Converting Strings to Numbers in C/C++

In C programming, converting strings to numbers is a common task that can be accomplished using several standard library functions. C provides multiple approaches to convert string representations of numbers into their corresponding numeric values.

Syntax

int atoi(const char *str);
long atol(const char *str);
double atof(const char *str);
long strtol(const char *str, char **endptr, int base);
double strtod(const char *str, char **endptr);

Method 1: Using atoi() for Integer Conversion

The atoi() function converts a string to an integer value −

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

int main() {
    char str1[] = "45";
    char str2[] = "3.14159";
    char str3[] = "31337 geek";
    
    int num1 = atoi(str1);
    int num2 = atoi(str2);
    int num3 = atoi(str3);
    
    printf("atoi("%s") is %d\n", str1, num1);
    printf("atoi("%s") is %d\n", str2, num2);
    printf("atoi("%s") is %d\n", str3, num3);
    
    return 0;
}

atoi("45") is 45
atoi("3.14159") is 3
atoi("31337 geek") is 31337

Method 2: Using atof() for Float Conversion

The atof() function converts a string to a floating-point number −

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

int main() {
    char str1[] = "45.67";
    char str2[] = "3.14159";
    char str3[] = "123.45abc";
    
    double num1 = atof(str1);
    double num2 = atof(str2);
    double num3 = atof(str3);
    
    printf("atof("%s") is %.2f\n", str1, num1);
    printf("atof("%s") is %.5f\n", str2, num2);
    printf("atof("%s") is %.2f\n", str3, num3);
    
    return 0;
}

atof("45.67") is 45.67
atof("3.14159") is 3.14159
atof("123.45abc") is 123.45

Method 3: Using strtol() for Advanced Integer Conversion

The strtol() function provides more control and error checking −

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

int main() {
    char str[] = "123abc456";
    char *endptr;
    
    long num = strtol(str, &endptr, 10);
    
    printf("Converted number: %ld\n", num);
    printf("Remaining string: "%s"\n", endptr);
    
    /* Converting hexadecimal */
    char hex[] = "FF";
    long hexNum = strtol(hex, NULL, 16);
    printf("Hex %s to decimal: %ld\n", hex, hexNum);
    
    return 0;
}

Converted number: 123
Remaining string: "abc456"
Hex FF to decimal: 255

Key Points

• atoi() and atof() stop at the first non-numeric character and return the converted portion.
• strtol() and strtod() provide better error handling and can detect conversion failures.
• These functions ignore leading whitespace but stop at the first invalid character.
• For invalid input, atoi() returns 0, while strtol() allows checking for errors.

Conclusion

C provides multiple functions for string-to-number conversion. Use atoi() and atof() for simple conversions, and strtol() or strtod() when you need error handling and more control over the conversion process.