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Strings

Work with text as arrays of characters and use the string.h functions.

C has no separate string type. A string is an array of char that ends with a special null character '\0'. That final zero tells functions such as printf where the text stops.

C
Open in playground
#include <stdio.h>

int main(void) {
    char word[] = "code";   /* stored as 'c' 'o' 'd' 'e' '\0' */
    printf("%s\n", word);
    printf("%c %c\n", word[0], word[3]);
    printf("array size: %zu\n", sizeof(word));
    return 0;
}
Output
code
c e
array size: 5

The array holds 5 characters, not 4, because of the hidden '\0'.

The string.h library

C
Open in playground
#include <stdio.h>
#include <string.h>

int main(void) {
    char first[20] = "Data";
    char second[] = "Science";

    printf("length: %zu\n", strlen(first));
    strcat(first, " ");        /* append */
    strcat(first, second);
    printf("joined: %s\n", first);

    char copy[20];
    strcpy(copy, second);      /* copy */
    printf("copy: %s\n", copy);

    printf("compare: %d\n", strcmp("apple", "apple") == 0);
    return 0;
}
Output
length: 4
joined: Data Science
copy: Science
compare: 1

Walking through a string

Because a string ends with '\0', you can loop until you reach it.

C
Open in playground
#include <stdio.h>
#include <ctype.h>

int main(void) {
    char text[] = "Hello World 2026";
    int vowels = 0, digits = 0;

    for (int i = 0; text[i] != '\0'; i++) {
        char ch = tolower(text[i]);
        if (ch == 'a' || ch == 'e' || ch == 'i' || ch == 'o' || ch == 'u') {
            vowels++;
        } else if (isdigit(ch)) {
            digits++;
        }
        text[i] = toupper(text[i]);
    }
    printf("%s\n", text);
    printf("vowels: %d, digits: %d\n", vowels, digits);
    return 0;
}
Output
HELLO WORLD 2026
vowels: 3, digits: 4

Building strings with snprintf

snprintf works like printf but writes into an array, and never writes more than the size you give it.

C
Open in playground
#include <stdio.h>

int main(void) {
    char label[32];
    snprintf(label, sizeof(label), "Roll no. %03d (%s)", 7, "B");
    printf("%s\n", label);
    return 0;
}
Output
Roll no. 007 (B)

GATE-style practice questions

GATE-styleMCQ · one correct answer

What does the following program print?

#include <stdio.h>
#include <string.h>
int main(void) {
    char s[] = "GATE2026";
    printf("%zu %zu\n", strlen(s), sizeof(s));
    return 0;
}
Show answer and explanationAnswer: B

strlen counts the characters before the terminating '\0': 8. The array also stores that '\0', so sizeof(s) is 9.

GATE-styleNAT · numerical answer

What does the following program print?

#include <stdio.h>
int main(void) {
    char s[] = "programming";
    char *p = s;
    while (*p) p++;
    printf("%d\n", (int)(p - s));
    return 0;
}
Show answer and explanationAnswer: 11

The loop moves p forward until it reaches the '\0' at the end, so p − s is the length of "programming", which is 11.

GATE-styleMCQ · one correct answer

What does the following program print?

#include <stdio.h>
int main(void) {
    char s[] = "abcdef";
    printf("%s\n", s + s[1] - s[0] + 2);
    return 0;
}
Show answer and explanationAnswer: B

s[1] − s[0] is 'b' − 'a' = 1, so the expression is s + 1 + 2 = s + 3, which points at "def".