C Academy · Lesson

strcmp and Comparison

Comparing strings.

strcmp and Comparison is a free C Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Comparing strings

You cannot compare strings with == in C. That only compares the addresses of two arrays, not their contents.

Use strcmp(a, b) from <string.h> to compare character by character.

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

int main(void) {
    char a[] = "apple";
    char b[] = "apple";
    if (strcmp(a, b) == 0)
        printf("Strings are equal\n");
    return 0;
}

What strcmp returns

strcmp returns an int:

0 if the strings are equal
a negative value if the first differing char in a is less than in b
a positive value otherwise

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

int main(void) {
    printf("%d\n", strcmp("abc", "abc"));
    printf("%d\n", strcmp("abc", "abd"));
    printf("%d\n", strcmp("abd", "abc"));
    return 0;
}

The zero-means-equal trap

A common bug: writing if (strcmp(a, b)) expecting it to mean equal.

Remember that strcmp returns 0 (falsy) when equal, so that condition is true when they differ. Always compare against 0 explicitly.

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

int main(void) {
    char input[] = "yes";
    if (strcmp(input, "yes") == 0)
        printf("Confirmed\n");
    else
        printf("Not confirmed\n");
    return 0;
}

How comparison works

strcmp compares the ASCII (byte) values of characters one at a time until they differ or a terminator is reached.

Uppercase letters (A=65) come before lowercase (a=97), so "Zebra" sorts before "apple".

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

int main(void) {
    printf("%d\n", strcmp("Zebra", "apple"));
    return 0;
}

Lexicographic ordering

The sign of strcmp lets you sort strings alphabetically. A negative result means the first argument comes earlier in dictionary order.

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

int main(void) {
    const char *x = "banana";
    const char *y = "cherry";
    if (strcmp(x, y) < 0)
        printf("%s comes before %s\n", x, y);
    return 0;
}

strncmp: limited comparison

strncmp(a, b, n) compares at most n characters. This is useful to check prefixes.

For example, testing whether a string starts with "http".

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

int main(void) {
    char url[] = "https://site.com";
    if (strncmp(url, "https", 5) == 0)
        printf("Secure URL\n");
    return 0;
}

Case-insensitive comparison

Standard strcmp is case-sensitive. To ignore case you can convert characters with tolower from <ctype.h> while comparing.

(POSIX systems also offer strcasecmp, but it is not in the C standard.)

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

int ci_cmp(const char *a, const char *b) {
    while (*a && (tolower(*a) == tolower(*b))) { a++; b++; }
    return tolower(*a) - tolower(*b);
}

int main(void) {
    printf("%d\n", ci_cmp("Hello", "hello"));
    return 0;
}

Searching with comparison

You can loop over an array of strings and use strcmp to find a match, like a simple lookup table.

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

int main(void) {
    const char *fruits[] = {"apple", "banana", "cherry"};
    const char *target = "banana";
    for (int i = 0; i < 3; i++) {
        if (strcmp(fruits[i], target) == 0) {
            printf("Found at index %d\n", i);
        }
    }
    return 0;
}

Sorting strings

Using strcmp as the engine, you can sort string arrays. Here is a tiny bubble sort that swaps pointers based on order.

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

int main(void) {
    const char *names[] = {"Carol", "Alice", "Bob"};
    int n = 3;
    for (int i = 0; i < n - 1; i++)
        for (int j = 0; j < n - 1 - i; j++)
            if (strcmp(names[j], names[j+1]) > 0) {
                const char *t = names[j];
                names[j] = names[j+1];
                names[j+1] = t;
            }
    for (int i = 0; i < n; i++) printf("%s\n", names[i]);
    return 0;
}

Don't compare with subtraction directly

You might try writing your own comparison with a[i] - b[i], but with raw char values, sign behavior is implementation-defined and can overflow.

Prefer the library strcmp, which handles these edge cases correctly.

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

int main(void) {
    int r = strcmp("file1", "file2");
    printf("Result sign: %s\n", r < 0 ? "negative" : "non-negative");
    return 0;
}

Comparing user input

A real-world use is checking a password or menu choice. Read input, then compare it against expected values with strcmp.

Always compare against 0 to test for equality.

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

int main(void) {
    char choice[] = "quit";
    if (strcmp(choice, "quit") == 0)
        printf("Exiting program\n");
    return 0;
}

Quick Check

Test your understanding of strcmp return values.

Recap

You learned how to compare strings in C:

strcmp returns 0 for equal, negative or positive for ordering.
Never use == on strings; it compares addresses.
Always test == 0 for equality (the zero-is-falsy trap).
strncmp compares a prefix; case-insensitive needs tolower.

Frequently asked questions

Is the “strcmp and Comparison” lesson free?

Yes — the full text of “strcmp and Comparison” is free to read here on the web, and the C Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C Academy course, upgrade to CoddyKit PRO.

What will I learn in “strcmp and Comparison”?

Comparing strings. You practise C Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C Academy?

No prior experience is required. C Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “strcmp and Comparison” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C Academy lesson?

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