Convert a String to an Integer in C

In C, we can convert a string to an integer using functions like atoi(), strtol(), and sscanf(). These functions help parse numerical values from a string representation and store them as an integer type.

In this tutorial, we will cover different methods to convert a string to an integer with step-by-step explanations and examples.

Methods to Convert a String to an Integer

1. Using atoi() Function to Convert a String to an Integer

The atoi() function (ASCII to integer) is a simple method to convert a string to an integer. It is part of the stdlib.h library. However, it does not provide error handling for invalid inputs.

main.c

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

int main() {
    char str[] = "12345";
    int num = atoi(str);

    printf("Converted integer: %d\n", num);
    return 0;
}

Explanation:

  1. We declare a character array str[] containing the numeric string "12345".
  2. The atoi() function from stdlib.h converts the string into an integer and stores it in num.
  3. The converted integer is printed using printf().

Output:

Converted integer: 12345

2. Using strtol() for Error Handling While Convert a String to an Integer

The strtol() (string to long) function is a safer alternative to atoi(). It allows error handling and supports different number bases.

main.c

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

int main() {
    char str[] = "98765";
    char *endptr;
    long num = strtol(str, &endptr, 10);

    if (*endptr != '\0') {
        printf("Conversion error, non-numeric characters found: %s\n", endptr);
    } else {
        printf("Converted integer: %ld\n", num);
    }
    
    return 0;
}

Explanation:

  1. We declare a character array str[] containing the numeric string "98765".
  2. The strtol() function converts the string into a long integer and stores it in num.
  3. The endptr pointer checks if any invalid characters are present after the number.
  4. If *endptr is not '\0', it means the string contains non-numeric characters, and an error message is printed.

Output:

Converted integer: 98765

3. Using sscanf() to Convert a String to an Integer

The sscanf() function can also be used to convert a string to an integer, similar to formatted input reading.

main.c

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#include <stdio.h>

int main() {
    char str[] = "54321";
    int num;

    if (sscanf(str, "%d", &num) == 1) {
        printf("Converted integer: %d\n", num);
    } else {
        printf("Conversion failed.\n");
    }

    return 0;
}

Explanation:

  1. We declare a character array str[] containing the numeric string "54321".
  2. The sscanf() function reads the string and converts it into an integer stored in num.
  3. If conversion is successful, it prints the integer; otherwise, it displays a failure message.

Output:

Converted integer: 54321

Conclusion

We explored different methods to convert a string to an integer in C:

  1. atoi(): Simple and fast but lacks error handling.
  2. strtol(): Provides error handling and supports different number bases.
  3. sscanf(): Useful for extracting multiple values from a string.

For better safety and error handling, strtol() is recommended over atoi().