Normalizing Strings (Stripping Whitespace, Unifying Case)

This lesson covers normalizing strings in C, so you can unify inconsistent formatting before comparing them. It's for anyone searching "C string normalization."

A string a user typed often has extra whitespace at the edges, or inconsistent uppercase/lowercase formatting. Detecting and stripping whitespace with isspace(), and unifying case, is called normalization.

The example's normalize() function detects and strips leading/trailing whitespace using isspace((unsigned char)*src), converting each character to lowercase one at a time. Notice using the character-checking functions from the ctype.h header.

A common early mistake is not realizing isspace() needs a cast. On a system where char is signed, passing part of a multi-byte character can cause undefined behavior, so casting to (unsigned char) is the safe convention.

In real projects, normalization is a frequently-used, essential technique for things like input validation and login handling.

📖 Reference code

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

void normalize(char *dst, const char *src) {
    while (isspace((unsigned char)*src)) src++;
    int len = strlen(src);
    while (len > 0 && isspace((unsigned char)src[len - 1])) len--;
    for (int i = 0; i < len; i++) {
        dst[i] = tolower((unsigned char)src[i]);
    }
    dst[len] = '\0';
}

int main(void) {
    char a[50], b[50];
    normalize(a, "  Hello World  ");
    normalize(b, "hello world");
    printf("%d\n", strcmp(a, b) == 0);
    printf("%s\n", a);
    return 0;
}

✍️ Your code

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🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).