chapter3

Decision Making and Looping Statements in C

Overview

In C, programs can dynamically choose which part of the code executes based on specific conditions.
This capability is known as decision making, where the statements employed are referred to as conditional statements.
Conditional statements evaluate one or more conditions and determine whether to execute a specific block of code based on the outcome (true or false).

Conditional Statements in C

If Statement
- Simplest form of decision-making in C.
- Definition: The if statement evaluates a condition and executes the corresponding block of code if the condition is true.
- Syntax:
```
 if (condition) {
     // code to execute if condition is true
 }
```
- Example:
```
 #include <stdio.h>
 int main() {
     int i = 10;
     if (i < 18) {
         printf("Eligible for vote");
     }
 }
 // Output: Eligible for vote
```
- The expression within parentheses () is considered the condition and must evaluate to true or false.
- The block of code within {} is the body of the if statement, which executes only if the condition evaluates to true.
- If the body contains a single statement, the braces {} may be omitted.

If-else Statement

Extends the if statement to include a block of code for when the condition is false.
Definition: An if-else statement consists of two blocks—one executed if the condition is true and the other if false.
Syntax:

 if (condition) {
     // code if condition is true
 } else {
     // code if condition is false
 }

Example:

 #include <stdio.h>
 void main() {
     int i = 10;
     if (i > 18) {
         printf("Eligible for vote");
     } else {
         printf("Not Eligible for vote");
     }
 }
 // Output: Not Eligible for vote

Nested If Statement

If a condition within an if statement is itself an if statement, this is known as a nested if.
Definition: A nested if statement allows placing one if statement inside another.
Example:

 #include <stdio.h>
 int main() {
     int i = 10;
     if (i == 10) {
         if (i < 18)
             printf("Still not eligible for vote");
         else
             printf("Eligible for vote\n");
     } else {
         if (i == 20) {
             if (i < 22)
                 printf("i is smaller than 22 too\n");
             else
                 printf("i is greater than 25");
         }
     }
 }
 // Output: Still not eligible for vote

If-else-if Ladder

This structure is used for multiple conditions.
Definition: The if-else-if statement executes the block of code for the first true condition, bypassing the rest; if none are true, the final else block executes.
Syntax:

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

Example:

 #include <stdio.h>
 int main() {
     int i = 20;
     if (i == 10)
         printf("Not Eligible");
     else if (i == 15)
         printf("wait for three years");
     else if (i == 20)
         printf("You can vote");
     else
         printf("Not a valid age");
     return 0;
 }
 // Output: You can vote

Switch Statement

An alternative to if-else-if for executing code based on a variable's value.
Definition: The switch statement executes the corresponding code block based on the value of the variable in the switch expression.
Syntax:

 switch (variable) {
     case value1:
         // execute if variable == value1
     case value2:
         // execute if variable == value2
     default:
         // executed if no case matches
 }

Example:

 #include <stdio.h>
 void main() {
     char var;
     int a, b, c;
     printf("Enter the character");
     scanf("%c", &var);
     printf("Enter the first number");
     scanf("%d", &a);
     printf("Enter the second number");
     scanf("%d", &b);
     switch (var) {
         case '+':
             c = a + b;
             printf("%d", c);
         default:
             printf("Default Case is executed");
             break;
     }
 }

Note: The switch expression should only evaluate to integers or characters, not other data types.

Conditional Operator (Ternary Operator)
- A shorthand for if-else that operates on three operands.
- Definition: The conditional operator ? : evaluates a condition and returns one of two values based on the truth of that condition.
- Syntax:
  c condition ? expression1 : expression2
- Example:
  c #include <stdio.h> int main() { int var; int flag = 0; var = flag == 0 ? 25 : -25; printf("Value of var when flag is 0: %d\n", var); return 0; } // Output: Value of var when flag is 0: 25

Jump Statements in C

Jump statements control the flow of execution within loops and functions.
- A) Break Statement
- Terminates the loop or switch statement.
- Control returns to the statement following the loop.
- Example:
  c #include <stdio.h> void main() { int arr[] = {1, 2, 3, 4, 5, 6}; int key = 3; int size = 6; for (int i = 0; i < size; i++) { if (arr[i] == key) { printf("Element found at position: %d", (i + 1)); break; } } } // Output: Element found at position: 3
- B) Continue Statement
- Opposite of break, it forces the loop to skip the current iteration and continue to the next.
- Example:
  c #include <stdio.h> int main() { for (int i = 1; i <= 10; i++) { if (i == 6) continue; else printf("%d ", i); } return 0; } // Output: 1 2 3 4 5 7 8 9 10
- C) Goto Statement
- Allows a jump to another point in the program marked by a label.
- Example:
  c #include <stdio.h> void main() { int n = 1; label: printf("%d ", n); n++; if (n <= 10) goto label; } // Output: 1 2 3 4 5 6 7 8 9 10

Loops in C

Allow repeating a block of code until a specific condition is met.
Types of loops in C:
1. For Loop
- Definition: An entry-controlled loop where the condition is checked before the loop executes.
- Syntax:
  c for (initialization; condition; update) { // body of the loop }
- Example:
  c #include <stdio.h> int main() { // Loop to print numbers from 1 to 5 for (int i = 0; i < 5; i++) { printf("%d ", i + 1); } return 0; } // Output: 1 2 3 4 5
1. While Loop
- Definition: Also an entry-controlled loop, checking the condition before the loop body runs.
- Syntax:
  c while (condition) { // Body of the loop }
- Example:
  c #include <stdio.h> int main() { int i = 0; // Initialization expression while (i <= 5) { // Test expression printf("%d ", i + 1); i++; // update expression } return 0; } // Output: 1 2 3 4 5 6
1. Do-While Loop
- Definition: An exit-controlled loop; it checks the condition after executing the loop body at least once.
- Syntax:
  c do { // Body of the loop } while (condition);
- Example:
  c #include <stdio.h> void main() { int i = 0; // Initialization expression do { printf("%d ", i); // loop body i++; // Update expression } while (i <= 10); // Condition to check } // Output: 0 1 2 3 4 5 6 7 8 9 10

Nested Loops

Nested loops involve placing one loop inside another.
Definition: The inner loop runs completely for each iteration of the outer loop.
Useful for tasks that require multiple iterations, such as operating on two-dimensional arrays.
Example:

  #include <stdio.h>
  void main() {
      for (int i = 0; i < 3; i++) { // Outer loop runs 3 times
          for (int j = 0; j < 2; j++) { // Inner loop runs 2 times for each outer iteration
              printf("i = %d, j = %d\n", i, j);
          }
      }
  }
  // Output: 
  // i = 0, j = 0
  // i = 0, j = 1
  // i = 1, j = 0
  // i = 1, j = 1
  // i = 2, j = 0
  // i = 2, j = 1