C Conditionals: if, else, and switch

Introduction

In C programming, conditionals let your program make decisions. They control which parts of your code run based on values or logic. This is essential for tasks like checking user input, handling errors, or choosing between actions.

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Why conditionals matter

Without conditionals, your program would run the same way every time. Conditionals let you:

• Respond to different inputs
• Branch into different logic paths
• Make your program interactive and robust

They keep code clear, testable, and efficient - especially when combined with good naming and small functions.

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if, else if, and else

Syntax

if (condition) {
  // do stuff if condition is true
} else if (another_condition) {
  // do stuff if another_condition is true
} else {
  // do stuff if none of the above are true
}

Rules of thumb

• Conditions are evaluated top to bottom; the first true branch runs, the rest are skipped.
• Always use braces {} even for one-line statements after e.g. if (x >1) y=1 but its not good practice as its easy to get confused when adding more lines later.
• Keep conditions simple; extract complex checks into well-named helper functions. i.e. better use if (CheckInput(input,limits) == 1 ) than if ( input < X && X < Y and input >= Z .......... )
• Enable warnings (-Wall -Wextra -Werror) to catch mistakes like = vs ==. I promise you will make this mistake at least once! :D

Example 1 — Simple if

#include <stdio.h>

int main(void) {
  int score = 85;
  if (score >= 50) {
    printf("Pass\n");
  }
  return 0;
}

Example 2 — if...else

#include <stdio.h>

int main(void) {
  int score = 45;
  if (score >= 50) {
    printf("Pass\n");
  } else {
    printf("Fail\n");
  }
  return 0;
}

Example 3 — if...else if...else chain

#include <stdio.h>

int main(void) {
  int grade = 72;
  if (grade >= 70) {
    printf("Class I\n");
  } else if (grade >= 60) {
    printf("Class II.i\n");
  } else if (grade >= 50) {
    printf("Class II.ii\n");
  } else if (grade >= 40) {
    printf("Class III\n");
  } else {
    printf("Fail\n");
  }
  return 0;
}

Example 4 — Nested if (use sparingly)

#include <stdio.h>

int main(void) {
  int x = 15;
  if (x > 0) {
    if (x % 2 == 0) {
      printf("positive even\n");
    } else {
      printf("positive odd\n");
    }
  } else {
    printf("non-positive\n");
  }
  return 0;
}

Notice things can get complicated quickly with many nested if statements! If you find yourself in this situation it might be better to reconsider the approach.

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switch statement

A more readable way to handle multiple branches is with a switch statement, which allow you to choose between several possible values of a single variable:

Syntax

switch (expression) {
  case VALUE_1:
    // statements
    break;
  case VALUE_2:
    // statements
    break;
  default:
    // statements if no case matches
    break;
}

• expression is evaluated once; control jumps to the matching case label.
• case labels must be integer constant expressions (e.g., literals, enum constants).
• Add break; to avoid the program continuing on to the next case, in some extremely rare cases you might want this but probably not!
• Use default for unexpected values. Here you might log or handle the errors

Example 1 — Switch-based menu

#include <stdio.h>

int main(void) {
  int choice = 0;
  printf("Menu:\n1. Start\n2. Stop\n3. Exit\nEnter choice: ");
  if (scanf("%d", &choice) != 1) return 1;

  switch (choice) {
    case 1:
      printf("Starting...\n");
      break;
    case 2:
      printf("Stopping...\n");
      break;
    case 3:
      printf("Exiting...\n");
      break;
    default:
      printf("Invalid choice.\n");
      break;
  }
  return 0;
}

Example 2 — Switch with enum

Switch statements are useful when combined with enums especially when using finite state machines in Semester 2. For example if we had a program that could be in three states, we can use a switch statement using our enum variables:

#include <stdio.h>

typedef enum {
  MODE_IDLE = 0,
  MODE_RUN  = 1,
  MODE_ERR  = 2
} mode_t;

int main(void) {
  mode_t m = MODE_RUN;

  switch (m) {
    case MODE_IDLE:
      printf("idle\n");
      break;
    case MODE_RUN:
      printf("run\n");
      break;
    case MODE_ERR:
      printf("error\n");
      break;
    default:
      printf("unknown mode\n");
      break;
  }
  return 0;
}

Example 3 — Declaring variables in a switch case safely

This is something I personally have got wrong many times! Each case within {} creates its own scope so that variables declared in the case are local to that case only.

#include <stdio.h>

int main(void) {
  int x = 2;
  switch (x) {
    case 1: {
      int a = 10;  // this 'a' belongs to the scope within {}
      printf("case 1: a = %d\n", a);
      break;
    }
    case 2: {
      int a = 20;  // this is ok as we are in a separate scope to case 1
      printf("case 2: a = %d\n", a);
      break;
    }
    default:
      printf("default\n");
      break;
  }
  return 0;
}

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When to use which?

• Use if / else if / else for:
  • Ranges or relational logic (e.g., x < 10, a <= b && b < c)
  • Complex boolean expressions
  • Situations where conditions are not small, fixed constants
• Use switch for:
  • Discrete values (menu options, key codes, protocol IDs)
  • Cleaner, flatter code when there are many exact-match branches
  • Enums, where each constant maps to a branch

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Summary table

Feature	if / else if / else	switch
Best for	Ranges, complex conditions	Discrete values, enums
Readability	Great for few branches	Great for many exact matches
Fall-through	Not applicable	Possible; requires break or comment
Expression types	Any expression yielding boolean truth	Integral types / enums
Maintenance	Can grow nested	Scales well with many cases

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Quick tips

• Prefer clear, simple conditions; extract complicated logic into helper functions.
• Always use braces {} for consistency and safety.
• In switch, break each case unless you intentionally fall through.
• Consider enabling -Wswitch and -Wswitch-enum to catch unhandled enum values.
• Compile with strict warnings: -Wall -Wextra -Werror.

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