Lesson 7 — Beginner

Arrays in C

Ek variable mein ek value — 100 students ke marks ke liye 100 variables? Bilkul nahi. Array ek line mein ek fixed-size group banata hai, jahan har element ek number (index) se milta hai. Is lesson mein hum array ke basics, operations aur 2D tak jaayenge.

Array kaise banta hai

#include <stdio.h>
int main() {
    int marks[5] = {90, 85, 78, 92, 88};

    // marks[0] = 90 (pehla), marks[4] = 88 (aakhri)
    // Total size = 5 * 4 = 20 bytes

    printf("Pehla = %d\n", marks[0]);
    printf("Aakhri = %d\n", marks[4]);
    return 0;
}

Output:
Pehla = 90
Aakhri = 88

• Size constant chahiye: int marks[5] ya int n; int a[n] (C99 VLA) — runtime mein badalna allowed nahi (fixed).
• Index 0 se: marks[5] for 5 elements = out of bounds (aakhri index 4).
• Memory contiguous: elements lagatar baithe hain — isliye (p + 1) seedha agle pe pahunch jaata hai.
• Partial init allowed: int a[5] = {10, 20}; → baaki 0 ho jaate hain.

Traversal — sum, max aur average

#include <stdio.h>
int main() {
    int a[5] = {12, 7, 19, 3, 25};
    int sum = 0, max = a[0];

    for (int i = 0; i < 5; i++) {
        sum += a[i];
        if (a[i] > max) max = a[i];
    }
    printf("Sum=%d Max=%d Avg=%.1f\n",
           sum, max, (float)sum / 5);
    return 0;
}

Output: Sum=66 Max=25 Avg=13.2

Traversal ka pattern: loop 0 se size tak, har i pe a[i] — baaki kaam wahi. Sum ke liye accumulator, max ke liye comparison, search ke liye if. Ek hi loop, alag-alag kaam.

2D arrays — matrix world

#include <stdio.h>
int main() {
    int a[2][3] = {{1, 2, 3}, {4, 5, 6}};
    int b[2][3] = {{7, 8, 9}, {10, 11, 12}};
    int c[2][3];

    for (int i = 0; i < 2; i++)
        for (int j = 0; j < 3; j++)
            c[i][j] = a[i][j] + b[i][j];

    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 3; j++)
            printf("%d ", c[i][j]);
        printf("\n");
    }
    return 0;
}

Output:
8 10 12
14 16 18

2D array = rows × columns, access a[i][j] (i = row, j = column). Memory mein phir bhi sab lagatar hi hai — rows one after another. Nested loop ka classic use: row-wise print aur matrix operations.

• Declaration: int a[rows][cols] — dono size compile time constant chahiye (VLA alag).
• Matrix transpose, multiplication, diagonal sum — sab nested loop pattern.
• int a[2][3] ka total size = 2*3*4 = 24 bytes.

Array ki traps aur real rules

GALAT (undefined behaviour):
  int a[5];
  printf("%d", a[5]);   // bahar ka index — garbage/crash
  printf("%d", a[-1]);  // peeche bhi bahar

SAHI (size nikalo):
  int n = sizeof(a) / sizeof(a[0]);   // = 5

// Array direct copy nahi hota:
  int b[5] = a;         // COMPILE ERROR (C mein)
  // chahiye toh loop ya memcpy() se copy karo

• C array bounds check NAHI karta — a[100] bhi chal jaata hai, chupke se memory kharab. Tumhari zimmedaari hai.
• Uninitialized array: bina value ke garbage — result kabhi kabhi naya hi aayega.
• Array = pointer decay: function ko array bhejne pe size nahi jaata (pointer ban jaata hai) — isi liye size alag se pass karte hain.
• Initialization zaroori: = {0} se sab zero kar do jab loop na chalao.

Exercise

Common mistakes

• a[size] ko valid samajhna: int a[5] mein valid index 0-4 hain. a[5] bahar hai — C check nahi karta, garbage/crash.
• Function ko array bhejte waqt size na dena: Array pointer ban jaata hai — sizeof size nahi deta. Size hamesha alag parameter se pass karo.
• Do arrays ko = se copy kar dena: C mein b[5] = a; compile error hai. Loop se copy karo ya memcpy() use karo.

#include <stdio.h>
int main() {
    int marks[4] = {66, 78, 91, 55};
    int pass = 0;
    for (int i = 0; i < 4; i++)
        if (marks[i] >= 33) pass++;
    printf("%d/4 pass\n", pass);
    return 0;
}
Output: 4/4 pass