if Statement?An if statement runs a block of code only when a condition is true. If the condition is false, that block is skipped. In C, 0 is false and any non-zero value is true.
if (condition) {
/* runs only when condition is true */
}
0 → false
non-zero → true
== compare = assign (do not mix them up)
use { } braces even for one statementif in CWrite a condition, put the optional work in braces, then compile.
n > 0 or x == y).if (condition) { ... } with braces around the body.==, !=, <, >, &&, ||, ! as needed.gcc file.c -std=c11 -o out.| Piece | Required? | Role |
|---|---|---|
| Condition | Yes | Expression tested before the body |
Body { } | Strongly recommended | Code that runs when true |
else | No | Optional false path — see if-else |
read or set values used in the test
if condition is true:
do the work
// otherwise skip the block
continue with the rest of the programKeep these facts handy while writing conditions.
| Topic | Detail |
|---|---|
| Syntax | if (cond) { ... } |
| Truth | 0 false; non-zero true |
| Compare | == != < > <= >= |
| Combine | && and, || or, ! not |
| Assign trap | if (x = 1) assigns — use == |
| Braces | Always use { } for clarity |
| Next step | if-else for a false branch |
Simulate simple if checks in the browser. Same ideas as the C examples below (compile those with gcc).
Trace if (n > 0) with two values. Same pattern as Example 1.
Case A: n = 5
check n > 0? true → run body → print "positive"
Case B: n = -2
check n > 0? false → skip body → nothing printed
if alone has no "else" path — false means skip.Compile every example with gcc file.c -std=c11 -o out. Next: if-else when you need a false branch.
The body runs only when n > 0. If n is zero or negative, nothing is printed.
#include <stdio.h>
int main(void) {
int n = 5;
if (n > 0) {
printf("positive\n");
}
return 0;
}n = 5 is what we will test.n > 0 is true.printf prints positive.n = -1, the body would be skipped entirely.Use == for equality and % for remainder. Never use = inside the condition by accident.
#include <stdio.h>
int main(void) {
int n = 8;
if (n == 0) {
printf("zero\n");
}
if (n % 2 == 0) {
printf("even\n");
}
return 0;
}n == 0 is false for 8 — that if is skipped.n % 2 is 0 when n is even.n % 2 == 0 is true → print even.== compares; = assigns.&&Both sides of && must be true. For an alternative path when false, use if-else.
#include <stdio.h>
int main(void) {
int age = 20;
int has_id = 1; /* 1 = true, 0 = false */
if (age >= 18 && has_id) {
printf("allowed\n");
}
return 0;
}age >= 18 is true for 20.has_id is 1 (non-zero → true).&& requires both true → body runs.Check these before relying on if in real projects.
= instead of ==if (x = 5) assigns 5 and is always true. Enable warnings (-Wall) and prefer ==.
Without { }, only the next statement is guarded. A second line you add later always runs — a classic bug.
==Floating-point rounding can make exact equality fail. Prefer a small epsilon range, or compare integers when possible.
if (ok); with a trailing semicolon is an empty if—the next line always runs.
| Approach | Cost | Best for |
|---|---|---|
Simple if | Negligible (branch) | Everyday decisions |
&& / || short-circuit | May skip right side | Cheap tests first; avoid side effects on the right |
Deep nested if | Harder to read | Prefer else-if / switch when clearer |
An if itself is cheap. Clarity matters more than micro-optimizing branches. Put the most likely or cheapest test first when combining with && / ||.
0 is false; anything else is true.==, never assign with = in a test.else when you need a false path.One line: if runs the body only when the condition is true—compare with ==, brace the body.
Learn how to run a different block when the condition is false.
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