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

TAKEAWAYS:

• Control structures (if, switch, loops) let programs make decisions and repeat tasks efficiently.
• Arrays store multiple values of the same type under a single name using indices (starting at 0).
• for loops are best for known iterations, while for unknown conditions, and do-while for "at least once" execution.
• Common mistakes include off-by-one errors in loops and forgetting array bounds.
• Arrays can be 1D (linear) or 2D (matrix-like), and must be declared with size.
• Always initialize arrays and validate loop conditions to avoid runtime errors.

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1. Control Structures: Making Decisions and Loops

Control structures change the flow of a program based on conditions or repetition needs. They are the "brain" of programming—they decide what to do next.

1.1 Decision-Making Statements

These statements execute different blocks of code based on conditions.

A. if Statement

• Checks a condition. If true, executes the block inside {}.
• Syntax:

  if (condition) {
      // code to execute if condition is true
  }
  

• Example: Check if a number is even.

  #include <stdio.h>
  int main() {
      int num = 4;
      if (num % 2 == 0) {
          printf("%d is even.", num);
      }
      return 0;
  }
  

  Output: 4 is even.

B. if-else Statement

• Executes one block if the condition is true, another if false.
• Syntax:

  if (condition) {
      // true block
  } else {
      // false block
  }
  

• Example: Check if a number is positive or negative.

  #include <stdio.h>
  int main() {
      int num = -5;
      if (num >= 0) {
          printf("%d is positive.", num);
      } else {
          printf("%d is negative.", num);
      }
      return 0;
  }
  

  Output: -5 is negative.

C. Nested if-else

• if statements inside other if statements for multiple conditions.
• Example: Check if a student passes, gets a distinction, or fails.

  #include <stdio.h>
  int main() {
      int marks = 85;
      if (marks >= 35) {
          if (marks >= 80) {
              printf("Distinction!");
          } else {
              printf("Pass.");
          }
      } else {
          printf("Fail.");
      }
      return 0;
  }
  

  Output: Distinction!

D. else if (Ladder)

• Multiple conditions checked in order.
• Syntax:

  if (condition1) {
      // code
  } else if (condition2) {
      // code
  } else {
      // default code
  }
  

• Example: Grade calculator.

  #include <stdio.h>
  int main() {
      int marks = 72;
      if (marks >= 90) {
          printf("Grade A");
      } else if (marks >= 80) {
          printf("Grade B");
      } else if (marks >= 70) {
          printf("Grade C");
      } else {
          printf("Grade D");
      }
      return 0;
  }
  

  Output: Grade C

E. switch-case Statement

• Tests a variable against multiple possible values.
• Syntax:

  switch (variable) {
      case value1:
          // code
          break;
      case value2:
          // code
          break;
      default:
          // default code
  }
  

• Example: Menu-driven program.

  #include <stdio.h>
  int main() {
      char choice = 'B';
      switch (choice) {
          case 'A':
              printf("Option A selected.");
              break;
          case 'B':
              printf("Option B selected.");
              break;
          default:
              printf("Invalid choice.");
      }
      return 0;
  }
  

  Output: Option B selected.

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1.2 Looping Statements

Loops repeat a block of code while a condition is true.

A. for Loop

• Used when the number of iterations is known.
• Syntax:

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

• Example: Print numbers 1 to 5.

  #include <stdio.h>
  int main() {
      for (int i = 1; i <= 5; i++) {
          printf("%d ", i);
      }
      return 0;
  }
  

  Output: 1 2 3 4 5

B. while Loop

• Repeats while the condition is true (may not execute at all).
• Syntax:

  while (condition) {
      // code to repeat
  }
  

• Example: Sum of numbers until user enters 0.

  #include <stdio.h>
  int main() {
      int num, sum = 0;
      printf("Enter numbers (0 to stop): ");
      scanf("%d", &num);
      while (num != 0) {
          sum += num;
          scanf("%d", &num);
      }
      printf("Sum = %d", sum);
      return 0;
  }
  

  Output (if inputs: 10, 20, 30, 0): Sum = 60

C. do-while Loop

• Executes at least once, then checks the condition.
• Syntax:

  do {
      // code to repeat
  } while (condition);
  

• Example: Simple calculator (runs once before checking).

  #include <stdio.h>
  int main() {
      int num;
      do {
          printf("Enter a positive number: ");
          scanf("%d", &num);
      } while (num <= 0);
      printf("Valid number entered: %d", num);
      return 0;
  }
  

  Output (if input: -5, then 10): Valid number entered: 10

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2. Arrays: Storing Multiple Values

Arrays store multiple values of the same data type in a single variable. Think of them as a list of boxes labeled array_name[0], array_name[1], etc.

