Data Modelling & AI Data Structure & Algorithm

Write your own atoi()

23 June 2025

1

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 of atoi()

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[] = "neveropen";
    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[] = "neveropen";
    val = atoi(strn2);
    printf("String value = %s\n", strn2);
    printf("Integer value = %d\n", val);
 
    return (0);
}

Java

import java.util.*;
 
public class Main {
    public static void main(String[] args)
    {
        int val;
        String strn1 = "12546";
 
        val = Integer.parseInt(strn1);
        System.out.println("String value = " + strn1);
        System.out.println("Integer value = " + val);
 
        String strn2 = "neveropen";
        try {
            val = Integer.parseInt(strn2);
            System.out.println("String value = " + strn2);
            System.out.println("Integer value = " + val);
        }
        catch (NumberFormatException e) {
            val = 0;
            System.out.println("String value = " + strn2);
            System.out.println("Integer value = " + val);
        }
    }
}

Python3

def main():
    strn1 = "12546"
    val = int(strn1)
    print("String value = ", strn1)
    print("Integer value = ", val)
 
    strn2 = "neveropen"
    try:
        val = int(strn2)
    except ValueError:
        val = 0
        print("String value = ", strn2)
        print("Integer value = ", val)
 
 
if __name__ == "__main__":
    main()

C#

using System;
 
namespace neveropen {
class Program {
    static void Main(string[] args)
    {
        String strn1 = "12546";
        int val = int.Parse(strn1);
        Console.WriteLine("String value = " + strn1);
        Console.WriteLine("Integer value = " + val);
 
        String strn2 = "neveropen";
        try {
            val = int.Parse(strn2);
        }
        catch (FormatException e) {
            val = 0;
            Console.WriteLine("String value = " + strn2);
            Console.WriteLine("Integer value = " + val);
        }
    }
}
}

Javascript

// Javascript code to convert string to integer
let val;
let strn1 = "12546";
 
val = parseInt(strn1);
console.log("String value = " + strn1);
console.log("Integer value = " + val);
 
let strn2 = "neveropen";
val = parseInt(strn2);
console.log("String value = " + strn2);
console.log("Integer value = " + val);
 
// This code is contributed by prasad264

Output

String value = 12546
Integer value = 12546
String value = neveropen
Integer value = 0

Complexity Analysis:

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

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

Approach 1

The following is a simple implementation of conversion without considering any special case.

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

Below is the implementation of the above approach:

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

Python3

# 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 range(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

Complexity Analysis:

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

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 the sign with it.

Below is the implementation of the above approach:

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

Python3

# 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 range(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

Complexity Analysis:

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

Approach 3

Four corner cases need to be handled:

Discard all leading whitespaces
Sign of the number
Overflow
Invalid Input

Below are the steps for the above approach:

To remove the leading whitespaces, run a loop and ignore the whitespaces until a character of the digit is reached.
It keeps a sign variable to keep track of the sign of the number.
It checks for valid input characters if all characters are from 0 to 9 and converts them into integers.
If an overflow occurs and if the number is greater than or equal to INT_MAX/10, 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 <limits.h>
#include <stdio.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:

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

Exercise:

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

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Write your own atoi()

Syntax of atoi()

Parameters

Return Value

Example

C++

C

Java

Python3

C#

Javascript

Complexity Analysis:

Approach 1

C++

C

Java

Python3

C#

Javascript

Approach 2

C++

C

Java

Python3

C#

Javascript

Complexity Analysis:

Approach 3

Dry Run:

C++

C

Java

Python3

C#

Javascript

Complexity Analysis:

Related Articles:

Exercise:

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