How to convert a string to a integer in C

In C programming, converting a string to an integer is a common task that can be accomplished using several built-in functions. The most commonly used functions are atoi(), strtol(), and sscanf().

Syntax

int atoi(const char *str);
long strtol(const char *str, char **endptr, int base);
int sscanf(const char *str, const char *format, ...);

Method 1: Using atoi() Function

The atoi() function converts a string to an integer. It stops reading when it encounters a non-digit character −

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

int main() {
    char str[] = "12345";
    int num = atoi(str);
    
    printf("String: %s
", str);
    printf("Integer: %d
", num);
    
    return 0;
}

String: 12345
Integer: 12345

Method 2: Using strtol() Function

The strtol() function provides better error handling and allows you to specify the base for conversion −

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

int main() {
    char str[] = "1999";
    char *endptr;
    long num = strtol(str, &endptr, 10);
    
    if (*endptr == '\0') {
        printf("String: %s
", str);
        printf("Integer: %ld
", num);
    } else {
        printf("Conversion failed
");
    }
    
    return 0;
}

String: 1999
Integer: 1999

Method 3: Extracting Numbers from Strings

Here's an example that extracts a year from a movie title string −

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

int main() {
    char *name = "The Matrix(1999)";
    char *ps;
    char year_str[5] = "";
    int year, i;
    
    /* Find the opening bracket */
    ps = strchr(name, '(');
    
    if (ps != NULL) {
        /* Extract characters between brackets */
        for (i = 1; i < strlen(ps) - 1 && i < 5; i++) {
            year_str[i-1] = ps[i];
        }
        year_str[4] = '\0';
        
        /* Convert to integer */
        year = atoi(year_str);
        printf("Movie: %s
", name);
        printf("Year: %d
", year);
    }
    
    return 0;
}

Movie: The Matrix(1999)
Year: 1999

Key Points

• atoi() is simple but has no error checking
• strtol() provides better error handling and base conversion
• Always ensure the string contains valid numeric characters
• Use strchr() to locate specific characters in strings

Conclusion

Converting strings to integers in C can be done using atoi() for simple cases or strtol() for better error handling. Choose the method based on your specific requirements for error checking and validation.