Conditionals

A program that always does the same thing isn't very interesting. Conditionals let a program look at a value and decide what to do next. They are the second pillar of computation, after sequence.

The if statement

The basic form is:

if (condition) {
    // run this if condition is true (non-zero)
}

The condition is any expression. C treats 0 as false and any non-zero value as true.

Change age to 15 and rerun. The line should disappear from the output.

if ... else

To do one or the other:

else if chains

For more than two branches, chain them:

The branches are checked top to bottom. The first one whose condition is true runs, and the rest are skipped.

Comparison operators

if (x = 5)  { /* ALWAYS true; also sets x to 5! */ }
if (x == 5) { /* what you meant */ }

Logical operators

Combine conditions with logical operators:

&& and || are short-circuiting: if the result is already determined after evaluating the left side, the right side is not evaluated. This lets you write safe checks like if (p != NULL && p->value == 0) — the dereference on the right never runs when p is null.

Truth values: what counts as true?

In C, anything non-zero is true. 0 is false. There is no separate boolean unless you #include <stdbool.h>.

if (count)       { ... }  // true if count != 0
if (!count)      { ... }  // true if count == 0
if (str)         { ... }  // true if str is not the NULL pointer

This is concise, but explicit comparisons are easier to read:

if (count != 0)  { ... }
if (str != NULL) { ... }

Prefer clarity over cleverness, especially while learning.

Nested conditionals

You can put if inside if:

Deep nesting is hard to read. When it grows past two levels, it's usually a sign to refactor — split a chunk into a separate function, or rewrite with && and ||.

The dangling-else pitfall

if (a)
    if (b)
        printf("both\n");
else
    printf("not a\n");      // BUG: this else binds to the inner if, not the outer

Indentation lies. The else always binds to the nearest unmatched if. Always use braces, even for single statements:

if (a) {
    if (b) {
        printf("both\n");
    }
} else {
    printf("not a\n");
}

This rule is so important that many style guides make braces mandatory. Make it your default.

The switch statement

When you have many branches on a single integer value, switch is sometimes clearer than a long if/else if chain.

Three things to remember:

• The case value must be a compile-time integer constant.
• Execution falls through to the next case unless you break. This is sometimes useful (notice cases 6 and 7 both run Weekend) and sometimes a bug. Always write break unless you want fall-through.
• default runs if none of the cases matched. It is optional but good practice to include one.

Decision tree, visualized

A diagram like this makes it obvious why order matters when the ranges overlap. If you wrote if (score >= 60) first, everyone with a score of 95 would get a D!

Challenge: leap year

A year is a leap year if it is divisible by 4, except that years divisible by 100 are not leap years, unless they are also divisible by 400.

#include <stdio.h>
int main(void) {
  int x = 5;
  if (x = 10) {
      printf("hello\n");
  }
  printf("x = %d\n", x);
  return 0;
}

switch (n) {
  case 1: printf("one ");
  case 2: printf("two ");  break;
  default: printf("other "); break;
}