If/Else — Making Decisions in C

C in 100 Seconds, Episode 5

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Programs need to make choices. In C, the if statement is how you branch your code based on conditions.

Basic If/Else If/Else

int score = 85;

if (score >= 90) {
  printf("Grade: A\n");
} else if (score >= 80) {
  printf("Grade: B\n");
} else if (score >= 70) {
  printf("Grade: C\n");
} else {
  printf("Grade: F\n");
}

C evaluates conditions top to bottom. The first one that's true wins, and the rest are skipped. If nothing matches, else catches everything that's left.

The Ternary Operator

For simple two-way decisions, C has a one-liner:

int age = 20;
printf(age >= 18 ? "Adult\n" : "Minor\n");

The syntax is condition ? value_if_true : value_if_false. It's compact, but don't nest them — readability drops fast.

How C Sees Conditions

C doesn't have a boolean type in the traditional sense. Conditions evaluate to integers: 0 is false, anything else is true. So if (1) always executes, and if (0) never does.

Why Does This Matter?

Every non-trivial program needs branching. Whether you're validating input, checking error codes, or routing logic, if/else is the foundation. The ternary operator is a shortcut for when the branch is simple enough to fit in one expression.

Full Code

#include <stdio.h>

int main() {
  int score = 85;

  if (score >= 90) {
    printf("Grade: A\n");
  } else if (score >= 80) {
    printf("Grade: B\n");
  } else if (score >= 70) {
    printf("Grade: C\n");
  } else {
    printf("Grade: F\n");
  }

  int age = 20;
  printf(age >= 18 ? "Adult\n" : "Minor\n");

  return 0;
}

Compile and Run

gcc ifelse.c -o ifelse
./ifelse

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Next episode: Switch — a cleaner way to handle multiple specific values.

Student code: github.com/GoCelesteAI/c-in-100-seconds/episode05