"String value = " << strn2 << endl;
    cout <<"Integer value = " << val <<endl;
 
    return (0);
}
 
// This code is contributed by shivanisinghss2110

C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
int main()
{
    int val;
    char strn1[] = "12546";
 
    val = atoi(strn1);
    printf("String value = %s\n", strn1);
    printf("Integer value = %d\n", val);
 
    char strn2[] = "GeeksforGeeks";
    val = atoi(strn2);
    printf("String value = %s\n", strn2);
    printf("Integer value = %d\n", val);
 
    return (0);
}

Output

String value = 12546
Integer value = 12546
String value = GeeksforGeeks
Integer value = 0

Now let’s understand various ways in which one can create there own atoi() function supported by various conditions:

Approach 1: Following is a simple implementation of conversion without considering any special case.

Initialize the result as 0.
Start from the first character and update result for every character.
For every character update the answer as result = result * 10 + (s[i] – ‘0’)

C++

// A simple C++ program for
// implementation of atoi
#include <bits/stdc++.h>
using namespace std;
 
// A simple atoi() function
int myAtoi(char* str)
{
    // Initialize result
    int res = 0;
 
    // Iterate through all characters
    // of input string and update result
    // take ASCII character of corresponding digit and
    // subtract the code from '0' to get numerical
    // value and multiply res by 10 to shuffle
    // digits left to update running total
    for (int i = 0; str[i] != '\0'; ++i)
        res = res * 10 + str[i] - '0';
 
    // return result.
    return res;
}
 
// Driver code
int main()
{
    char str[] = "89789";
   
    // Function call
    int val = myAtoi(str);
    cout << val;
    return 0;
}
 
// This is code is contributed by rathbhupendra

C

// Program to implement atoi() in C
#include <stdio.h>
 
// A simple atoi() function
int myAtoi(char* str)
{
    // Initialize result
    int res = 0;
 
    // Iterate through all characters
    // of input string and update result
    // take ASCII character of corresponding digit and
    // subtract the code from '0' to get numerical
    // value and multiply res by 10 to shuffle
    // digits left to update running total
    for (int i = 0; str[i] != '\0'; ++i)
        res = res * 10 + str[i] - '0';
 
    // return result.
    return res;
}
 
// Driver Code
int main()
{
    char str[] = "89789";
   
    // Function call
    int val = myAtoi(str);
    printf("%d ", val);
    return 0;
}

Java

// A simple Java program for
// implementation of atoi
class GFG {
 
    // A simple atoi() function
    static int myAtoi(String str)
    {
        // Initialize result
        int res = 0;
 
        // Iterate through all characters
        // of input string and update result
        // take ASCII character of corresponding digit and
        // subtract the code from '0' to get numerical
        // value and multiply res by 10 to shuffle
        // digits left to update running total
        for (int i = 0; i < str.length(); ++i)
            res = res * 10 + str.charAt(i) - '0';
 
        // return result.
        return res;
    }
 
    // Driver code
    public static void main(String[] args)
    {
        String str = "89789";
       
         
        // Function call
        int val = myAtoi(str);
        System.out.println(val);
    }
}
 
// This code is contributed by PrinciRaj1992

Python

# Python program for implementation of atoi
 
# A simple atoi() function
 
 
def myAtoi(string):
    res = 0
 
    # Iterate through all characters of
    #  input string and update result
    for i in xrange(len(string)):
        res = res * 10 + (ord(string[i]) - ord('0'))
 
    return res
 
 
# Driver program
string = "89789"
 
# Function call
print myAtoi(string)
 
# This code is contributed by BHAVYA JAIN

C#

// A simple C# program for implementation
// of atoi
using System;
 
class GFG {
 
    // A simple atoi() function
    static int myAtoi(string str)
    {
        int res = 0; // Initialize result
 
        // Iterate through all characters
        // of input string and update result
        // take ASCII character of corresponding digit and
        // subtract the code from '0' to get numerical
        // value and multiply res by 10 to shuffle
        // digits left to update running total
        for (int i = 0; i < str.Length; ++i)
            res = res * 10 + str[i] - '0';
 
        // return result.
        return res;
    }
 
    // Driver code
    public static void Main()
    {
        string str = "89789";
       
        // Function call
        int val = myAtoi(str);
        Console.Write(val);
    }
}
 
// This code is contributed by Sam007.

