Control Structures & Arrays in C: Loops, Decisions & Data Storage

TAKEAWAYS:

• Control structures (if-else, loops) make programs decide and repeat actions without writing the same code again.
• Arrays let you store lists of values (like grades of 50 students) using a single variable name with an index.
• The for loop is best for count-controlled repetition (e.g., "run 10 times"), while while/do-while are for condition-controlled loops (e.g., "keep asking until correct").
• 2D arrays store tables (e.g., a 3×3 matrix), accessed with array[row][column].
• Common mistakes: off-by-one errors in loops, uninitialized arrays, and forgetting array bounds.

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## **1. Control Structures: Making Decisions and Loops**

### **1.1 Decision-Making: `if`, `if-else`, `else-if` (Ladder)**
Programs often need to **choose** between actions based on conditions. C uses:
- `if (condition)` → Executes if `condition` is **true** (non-zero).
- `else` → Runs if `if` is false.
- `else-if` → Checks multiple conditions (like a ladder).

```c
// Example: Check if a number is positive, negative, or zero
#include <stdio.h>
int main() {
    int num = -5;
    if (num > 0)
        printf("%d is positive.", num);
    else if (num < 0)
        printf("%d is negative.", num);
    else
        printf("Number is zero.");
    return 0;
}

Output: -5 is negative.

How it works:

1. Check num > 0 → False → go to else-if.
2. Check num < 0 → True → print and exit.

1.2 Loops: Repeating Actions

Loops repeat a block of code until a condition is met.

A. for Loop (Count-Controlled)

Best for known iterations (e.g., "print numbers 1 to 10"). Syntax:

for (initialization; condition; increment) {
    // Code to repeat
}

Example: Print numbers 1 to 5.

for (int i = 1; i <= 5; i++) {
    printf("%d ", i);
}

Output: 1 2 3 4 5

B. while Loop (Condition-Controlled)

Runs while a condition is true. Use when you don’t know how many iterations are needed. Example: Keep asking for input until the user enters 0.

int num;
while (num != 0) {
    printf("Enter a number (0 to stop): ");
    scanf("%d", &num);
}

C. do-while Loop (Post-Test Loop)

Runs at least once, then checks the condition. Example: Menu that runs until the user chooses 3.

int choice;
do {
    printf("\n1. Add\n2. Subtract\n3. Exit\nEnter choice: ");
    scanf("%d", &choice);
    // Perform action based on choice
} while (choice != 3);

flowchart TD
    A["Start"] --> B["Run code block"]
    B --> C["Check condition (choice != 3)"]
    C -->|"True"| B
    C -->|"False"| D["Exit"]

1.3 Nested Loops (Loops Inside Loops)

Used for multi-dimensional tasks (e.g., printing a multiplication table). Example: Print a 3×3 multiplication table.

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        printf("%d ", i * j);
    }
    printf("\n");
}

Output:

1 2 3
2 4 6
3 6 9

2. Arrays: Storing Multiple Values

Arrays store multiple values of the same type under one name. Syntax:

data_type array_name[size];

Example: Store 5 student marks.

int marks[5] = {85, 90, 78, 92, 88};

2.1 1D Arrays (Single-Dimensional)

• Access elements using index (starts at 0).
• Example: Print all marks.

for (int i = 0; i < 5; i++) {
    printf("Marks %d: %d\n", i+1, marks[i]);
}

Output:

Marks 1: 85
Marks 2: 90
...

2.2 2D Arrays (Multi-Dimensional)

• Store tables (rows × columns).
• Example: 2×3 matrix.

int matrix[2][3] = {{1, 2, 3}, {4, 5, 6}};

• Access: matrix[row][column].
• Example: Print the matrix.

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

Output:

1 2 3
4 5 6

2.3 Common Array Operations

Example: Find the sum of array elements.

int sum = 0;
for (int i = 0; i < 5; i++) {
    sum += marks[i];
}
printf("Total = %d", sum); // Output: 433

3. Common Mistakes and How to Avoid Them

4. NEB-Style Solved Examples

Example 1: Find the Largest Number in an Array

#include <stdio.h>
int main() {
    int numbers[5] = {12, 45, 67, 23, 89};
    int largest = numbers[0];
    for (int i = 1; i < 5; i++) {
        if (numbers[i] > largest)
            largest = numbers[i];
    }
    printf("Largest number: %d", largest);
    return 0;
}

Output: 89

Example 2: Print a Pyramid Pattern

#include <stdio.h>
int main() {
    for (int i = 1; i <= 4; i++) {
        for (int j = 1; j <= i; j++) {
            printf("* ");
        }
        printf("\n");
    }
    return 0;
}

Output:

*
* *
* * *
* * * *

Example 3: 2D Array (Matrix Addition)

int A[2][2] = {{1, 2}, {3, 4}};
int B[2][2] = {{5, 6}, {7, 8}};
int C[2][2];
for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 2; j++) {
        C[i][j] = A[i][j] + B[i][j];
    }
}
// C = {{6, 8}, {10, 12}}

5. NEB Board-Style Questions

Short Answer (2 marks each)

1. What is the output of the following code?

   int x = 5;
   while (x > 0) {
       printf("%d ", x--);
   }
   

   Answer: 5 4 3 2 1

2. Write a for loop to print even numbers from 10 to 20. Answer:

   for (int i = 10; i <= 20; i += 2) {
       printf("%d ", i);
   }
   

3. What is the difference between while and do-while? Answer:

   Feature	while	do-while
   Check	Before execution	After execution
   Guarantee	May not run once	Runs at least once
   Use case	Unknown iterations	Menu-driven programs

Programming (5 marks)

Write a program to find the sum of all even numbers in a 1D array of 10 elements.

#include <stdio.h>
int main() {
    int arr[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    int sum = 0;
    for (int i = 0; i < 10; i++) {
        if (arr[i] % 2 == 0)
            sum += arr[i];
    }
    printf("Sum of even numbers: %d", sum);
    return 0;
}

Output: 30 (2 + 4 + 6 + 8 + 10)

Explain (3 marks)

Explain the use of nested loops with an example. Answer: Nested loops are used when a task requires repetition inside repetition. For example:

• Printing a multiplication table (outer loop for rows, inner loop for columns).
• Processing a 2D array (e.g., matrix operations). Example: Print a 2×2 identity matrix.

for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 2; j++) {
        printf("%d ", (i == j) ? 1 : 0);
    }
    printf("\n");
}

Output:

1 0
0 1

Exam Tip

1. For loops:

   • Always initialize, test, and update the loop variable.
   • Common mistake: for (i=0; i=5; i++) (use i<=5 or i<5).

2. Arrays:

   • Remember index starts at 0 (e.g., arr[3] is the 4th element).
   • Use sizeof(array)/sizeof(array[0]) to find array size dynamically.

3. Nested loops:

   • The outer loop controls rows, the inner loop controls columns.
   • Example: For a 3×3 matrix, outer loop runs 3 times, inner loop runs 3 times per outer iteration.

4. NEB loves:

   • Pattern printing (pyramids, stars).
   • Array traversal (find max/min, sum).
   • Matrix operations (addition, multiplication).

5. Avoid:

   • Magic numbers (use const int SIZE = 5; instead of hardcoding 5).
   • Uninitialized variables (always set default values).