Strings: char arrays and the null terminator

Text is an array of chars

• C has no separate string type. A string is just an array of char.
• char word[] = "hi"; actually stores 'h', 'i', and a hidden '\0' at the end.
• You can read one character at a time with an index, exactly like any array.

The invisible terminator '\0'

• Every C string ends with a special null terminator, written '\0' (a byte equal to 0).
• It marks where the text stops. The word "hi" takes three bytes: 'h', 'i', '\0'.
• Functions know a string has ended when they reach '\0' — there is no stored length.

Walking a string

• To visit each character, loop an index while s[i] != '\0'.
• When s[i] becomes '\0', you have reached the end — stop.
• This is how you count characters, search, or copy a string by hand.

#include <stdio.h>

int main(void) {
    char word[] = "hello";
    int len = 0;
    while (word[len] != '\0') {   // stop at the terminator
        len++;
    }
    printf("%d\n", len);          // 5
    return 0;
}

The <string.h> toolbox

• <string.h> has ready-made helpers: strlen(s) (length), strcpy(dst, src) (copy), strcmp(a, b) (compare), strcat(dst, src) (join).
• strcmp(a, b) returns 0 when the two strings are equal — handy in tests.
• These all rely on the '\0' terminator to know where each string ends.

Common mistakes

• A string needs one extra byte for \0: a 5-letter word needs char[6].
• Compare strings with strcmp, not ==.

Now you try

• To change a string in place, the caller must pass a writable char array, not a fixed text.
• Stop your loops at '\0'. Do not write a main — the checker provides one.