Decision Making in C

In programming, we often need to perform different sets of actions depending on certain conditions. For example:

• If the weather is fine, go for a walk.
• If the highway is busy, take a diversion.

In C, decision-making is done using conditional statements, which enable the program to decide which path of execution to follow. These include:

• if statement,
• if-else statement,
• Conditional (ternary) operator,
• switch statement

Let's dive into the C version of decision control instructions and how we can use them in different scenarios.

---

if statement

• In C, we use the if statement to make decisions. The general form of the if statement looks like this:
• Structure of if statement:

  if(conditional_statement){
      // execute is condition is true
  }
  

  • conditional_statement:
    • Anything that returns true(non-zero) or false(0).
    • Can be single condition (i < 10) or combination of multiple condition (i>5 && i < 10), or any function that returns boolean.
  • The conditional_statement is enclosed in parentheses ().
  • If the conditional_statement evaluates to true, the block of code inside the braces {} is executed.
  • If the condition is false, the program skips the code inside the braces and continues with the next line.
• Example:
  • Code:

    int quantity = 2000;
    
    if (quantity > 1000) {
        printf("10%% discount\n");
    }
    

  • Output:

    10% discount
    

if-else statement

• The if-else statement allows you to execute one block of code if the condition is true and another block if the condition is false.
• Structure of if-else statement is:

  if(conditional_statement){
      // execute is condition is true
  }
  else{
      // execute if false
  }
  

• Let’s take a scenario where a discount of 10% is given if the quantity purchased is greater than 1000.
  • Code:

    int quantity = 2000;
    
    if (quantity > 1000) {
        printf("10%% discount\n");
    }
    else{
        printf("No discount\n");
    }
    

  • Output:

    10% discount
    

else-if Ladder

• The else-if ladder allows us to check multiple conditions in sequence.
• Once a condition is true, the corresponding block of code is executed, and the rest of the ladder is skipped.
• Structure of else-if ladder:

  if (condition1) {
      // execute if condition1 is true
  }
  else if (condition2) {
      // execute if condition2 is true
  }
  else if (condition3) {
      // execute if condition3 is true
  }
  else {
      // execute if none of the above conditions are true
  }
  

• Let's consider a scenario where:
  • A 20% discount is given if the quantity is above 2000.
  • A 10% discount if the quantity is above 1000 but 2000 or below.
  • No discount if the quantity is 1000 or below.
  • Example code:

    int quantity = 1500;
    
    if (quantity > 2000) {
        printf("20%% discount\n");
    }
    else if (quantity > 1000 && quantity <= 2000) {
        printf("10%% discount\n");
    }
    else {
        printf("No discount\n");
    }
    

  • Output:

    10% discount
    

  • In this example:
    • If quantity is more than 2000, it prints "20% discount".
    • If quantity is between 1001 and 2000, it prints "10% discount".
    • If quantity is 1000 or below, it prints "No discount".

Nested if-else Statements in C

• Sometimes we may need to test multiple conditions, one inside another. This is called nesting of if-else statements.
• All of the above statements can be nested.
• Example:

  int number = 0;
  if (number >= 0) {
      if (number == 0) {
          printf("The number is zero.\n");
      } else {
          printf("The number is positive.\n");
      }
  } else {
      printf("The number is negative.\n");
  }
  

• Output:

  The number is zero.
  

Ternary Operator

• The conditional or ternary operator (? :) offers a quick way to check a condition and choose between two values.

• This operator is often used to replace simple if-else statements for readability, especially in assignments and quick checks.

• Structure of ternary operator:

  condition ? expression_if_true : expression_if_false;
  

• How the Ternary Conditional Operator Works

  • condition is evaluated first.
  • If the condition is true (non-zero), expression_if_true is evaluated and becomes the result.
  • If the condition is false (0), expression_if_false is evaluated and becomes the result.

• Example:

  int age = 18;
  const char* eligibility = (age >= 18) ? "Eligible to vote" : "Not eligible to vote";
  printf("%s\n", eligibility);
  

• Output:

  Eligible to vote
  

• Explaination:

  Here, the condition (age >= 18) is evaluated.
  As age >= 18 is true, eligibility is set to "Eligible to vote". If age < 18, the condition would be false, and eligibility would be set to "Not eligible to vote".
  

Summary

• if statement: Executes a block if a condition is true.
• if-else statement: Executes one block if the condition is true and another if it's false.
• Nested if-else: Used when multiple conditions need to be checked, each depending on the previous.
• Ternary is nothing but a shorter version of if-else.
• As our programs grow more complex, mastering these control statements will help us handle different situations effectively.

switch statement

• The switch statement allows the program to evaluate an expression (typically an integer or character) and jump directly to the corresponding case label.

• Each case represents a possible value of the expression, and the corresponding block of code is executed when the value matches.

• If no match is found, an optional default block is executed.

• Structure of switch statement:

  switch (expression) {
      case constant1:
          // code to execute if expression matches constant1
          break;
      case constant2:
          // code to execute if expression matches constant2
          break;
      case constant3:
          // code to execute if expression matches constant3
          break;
      default:
          // code to execute if no cases match
  }
  

• How the switch Statement Works

  • The expression inside the switch is evaluated.
  • The program compares the result with each case constant, starting from the top.
  • When a match is found, the code inside that case block is executed.
  • The break statement ensures that the program exits the switch after executing the matching case.
  • Without the break, execution continues to the next case block.
  • If no case matches, the default block (if present) is executed.

• Example:

  int day = 3;
  
  switch (day) {
      case 1:
          printf("Monday\n");
          break;
      case 2:
          printf("Tuesday\n");
          break;
      case 3:
          printf("Wednesday\n");
          break; // <----------------------(1)
      default:
          printf("Other day\n");
  }
  

• Output:

  Wednesday
  

• Output if the break (1) is not used:

  Wednesday
  Other day