C Program To Write Your Own atoi()

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

int main()
{
    int val;
    char strn1[] = "12546";

    val = atoi(strn1);
    printf("String value = %s
", strn1);
    printf("Integer value = %d
", val);

    char strn2[] = "GeeksforGeeks";
    val = atoi(strn2);
    printf("String value = %s
", strn2);
    printf("Integer value = %d
", val);

    return (0);
}

// 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] != ''; ++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;
}

// 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] != ''; ++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;
}

// 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.