Javascript

<script>
// A simple Javascript program for
// implementation of atoi
 
// A simple atoi() function
function myAtoi(str)
{
    // Initialize result
        let res = 0;
  
        // Iterate through all characters
        // of input string and update result
        // take ASCII character of corresponding digit and
        // subtract the code from '0' to get numerical
        // value and multiply res by 10 to shuffle
        // digits left to update running total
        for (let i = 0; i < str.length; ++i)
            res = res * 10 + str[i].charCodeAt(0) - '0'.charCodeAt(0);
  
        // return result.
        return res;
}
 
// Driver code
let str = "89789";
        
          
// Function call
let val = myAtoi(str);
document.write(val);
 
 
// This code is contributed by rag2127
</script>

Output

Approach 2: This implementation handles the negative numbers. If the first character is ‘-‘ then store the sign as negative and then convert the rest of the string to number using the previous approach while multiplying sign with it.

C++

// A C++ program for
// implementation of atoi
#include <bits/stdc++.h>
using namespace std;
 
// A simple atoi() function
int myAtoi(char* str)
{
    // Initialize result
    int res = 0;
 
    // Initialize sign as positive
    int sign = 1;
 
    // Initialize index of first digit
    int i = 0;
 
    // If number is negative,
    // then update sign
    if (str[0] == '-') {
        sign = -1;
 
        // Also update index of first digit
        i++;
    }
 
    // Iterate through all digits
    // and update the result
    for (; str[i] != '\0'; ++i)
        res = res * 10 + str[i] - '0';
 
    // Return result with sign
    return sign * res;
}
 
// Driver code
int main()
{
    char str[] = "-123";
 
    // Function call
    int val = myAtoi(str);
    cout << val;
    return 0;
}
 
// This is code is contributed by rathbhupendra

C

// A C program for
// implementation of atoi
#include <stdio.h>
 
// A simple atoi() function
int myAtoi(char* str)
{
    // Initialize result
    int res = 0;
 
    // Initialize sign as positive
    int sign = 1;
 
    // Initialize index of first digit
    int i = 0;
 
    // If number is negative,
    // then update sign
    if (str[0] == '-') {
        sign = -1;
 
        // Also update index of first digit
        i++;
    }
 
    // Iterate through all digits
    // and update the result
    for (; str[i] != '\0'; ++i)
        res = res * 10 + str[i] - '0';
 
    // Return result with sign
    return sign * res;
}
 
// Driver code
int main()
{
    char str[] = "-123";
   
    // Function call
    int val = myAtoi(str);
    printf("%d ", val);
    return 0;
}

Java

// Java program for
// implementation of atoi
class GFG {
 
    // A simple atoi() function
    static int myAtoi(char[] str)
    {
 
        // Initialize result
        int res = 0;
 
        // Initialize sign as positive
        int sign = 1;
 
        // Initialize index of first digit
        int i = 0;
 
        // If number is negative, then
        // update sign
        if (str[0] == '-') {
            sign = -1;
 
            // Also update index of first
            // digit
            i++;
        }
 
        // Iterate through all digits
        // and update the result
        for (; i < str.length; ++i)
            res = res * 10 + str[i] - '0';
 
        // Return result with sign
        return sign * res;
    }
 
    // Driver code
    public static void main(String[] args)
    {
        char[] str = "-123".toCharArray();
       
        // Function call
        int val = myAtoi(str);
        System.out.println(val);
    }
}
 
// This code is contributed by 29AjayKumar

Python

# Python program for implementation of atoi
 
# A simple atoi() function
 
 
def myAtoi(string):
    res = 0
    # initialize sign as positive
    sign = 1
    i = 0
 
