atoi() Function

The atoi() function in C converts a string representing a number into its corresponding integer value. It processes the string by skipping any leading whitespace, handling an optional sign, and then reading in the digits to form the integer.

Syntax of atoi()

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int atoi(const char *str);

Parameters

ParameterDescription
strC-string beginning with the representation of an integral number.

Return Value

The function returns the converted integral number as an int value. If no valid conversion is performed, it returns 0.

Notes

The atoi() function begins by discarding any leading whitespace characters. It then checks for an optional plus or minus sign and converts the subsequent digits into an integer value. Any characters following the number are ignored and do not affect the conversion.

Examples for atoi()

Example 1: Converting a Basic Numeric String

This example demonstrates how atoi() converts a simple numeric string to an integer.

Program

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

int main() {
    char numStr[] = "12345";
    int num = atoi(numStr);
    printf("Converted number: %d\n", num);
    return 0;
}

Explanation

  1. The string "12345" is converted to the integer 12345.
  2. The program prints the converted number.

Output

Converted number: 12345

Example 2: Converting a String with Leading Whitespaces and a Negative Sign

This example illustrates how atoi() handles strings that contain leading whitespaces and a negative sign.

Program

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

int main() {
    char numStr[] = "   -9876xyz";
    int num = atoi(numStr);
    printf("Converted number: %d\n", num);
    return 0;
}

Explanation

  1. The function skips the leading spaces and detects the negative sign.
  2. The digits are then converted into the integer -9876, ignoring the non-numeric characters that follow.

Output

Converted number: -9876

Example 3: Handling an Invalid Numeric Conversion

This example shows the behavior of atoi() when the input string does not begin with a valid numeric value.

Program

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

int main() {
    char numStr[] = "abc985";
    int num = atoi(numStr);
    printf("Converted number: %d\n", num);
    return 0;
}

Explanation

  1. The string "abc985" does not start with a numeric value.
  2. atoi() fails to convert any portion of the string, resulting in a return value of 0.

Output

Converted number: 0