String Without string.h

Why write string functions by hand?

C gives you ready-made string tools in <string.h> — strlen, strcpy, strrev and more. But writing your own versions is one of the best ways to truly understand strings, and it is a very common interview exercise.

Everything below uses only loops and arrays — no string.h. Once you see how these work, the library functions will never feel like magic again.

The secret: the null terminator

Every C string secretly ends with a special character, '\0' (the null terminator). The word "Hi" is really stored as three characters: 'H', 'i', '\0'. That final '\0' is how every function knows where the string ends.

Length without strlen

#include <stdio.h>
int main() {
    char str[] = "CodingEasily";
    int len = 0;
    while (str[len] != '\0')   // stop at the terminator
        len++;
    printf("Length = %d\n", len);
    return 0;
}

The loop counts characters until it meets '\0' — that count is the length.

Copy without strcpy

#include <stdio.h>
int main() {
    char src[] = "Hello";
    char dest[20];
    int i = 0;
    while (src[i] != '\0') { dest[i] = src[i]; i++; }
    dest[i] = '\0';            // copy the terminator too!
    printf("Copied: %s\n", dest);
    return 0;
}

Reverse a string

Find the length, then swap the ends inward until they meet.

#include <stdio.h>
int main() {
    char str[] = "coding";
    int len = 0;
    while (str[len] != '\0') len++;

    for (int i = 0, j = len - 1; i < j; i++, j--) {
        char t = str[i]; str[i] = str[j]; str[j] = t;  // swap
    }
    printf("Reversed: %s\n", str);
    return 0;
}

The two indices i and j move toward each other, swapping as they go. When they meet, the string is fully reversed.

Compare two strings

int i = 0;
while (a[i] != '\0' && b[i] != '\0') {
    if (a[i] != b[i]) break;   // first difference
    i++;
}
if (a[i] == b[i]) printf("Equal\n");
else              printf("Not equal\n");

Walk both strings together; the moment two characters differ, they are not equal. If you reach the end of both at once, they match.

Common mistakes

• Forgetting to add '\0' after copying, leaving an invalid string.
• Making the destination array too small to hold the copy plus '\0'.
• Looping with <= length and reading one character too far.
• In reverse, using i <= j and needlessly swapping the middle character with itself.

Summary

• Every C string ends with the null terminator '\0'.
• Length: count characters until '\0'.
• Copy: assign each character, then add '\0' yourself.
• Reverse: swap characters from both ends inward.
• Compare: walk both strings until they differ or both end.