    # if number is negative then update sign
    if string[0] == '-':
        sign = -1
        i += 1
 
    # Iterate through all characters
    # of input string and update result
    for j in xrange(i, len(string)):
        res = res*10+(ord(string[j])-ord('0'))
 
    return sign * res
 
 
# Driver code
string = "-123"
 
# Function call
print myAtoi(string)
 
# This code is contributed by BHAVYA JAIN

C#

// C# program for implementation of atoi
using System;
 
class GFG {
 
    // A simple atoi() function
    static int myAtoi(string str)
    {
 
        // Initialize result
        int res = 0;
 
        // Initialize sign as positive
        int sign = 1;
 
        // Initialize index of first digit
        int i = 0;
 
        // If number is negative, then
        // update sign
        if (str[0] == '-') {
            sign = -1;
 
            // Also update index of first
            // digit
            i++;
        }
 
        // Iterate through all digits
        // and update the result
        for (; i < str.Length; ++i)
            res = res * 10 + str[i] - '0';
 
        // Return result with sign
        return sign * res;
    }
 
    // Driver code
    public static void Main()
    {
        string str = "-123";
       
        // Function call
        int val = myAtoi(str);
        Console.Write(val);
    }
}
 
// This code is contributed by Sam007.

Javascript

<script>
 
    // JavaScript program for implementation of atoi
 
    // A simple atoi() function
    function myAtoi(str)
    {
  
        // Initialize result
        var res = 0;
  
        // Initialize sign as positive
        var sign = 1;
  
        // Initialize index of first digit
        var i = 0;
  
        // If number is negative, then
        // update sign
        if (str[0] == '-') {
            sign = -1;
  
            // Also update index of first
            // digit
            i++;
        }
  
        // Iterate through all digits
        // and update the result
        for (; i < str.length; ++i)
            res = res * 10 + str[i].charCodeAt(0) - '0'.charCodeAt(0);
  
        // Return result with sign
        return sign * res;
    }
  
      // Driver code
          var str = "-129";
          var val=myAtoi(str);
        document.write(val);
 
</script>
 <! --This code is contributed by nirajgusain5 -->

Output

-123

Approach 3: This implementation handles various type of errors. If str is NULL or str contains non-numeric characters then return 0 as the number is not valid.

C++

// A simple C++ program for
// implementation of atoi
#include <iostream>
using namespace std;
 
// A utility function to check
// whether x is numeric
bool isNumericChar(char x)
{
    return (x >= '0' && x <= '9') ? true : false;
}
 
// A simple atoi() function.
// If the given string contains
// any invalid character, then
// this function returns 0
int myAtoi(char* str)
{
    if (*str == '\0')
        return 0;
 
    // Initialize result
    int res = 0;
 
    // Initialize sign as positive
    int sign = 1;
 
    // Initialize index of first digit
    int i = 0;
 
    // If number is negative,
    // then update sign
    if (str[0] == '-')
    {
        sign = -1;
 
        // Also update index of first digit
        i++;
    }
 
    // Iterate through all digits of
    // input string and update result
    for (; str[i] != '\0'; ++i)
    {
 
        // You may add some lines
        // to write error message
        if (isNumericChar(str[i]) == false)
            break;
 
        // to error stream
        res = res * 10 + str[i] - '0';
    }
 
    // Return result with sign
    return sign * res;
}
 
// Driver program to test above function
int main()
{
    char str[] = "-134";
    int val = myAtoi(str);
    cout << " "<< val;
    return 0;
}
 
// this code is contributed by shivanisinghss2110

C

// A simple C++ program for
// implementation of atoi
 
#include <stdio.h>
#include <stdbool.h>
// A utility function to check
// whether x is numeric
bool isNumericChar(char x)
{
    return (x >= '0' && x <= '9') ? true : false;
}
 
// A simple atoi() function.
// If the given string contains
// any invalid character, then
// this function returns 0
int myAtoi(char* str)
{
    if (*str == '\0')
        return 0;
 
    // Initialize result
    int res = 0;
 
    // Initialize sign as positive
    int sign = 1;
 
    // Initialize index of first digit
    int i = 0;
 
    // If number is negative,
    // then update sign
    if (str[0] == '-')
    {
        sign = -1;
 
