Lesson 8 — Beginner

Strings in C

C mein string koi alag type nahi hai — bas char array jo ek special character par rukta hai: '\0'. Is lesson mein hum isi detail ko dekhenge — string kaise banti hai, safely input kaise lete hain, string.h ke functions kaise chalte hain, aur palindrome jaise programs kaise bante hain.

String = char array + null terminator

#include <stdio.h>
int main() {
    char name[20] = "DSWallah";
    // D  S  W  a  l  l  a  h  \0
    // 0  1  2  3  4  5  6  7  8

    printf("%s\n", name);        // DSWallah
    printf("Size = %zu\n", sizeof(name));  // 20 (poora array)
    return 0;
}

Output:
DSWallah
Size = 20

• Length vs size: "DSWallah" ki length 8, array size 20. '\0' length mein count nahi hota, size mein jagah le leta hai.
• char name[8] = "DSWallah"; — problem! '\0' ke liye jagah nahi bachti. Hamesha 1 extra rakho (name[9]).
• Char wise access: name[0] = 'D', name[7] = 'h' — array jaisi hi zindagi.
• String literal vs array: char *s = "abc" string literal hai (read-only, badal mat), char s[] = "abc" array hai (editable).

String ka input aur output

#include <stdio.h>
int main() {
    char name[50];

    printf("Ek shabd: ");
    scanf("%49s", name);          // space pe rok jaata hai
    printf("Shabd = %s\n", name);

    printf("Poori line: ");
    scanf(" %[^\n]", name);      // space sahit (ek trick)
    printf("Line = %s\n", name);
    return 0;
}

Ek shabd: Rahul
Shabd = Rahul

• %s ke saath width: scanf("%49s", buf) — buffer overflow se bachne ka standard tareeka.
• Space chahiye: "%[^ ]" poora line leta hai (newline tak), par width na bhoolo.
• Safest: fgets(buf, sizeof(buf), stdin) — newline ke saath, size limited.
• Print: printf("%s", s) ya puts(s) — puts khud newline add karta hai.

string.h ke 4 must-know functions

#include <stdio.h>
#include <string.h>
int main() {
    char a[30] = "Hello";
    char b[30] = "World";

    printf("Len = %d\n", (int)strlen(a));   // 5
    strcat(a, " ");
    strcat(a, b);
    printf("%s\n", a);                       // Hello World
    printf("cmp = %d\n", strcmp("abc", "abd"));  // -1 (negative)
    strcpy(a, "Bye");
    printf("%s\n", a);                       // Bye
    return 0;
}

Output:
Len = 5
Hello World
cmp = -1
Bye

• strlen(s): kitne characters (\0 nahi). Return type size_t — printf ke liye %zu ya cast.
• strcpy(dst, src): copy (destination pehle). Destination ki size check tumhe khud karni hai!
• strcat(dst, src): jodna (append) — dst mein jagah honi chahiye dono ka.
• strcmp(a, b): 0 = barabar, negative = pehla chhota, positive = pehla bada (lexicographic).

Palindrome program — string manipulation

#include <stdio.h>
#include <string.h>
int main() {
    char s[] = "madam";
    int n = strlen(s), flag = 1;

    for (int i = 0; i < n / 2; i++)
        if (s[i] != s[n - 1 - i]) { flag = 0; break; }

    printf("%s is %s\n", s, flag ? "Palindrome" : "Not Palindrome");
    return 0;
}

Output: madam is Palindrome

// String ↔ number conversion (bonus, interview favourite)
#include <stdio.h>
#include <stdlib.h>
int main() {
    char num[] = "42";
    int x = atoi(num);           // string → int
    printf("%d + 8 = %d\n", x, x + 8);

    char back[10];
    sprintf(back, "%d", x * 2);  // int → string
    printf("back = %s\n", back);
    return 0;
}
Output:
42 + 8 = 50
back = 84

String functions ka pattern: length nikalo, loop chalao, compare/transform karo. Reversal, palindrome, vowel count, word count — sab isi formula se bante hain. Ek baar likh lo, baaki sab apne aap aayenge.

Exercise

Common mistakes

• == se string compare kar lena: s1 == s2 address compare karta hai, content nahi. Comparison ke liye strcmp(s1, s2) == 0 — hamesha.
• Array size aur string length mein confusion: char s[20] = "abc" — length 3, size 20. Buffer operations mein size se kaam hota hai, strlen se logic se.
• strcat/strcpy pe buffer size na sochna: Destination ki jagah check kiye bina = memory overwrite. Hamesha socho: dst mein total length kitni fit hogi?

#include <stdio.h>
#include <string.h>
int main() {
    char s[] = "121";
    int n = strlen(s), ok = 1;
    for (int i = 0; i < n / 2; i++)
        if (s[i] != s[n-1-i]) ok = 0;
    printf("%s palindrome? %s\n", s, ok ? "YES" : "NO");
    return 0;
}
Output: 121 palindrome? YES