How to Check if a String Contains Non-ASCII Characters in C

Check if a String Contains Non-ASCII Characters in C

In this tutorial, you will learn how to check if a string contains non-ASCII characters in C. We will explore multiple approaches to detect characters outside the ASCII range (0 to 127) using different functions and methods.

---

Example 1: Checking Non-ASCII Characters using a Simple Loop

In this example, we will use a simple for loop to iterate through each character in a string and check if its ASCII value is greater than 127. If any character exceeds this value, we determine that the string contains non-ASCII characters.

main.c

#include <stdio.h>

int main() {
    char str[] = "Hello, Wörld!"; // String with a non-ASCII character (ö)
    int i;
    int flag = 0;

    // Check each character in the string
    for (i = 0; str[i] != '\0'; i++) {
        if ((unsigned char)str[i] > 127) {
            flag = 1;
            break;
        }
    }

    if (flag) {
        printf("The string contains non-ASCII characters.\n");
    } else {
        printf("The string contains only ASCII characters.\n");
    }

    return 0;
}

Explanation:

1. The string str is initialized with “Hello, Wörld!”, which includes the non-ASCII character ö.
2. A for loop iterates over the string until the null terminator '\0' is encountered.
3. Each character is cast to unsigned char and its ASCII value is compared with 127.
4. If a character’s ASCII value is greater than 127, the flag variable is set to 1 and the loop breaks.
5. After the loop, the flag is checked to determine whether to print a message indicating the presence of non-ASCII characters.

Output:

The string contains non-ASCII characters.

Example 2: Checking Non-ASCII Characters using the isascii Function

In this example, we will use the isascii function from the <ctype.h> library to check whether each character in the string is an ASCII character. The function returns a non-zero value if the character is within the ASCII range and 0 otherwise.

main.c

#include <stdio.h>
#include <ctype.h>

int main() {
    char str[] = "Hello, 世界!"; // String with non-ASCII characters (世界)
    int i;
    int flag = 0;

    // Iterate over each character and use isascii() to check
    for (i = 0; str[i] != '\0'; i++) {
        if (!isascii((unsigned char)str[i])) {
            flag = 1;
            break;
        }
    }

    if (flag) {
        printf("The string contains non-ASCII characters.\n");
    } else {
        printf("The string contains only ASCII characters.\n");
    }

    return 0;
}

Explanation:

1. The string str is defined as “Hello, 世界!” which includes non-ASCII characters (the Chinese characters for “world”).
2. A for loop is used to traverse the string until the null terminator '\0' is reached.
3. The isascii function, after casting each character to unsigned char, checks if the character is within the ASCII range.
4. If isascii returns 0 for any character, the flag variable is set to 1 and the loop terminates early.
5. Finally, the flag is evaluated to print the appropriate message indicating whether non-ASCII characters are present.

Output:

The string contains non-ASCII characters.

Conclusion

In this tutorial, we learned two different methods to check if a string contains non-ASCII characters in C. The first method manually compares each character’s ASCII value using a loop, while the second method leverages the isascii function from the <ctype.h> library for clarity and simplicity.