Standard Library: string.h

The string.h header provides functions for manipulating C strings (null-terminated character arrays).

---

String Length

#include <string.h>

char s[] = "Hello";
int len = strlen(s);   /* 5 — does not count \0 */

---

String Copy

char dest[20];
char src[] = "World";

strcpy(dest, src);               /* copies src to dest */
strncpy(dest, src, 3);           /* copies at most 3 chars */
printf("%s\n", dest);            /* Wor */

---

String Concatenation

char s1[30] = "Hello";
char s2[] = " World";

strcat(s1, s2);          /* s1 = "Hello World" */
strncat(s1, s2, 3);      /* appends at most 3 chars */

---

String Comparison

strcmp(s1, s2)    /* 0 if equal, <0 if s1 < s2, >0 if s1 > s2 */
strncmp(s1, s2, n)  /* compare first n characters */
strcmpi(s1, s2)   /* case-insensitive compare (non-standard) */

---

String Search

char *p;

p = strchr("Hello World", 'o');    /* pointer to first 'o' */
p = strrchr("Hello World", 'o');   /* pointer to last 'o' */
p = strstr("Hello World", "World"); /* pointer to "World" */

---

String Transformation

strupr(str);    /* convert to uppercase (non-standard) */
strlwr(str);    /* convert to lowercase (non-standard) */
strrev(str);    /* reverse string (non-standard) */

Portable alternatives:

#include <ctype.h>

for (int i = 0; str[i]; i++) {
    str[i] = toupper(str[i]);   /* ANSI-portable uppercase */
}

---

Memory Functions (also in string.h)

char buf[10];
memset(buf, 0, sizeof(buf));    /* zero out buffer */

---

Common String Programs

/* Check if palindrome */
#include <stdio.h>
#include <string.h>

int isPalindrome(char s[]) {
    int n = strlen(s);
    for (int i = 0; i < n/2; i++) {
        if (s[i] != s[n-1-i]) return 0;
    }
    return 1;
}

int main() {
    printf("%d\n", isPalindrome("madam"));   /* 1 */
    printf("%d\n", isPalindrome("hello"));   /* 0 */
}