        // Also update index of first digit
        i++;
    }
 
    // Iterate through all digits of
    // input string and update result
    for (; str[i] != '\0'; ++i)
    {
 
        // You may add some lines
        // to write error message
        if (isNumericChar(str[i]) == false)
            return 0;
 
        // to error stream
        res = res * 10 + str[i] - '0';
    }
 
    // Return result with sign
    return sign * res;
}
 
// Driver program to test above function
int main()
{
    char str[] = "-134";
    int val = myAtoi(str);
    printf("%d ", val);
    return 0;
}

Java

// A simple Java program for
// implementation of atoi
class Atoi {
 
    // A simple atoi() function.
    // If the given string contains
    // any invalid character, then
    // this function returns 0
    public static int atoi(String str)
    {
 
        if (str.length() == 0)
        {
            return -1;
        }
        // Initialize sign as positive
        int sign = 1;
 
        // Initialize result
        int res = 0;
 
        // Initialize index of first digit
        int i = 0;
 
        // If number is negative,
        // then update sign
        if (str.charAt(i) == '-')
        {
            sign = -1;
 
            // Also update index of first digit
            i++;
        }
 
        // Iterate through all digits of input
        // string and update result
        for (; i < str.length(); i++) {
            if (Character.isDigit(str.charAt(i)) == false)
            {
                return -1;
            }
            res = res * 10
                  + Character.getNumericValue(
                        str.charAt(i));
        }
 
        return sign * res;
    }
 
    // Driver code
    public static void main(String[] args)
    {
        String str = "-134";
       
        // Function call
        int num = atoi(str);
        System.out.println(num);
    }
}
 
// This code is contributed by Anonymous_Geek

Python

# Python program for
# implementation of atoi
 
# A utility function to check
# whether x is numeric
 
 
def isNumericChar(x):
    if (x >= '0' and x <= '9'):
        return True
    return False
 
# A simple atoi() function.
# If the given string contains
# any invalid character,
# then this function returns 0
 
 
def myAtoi(string):
    if len(string) == 0:
        return 0
 
    res = 0
    # initialize sign as positive
    sign = 1
    i = 0
 
    # if number is negative then update sign
    if string[0] == '-':
        sign = -1
        i += 1
 
    # Iterate through all characters
    # of input string and update result
    for j in xrange(i, len(string)):
         
        # You may add some lines to
        # write error message to error stream
        if isNumericChar(string[j] == False):
            return 0
 
        res = res * 10 + (ord(string[j]) - ord('0'))
 
    return sign * res
 
 
# Driver code
string = "-134"
 
# Function call
print myAtoi(string)
 
# This code is contributed by BHAVYA JAIN

C#

// A simple C# program for implementation of atoi
using System;
 
class Atoi {
 
    // A simple atoi() function. If the given string
    // contains any invalid character, then this function
    // returns 0
    public static int atoi(String str)
    {
 
        if (str.Length == 0) {
            return -1;
        }
        int sign = 1; // Initialize sign as positive
        int res = 0; // Initialize result
        int i = 0; // Initialize index of first digit
 
        // If number is negative, then update sign
        if (str[i] == '-') {
            sign = -1;
            i++; // Also update index of first digit
        }
 
        // Iterate through all digits of input
        // string and update result
        for (; i < str.Length; i++) {
            if (char.IsDigit(str[i]) == false) {
                return -1;
            }
            res = res * 10 + int.Parse(str[i].ToString());
        }
 
        return sign * res;
    }
 
    // Driver code
    public static void Main(String[] args)
    {
        String str = "-134";
       
        // Function call
        int num = atoi(str);
        Console.WriteLine(num);
    }
}
 
// This code is contributed by 29AjayKumar

Javascript

<script>
 
// A simple Javascript program for
// implementation of atoi
 
// A simple atoi() function.
// If the given string contains
// any invalid character, then
// this function returns 0
function atoi(str)
{
    if (str.length == 0)
    {
        return -1;
    }
     
    // Initialize sign as positive
    let sign = 1;
 
