Write your own atoi()

The atoi() function takes a string (which represents an integer) as an argument and returns its value.

Following is a simple implementation. We initialize result as 0. We start from the first character and update result for every character.

// A simple C++ program for implementation of atoi #include <stdio.h>   // A simple atoi() function int myAtoi(char *str) {     int res = 0; // Initialize result       // Iterate through all characters of input string and update result     for (int i = 0; str[i] != '\0'; ++i)         res = res*10 + str[i] - '0';       // return result.     return res; }   // Driver program to test above function int main() {     char str[] = "89789";     int val = myAtoi(str);     printf ("%d ", val);     return 0; }

Output:

89789

The above function doesn’t handle negative numbers. Following is a simple extension to handle negative numbers.

// A C++ program for implementation of atoi #include <stdio.h>   // A simple atoi() function int myAtoi(char *str) {     int res = 0;  // Initialize result     int sign = 1;  // Initialize sign as positive     int i = 0;  // Initialize index of first digit           // If number is negative, then update sign     if (str[0] == '-')     {         sign = -1;          i++;  // Also update index of first digit     }           // 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 program to test above function int main() {     char str[] = "-123";     int val = myAtoi(str);     printf ("%d ", val);     return 0; }

Output:

-123

The above implementation doesn’t handle errors. What if str is NULL or str contains non-numeric characters.Following implementation handles errors.

// A simple C++ program for implementation of atoi   #include <stdio.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 == NULL)        return 0;       int res = 0;  // Initialize result     int sign = 1;  // Initialize sign as positive     int i = 0;  // Initialize index of first digit       // If number is negative, then update sign     if (str[0] == '-')     {         sign = -1;         i++;  // Also update index of first digit     }       // Iterate through all digits of input string and update result     for (; str[i] != '\0'; ++i)     {         if (isNumericChar(str[i]) == false)             return 0; // You may add some lines to write error message                       // 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; }

Time Complexity: O(n) where n is the number of characters in input string.

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.