2.1 One-Dimensional (1D) Arrays

• A list of elements accessed by index (starting at 0).
• Declaration:

  data_type array_name[size];
  

• Example: Store 5 integers.

  int marks[5]; // Declares an array of 5 integers
  

A. Initializing Arrays

• Assign values at declaration.

  int numbers[3] = {10, 20, 30};
  

• Partial initialization (rest are 0):

  int arr[5] = {1, 2}; // arr = {1, 2, 0, 0, 0}
  

B. Accessing Array Elements

• Use indices (0 to size-1).

  printf("%d", numbers[1]); // Output: 20
  

C. Traversing an Array

• Loop through all elements.

  #include <stdio.h>
  int main() {
      int arr[3] = {5, 10, 15};
      for (int i = 0; i < 3; i++) {
          printf("%d ", arr[i]);
      }
      return 0;
  }
  

  Output: 5 10 15

D. Input and Output in Arrays

• Input:

  for (int i = 0; i < 5; i++) {
      scanf("%d", &marks[i]);
  }
  

• Output:

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

Index:  0   1   2   3   4
Value: [10, 20, 30, 40, 50]

Label: "Array 'arr' with 5 elements"

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2.2 Two-Dimensional (2D) Arrays

• A table (rows × columns) of values.
• Declaration:

  data_type array_name[rows][columns];
  

• Example: 2×3 matrix.

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

A. Accessing 2D Array Elements

• Use row[column].

  printf("%d", matrix[1][2]); // Output: 6
  

B. Traversing a 2D Array

• Use nested for loops.

  #include <stdio.h>
  int main() {
      int mat[2][3] = {{1, 2, 3}, {4, 5, 6}};
      for (int i = 0; i < 2; i++) {
          for (int j = 0; j < 3; j++) {
              printf("%d ", mat[i][j]);
          }
          printf("\n");
      }
      return 0;
  }
  

  Output:

  1 2 3
  4 5 6
  

Row 0: [1, 2, 3]
Row 1: [4, 5, 6]

Label: "2D array 'matrix' with 2 rows and 3 columns"

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2.3 Common Array Operations

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3. Common Mistakes and How to Avoid Them

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4. Practical Applications

1. Menu-Driven Programs Use switch-case to create interactive menus (e.g., calculator, student records).
2. Data Analysis Store exam marks in an array, then compute average, highest, or lowest score.
3. Pattern Printing Use nested loops to print patterns like pyramids or stars.

   #include <stdio.h>
   int main() {
       for (int i=1; i<=3; i++) {
           for (int j=1; j<=i; j++) {
               printf("* ");
           }
           printf("\n");
       }
       return 0;
   }
   

   Output:

   *
   * *
   * * *
   

4. Game Development Arrays store game boards (e.g., Tic-Tac-Toe) or player scores.

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5. Exam Tips

What to Remember

✅ Loop Conditions:

• for loops are best for known iterations (e.g., "print 1 to 10").
• while loops are for unknown conditions (e.g., "read until user enters 0").
• do-while ensures the loop runs at least once.

✅ Array Indexing:

• Arrays start at index 0, not 1.
• Always declare the correct size to avoid overflow.

✅ Common Questions:

• Trace the output of a given loop or if-else block.
• Find errors in a snippet (e.g., missing break in switch).
• Write code to perform tasks like:
  • Sum of array elements.
  • Find the largest/smallest number in an array.
  • Print a pattern using loops.

NEB-Style Questions to Practice

1. Short Answer:

   • What is the output of the following code?

     int x = 5;
     if (x > 3) {
         printf("A");
     } else {
         printf("B");
     }
     

     Answer: A

2. Programming:

   • Write a program to print the multiplication table of a given number using a for loop.

     #include <stdio.h>
     int main() {
         int num = 5;
         for (int i=1; i<=10; i++) {
             printf("%d x %d = %d\n", num, i, num*i);
         }
         return 0;
     }
     

3. Trace the Output:

   • What will be printed?

     int arr[3] = {10, 20, 30};
     for (int i=0; i<=2; i++) {
         printf("%d ", arr[i]);
     }
     

     Answer: 10 20 30

4. Error Identification:

   • Find the error in this code:

     int arr[5];
     arr[5] = 100; // Error: Index out of bounds
     

     Fix: Use arr[0] to arr[4].

5. Array Application:

   • Write a program to find the sum of all even numbers in an array.

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

     Output: Sum of evens: 6 (2 + 4)

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Final Note: Control structures and arrays are the backbone of programming. Master them by:

1. Writing small programs (e.g., calculators, games).
2. Tracing loops and if-else blocks on paper.
3. Practicing NEB-style questions to recognize patterns.

Keep coding! 🚀