    // Initialize result
    let res = 0;
 
    // Initialize index of first digit
    let i = 0;
 
    // If number is negative,
    // then update sign
    if (str[i] == '-')
    {
        sign = -1;
         
        // Also update index of first digit
        i++;
    }
      
    // Iterate through all digits of input
    // string and update result
    for(; i < str.length; i++)
    {
        if (!isNaN(parseInt(str[i])) == false)
        {
            return -1;
        }
        res = res * 10 + parseInt(str[i]);
    }
    return sign * res;
}
 
// Driver code
let str = "-134";
 
// Function call
let num = atoi(str);
 
document.write(num);
 
// This code is contributed by avanitrachhadiya2155
 
</script>

Output

 -134

Approach 4: Four corner cases needs to be handled:

Discards all leading whitespaces
Sign of the number
Overflow
Invalid input

To remove the leading whitespaces run a loop until a character of the digit is reached. If the number is greater than or equal to INT_MAX/10. Then return INT_MAX if the sign is positive and return INT_MIN if the sign is negative. The other cases are handled in previous approaches.

Dry Run:

Below is the implementation of the above approach:

C++

// A simple C++ program for
// implementation of atoi
#include <bits/stdc++.h>
using namespace std;
 
int myAtoi(const char* str)
{
    int sign = 1, base = 0, i = 0;
     
    // if whitespaces then ignore.
    while (str[i] == ' ')
    {
        i++;
    }
     
    // sign of number
    if (str[i] == '-' || str[i] == '+')
    {
        sign = 1 - 2 * (str[i++] == '-');
    }
   
    // checking for valid input
    while (str[i] >= '0' && str[i] <= '9')
    {
        // handling overflow test case
        if (base > INT_MAX / 10
            || (base == INT_MAX / 10
            && str[i] - '0' > 7))
        {
            if (sign == 1)
                return INT_MAX;
            else
                return INT_MIN;
        }
        base = 10 * base + (str[i++] - '0');
    }
    return base * sign;
}
 
 
// Driver Code
int main()
{
    char str[] = "  -123";
   
    // Functional Code
    int val = myAtoi(str);
    cout <<" "<< val;
    return 0;
}
 
// This code is contributed by shivanisinghss2110

C

// A simple C++ program for
// implementation of atoi
#include <stdio.h>
#include <limits.h>
 
int myAtoi(const char* str)
{
    int sign = 1, base = 0, i = 0;
     
    // if whitespaces then ignore.
    while (str[i] == ' ')
    {
        i++;
    }
     
    // sign of number
    if (str[i] == '-' || str[i] == '+')
    {
        sign = 1 - 2 * (str[i++] == '-');
    }
   
    // checking for valid input
    while (str[i] >= '0' && str[i] <= '9')
    {
        // handling overflow test case
        if (base > INT_MAX / 10
            || (base == INT_MAX / 10
            && str[i] - '0' > 7))
        {
            if (sign == 1)
                return INT_MAX;
            else
                return INT_MIN;
        }
        base = 10 * base + (str[i++] - '0');
    }
    return base * sign;
}
 
 
// Driver Code
int main()
{
    char str[] = "  -123";
   
    // Functional Code
    int val = myAtoi(str);
    printf("%d ", val);
    return 0;
}
// This code is contributed by Yogesh shukla.

Java

// A simple Java program for
// implementation of atoi
class GFG {
    static int myAtoi(char[] str)
    {
        int sign = 1, base = 0, i = 0;
 
        // if whitespaces then ignore.
        while (str[i] == ' ')
        {
            i++;
        }
 
        // sign of number
        if (str[i] == '-' || str[i] == '+')
        {
            sign = 1 - 2 * (str[i++] == '-' ? 1 : 0);
        }
 
        // checking for valid input
        while (i < str.length
               && str[i] >= '0'
               && str[i] <= '9') {
 
            // handling overflow test case
            if (base > Integer.MAX_VALUE / 10
                || (base == Integer.MAX_VALUE / 10
                    && str[i] - '0' > 7))
            {
                if (sign == 1)
                    return Integer.MAX_VALUE;
                else
                    return Integer.MIN_VALUE;
            }
            base = 10 * base + (str[i++] - '0');
        }
        return base * sign;
    }
 
