🟡 Intermediate · Lesson 23

String Functions (string.h)

String Functions

String Functions Overview

C provides many useful string functions in the <string.h> header file. These functions help you find length, copy, join, compare and search strings.

Important Standard C Note

strrev(), strupr() and strlwr() are not standard C functions. Some compilers support them, but GCC and many online compilers may not. Use manual loops or <ctype.h> for portable code.

Standard String Functions

Function	Syntax	Purpose
`strlen()`	`strlen(s)`	Returns length excluding null character `'\0'`
`strcpy()`	`strcpy(dest, src)`	Copies source string into destination
`strncpy()`	`strncpy(dest, src, n)`	Copies at most n characters
`strcat()`	`strcat(dest, src)`	Appends source string to destination
`strncat()`	`strncat(dest, src, n)`	Appends at most n characters
`strcmp()`	`strcmp(s1, s2)`	Compares two strings
`strncmp()`	`strncmp(s1, s2, n)`	Compares first n characters
`strchr()`	`strchr(s, ch)`	Finds first occurrence of a character
`strstr()`	`strstr(s, sub)`	Finds first occurrence of a substring

Complete Demo Program

C Language – Standard String Functions

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

void reverseString(char s[]) {
    int i = 0, j = strlen(s) - 1;
    while (i < j) {
        char temp = s[i];
        s[i] = s[j];
        s[j] = temp;
        i++;
        j--;
    }
}

void toUpperCase(char s[]) {
    for (int i = 0; s[i] != '\0'; i++) {
        s[i] = toupper((unsigned char)s[i]);
    }
}

int main() {
    char s1[80] = "Hello";
    char s2[80] = "World";
    char copy[80];
    char rev[80] = "Gagan";
    char up[80] = "codekafunda";

    printf("Length = %d\n", (int)strlen(s1));

    strcpy(copy, s1);
    printf("Copy = %s\n", copy);

    strcat(s1, " ");
    strcat(s1, s2);
    printf("Joined = %s\n", s1);

    printf("Compare = %d\n", strcmp("abc", "abc"));
    printf("Substring = %s\n", strstr("Hello World", "World"));

    reverseString(rev);
    printf("Reverse = %s\n", rev);

    toUpperCase(up);
    printf("Uppercase = %s\n", up);

    return 0;
}

Length = 5 Copy = Hello Joined = Hello World Compare = 0 Substring = World Reverse = nagaG Uppercase = CODEKAFUNDA

Practice Questions

Write a program to count vowels and consonants in a string.
Write a program to check whether two strings are equal without using strcmp().
Write a program to reverse a string manually.
Write a program to count words in a sentence.

String Functions Overview

C में string functions के लिए <string.h> header file use की जाती है। इससे length निकालना, copy करना, join करना, compare करना और substring search करना आसान हो जाता है।

Important Standard C Note

strrev(), strupr() और strlwr() standard C functions नहीं हैं। ये कुछ compilers में चलते हैं, लेकिन GCC/online compilers में error दे सकते हैं। Portable code के लिए manual loop और <ctype.h> use करें।

Standard String Functions

Function	काम
`strlen(s)`	String की length देता है
`strcpy(d,s)`	s को d में copy करता है
`strcat(d,s)`	s को d के end में जोड़ता है
`strcmp(s1,s2)`	दो strings compare करता है
`strchr(s,ch)`	character search करता है
`strstr(s,sub)`	substring search करता है

Complete Demo Program

C Language – Standard String Functions

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

void reverseString(char s[]) {
    int i = 0, j = strlen(s) - 1;
    while (i < j) {
        char temp = s[i];
        s[i] = s[j];
        s[j] = temp;
        i++;
        j--;
    }
}

void toUpperCase(char s[]) {
    for (int i = 0; s[i] != '\0'; i++) {
        s[i] = toupper((unsigned char)s[i]);
    }
}

int main() {
    char s1[80] = "Namaste";
    char s2[80] = "Duniya";
    char rev[80] = "Code";
    char up[80] = "codekafunda";

    printf("Length = %d\n", (int)strlen(s1));
    strcat(s1, " ");
    strcat(s1, s2);
    printf("Joined = %s\n", s1);
    printf("Compare = %d\n", strcmp("abc", "abc"));

    reverseString(rev);
    printf("Reverse = %s\n", rev);

    toUpperCase(up);
    printf("Uppercase = %s\n", up);
    return 0;
}

Length = 7 Joined = Namaste Duniya Compare = 0 Reverse = edoC Uppercase = CODEKAFUNDA

Practice

String में vowels और consonants count करें।
strcmp() के बिना दो strings compare करें।
Manual loop से string reverse करें।
Sentence में words count करें।

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