atoi() Function in C

The atoi() function in C is used to convert a string (array of characters) into its integer representation. The name atoi stands for “ASCII to Integer”. This function is declared in the stdlib.h header file.

Syntax:

int atoi(const char *str);

Parameters:

• str: A pointer to a null-terminated string representing the number to be converted.

Return Value:

• Returns the integer value represented by the string.
• If the string doesn’t represent a valid number, the function returns 0.

Key Points:

1. Whitespace Handling: Leading whitespaces (spaces, tabs, etc.) are ignored.
2. Sign Handling: It can handle both positive and negative numbers (e.g., "-123" will return -123).
3. Non-Numeric Characters: If the string contains non-numeric characters (other than the optional leading ‘+’ or ‘-‘ and the digits), conversion stops at the first invalid character, and the integer value up to that point is returned.
4. Overflow and Underflow: In case of overflow or underflow, the result is implementation-dependent, though it may return a value that fits the limits of int.

Example:

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

int main() {
    const char *str1 = "12345";
    const char *str2 = "   -5678";
    const char *str3 = "abc123"; // Invalid string

    // Using atoi() to convert strings to integers
    int num1 = atoi(str1);
    int num2 = atoi(str2);
    int num3 = atoi(str3);  // Will return 0 because the string starts with non-numeric characters

    printf("String '%s' converts to %d\n", str1, num1);
    printf("String '%s' converts to %d\n", str2, num2);
    printf("String '%s' converts to %d\n", str3, num3);

    return 0;
}

Output:

String '12345' converts to 12345
String '   -5678' converts to -5678
String 'abc123' converts to 0

Notes:

• atoi() does not handle errors robustly. If you want better error handling (e.g., checking if the conversion succeeded), you can use strtol() instead, which provides more control over the conversion process.