    // Driver code
    public static void main(String[] args)
    {
        char str[] = " -123".toCharArray();
       
        // Function call
        int val = myAtoi(str);
        System.out.printf("%d ", val);
    }
}
 
// This code is contributed by 29AjayKumar

Python3

# A simple Python3 program for
# implementation of atoi
import sys
 
def myAtoi(Str):
 
    sign, base, i = 1, 0, 0
     
    # If whitespaces then ignore.
    while (Str[i] == ' '):
        i += 1
     
    # Sign of number
    if (Str[i] == '-' or Str[i] == '+'):
        sign = 1 - 2 * (Str[i] == '-')
        i += 1
 
    # Checking for valid input
    while (i < len(Str) and
          Str[i] >= '0' and Str[i] <= '9'):
               
        # Handling overflow test case
        if (base > (sys.maxsize // 10) or
           (base == (sys.maxsize // 10) and
           (Str[i] - '0') > 7)):
            if (sign == 1):
                return sys.maxsize
            else:
                return -(sys.maxsize)
         
        base = 10 * base + (ord(Str[i]) - ord('0'))
        i += 1
     
    return base * sign
 
# Driver Code
Str = list(" -123")
 
# Functional Code
val = myAtoi(Str)
 
print(val)
 
# This code is contributed by divyeshrabadiya07

C#

// A simple C# program for implementation of atoi
using System;
 
class GFG {
    static int myAtoi(char[] str)
    {
        int sign = 1, Base = 0, i = 0;
 
        // if whitespaces then ignore.
        while (str[i] == ' ') {
            i++;
        }
 
        // sign of number
        if (str[i] == '-' || str[i] == '+') {
            sign = 1 - 2 * (str[i++] == '-' ? 1 : 0);
        }
 
        // checking for valid input
        while (
            i < str.Length
            && str[i] >= '0'
            && str[i] <= '9') {
 
            // handling overflow test case
            if (Base > int.MaxValue / 10 || (Base == int.MaxValue / 10 && str[i] - '0' > 7)) {
                if (sign == 1)
                    return int.MaxValue;
                else
                    return int.MinValue;
            }
            Base = 10 * Base + (str[i++] - '0');
        }
        return Base * sign;
    }
 
    // Driver code
    public static void Main(String[] args)
    {
        char[] str = " -123".ToCharArray();
        int val = myAtoi(str);
        Console.Write("{0} ", val);
    }
}
 
// This code is contributed by 29AjayKumar

Javascript

<script>
// A simple JavaScript program for
// implementation of atoi
    function myAtoi(str)
   {
    var sign = 1, base = 0, i = 0;
     
    // if whitespaces then ignore.
    while (str[i] == ' ')
    {
        i++;
    }
     
    // sign of number
    if (str[i] == '-' || str[i] == '+')
    {
        sign = 1 - 2 * (str[i++] == '-');
    }
   
    // checking for valid input
    while (str[i] >= '0' && str[i] <= '9')
    {
        // handling overflow test case
        if (base > Number.MAX_VALUE/ 10
            || (base == Number.MAX_VALUE / 10
            && str[i] - '0' > 7))
        {
            if (sign == 1)
                return Number.MAX_VALUE;
            else
                return Number.MAX_VALUE;
        }
        base = 10 * base + (str[i++] - '0');
    }
    return base * sign;
}
 
    // Driver code
        var str = " -123";
       
        // Function call
        var val = myAtoi(str);
        document.write(" ", val);
     
// This code is contributed by shivanisinghss2110
</script>

Output

 -123

Complexity Analysis for all the above Approaches:

Time Complexity: O(n).
Only one traversal of string is needed.
Space Complexity: O(1).
As no extra space is required.

Recursive program for atoi().

Exercise:
Write your won atof() that takes a string (which represents an floating point value) as an argument and returns its value as double.

This article is compiled by Abhay Rathi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
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