if-else?if-else is a two-way branch: one block runs when the condition is true, and a different block runs when it is false. Exactly one path executes—never both, never neither.
if (condition) {
/* true path */
} else {
/* false path */
}
true → if body only
false → else body only
use { } on both sides
need 3+ choices → else-if / switchif-else in CWrite the condition, then both outcomes in braced blocks.
n % 2 == 0 or score >= 40).if (condition) { ... }.else { ... } right after.gcc file.c -std=c11 -o out.| Piece | Required? | Role |
|---|---|---|
| Condition | Yes | Chooses which branch runs |
if body | Yes | Runs when condition is true |
else body | Yes (for if-else) | Runs when condition is false |
set or read the value to test
if condition is true:
do the true work
else:
do the false work
continue after the branchKeep these facts handy while writing two-way branches.
| Topic | Detail |
|---|---|
| Syntax | if (cond) { ... } else { ... } |
| Branches | Exactly one of the two runs |
vs if | else covers the false case |
| Braces | Use { } on both sides |
| Dangling else | else binds to nearest if |
| More choices | else-if chain |
| Sample | even/odd, pass/fail, sign of n |
Simulate if-else choices in the browser. Same ideas as the C examples below (compile those with gcc).
Trace if (n % 2 == 0) … else … with two values. Same pattern as Example 1.
Case A: n = 8
8 % 2 == 0? true → if body → "even"
else skipped
Case B: n = 7
7 % 2 == 0? false → else body → "odd"
if body skipped
One branch only — always.Compile every example with gcc file.c -std=c11 -o out. Review if; next try else-if for more than two outcomes.
Classic two-way branch: remainder 0 means even; otherwise odd.
#include <stdio.h>
int main(void) {
int n = 8;
if (n % 2 == 0) {
printf("even\n");
} else {
printf("odd\n");
}
return 0;
}n = 8 is what we test.n % 2 is 0 for even numbers.even.n = 7, the else branch would print odd.Use a threshold: score at or above 40 passes; otherwise fail.
#include <stdio.h>
int main(void) {
int score = 55;
if (score >= 40) {
printf("pass\n");
} else {
printf("fail\n");
}
return 0;
}score >= 40.pass.else and print fail.else covers every case that is not positive (zero and negative). For three labels (positive / zero / negative), use else-if.
#include <stdio.h>
int main(void) {
int n = -2;
if (n > 0) {
printf("positive\n");
} else {
printf("not positive\n");
}
return 0;
}n > 0 is false for -2.not positive (covers zero and negatives).else if when needed.Check these before relying on if-else in real projects.
elseWithout braces, else pairs with the nearest if—often not the one you meant. Always brace both bodies.
= instead of ==if (x = 1) assigns and takes the true branch. Compare with ==; compile with -Wall.
else means “everything else.” For positive / zero / negative, use else-if, not a single if-else.
if (ok); else … makes an empty if. The following else may not match what you expect—avoid the semicolon after the condition.
| Approach | Cost | Best for |
|---|---|---|
if-else | One branch taken | Binary decisions |
Long else-if chains | Checks until a match | Put common cases first |
switch on ints | Often jump table | Many discrete values — see switch |
An if-else is cheap. Prefer the form that reads clearest. For many constant cases, switch can be clearer and sometimes faster.
else covers the false case explicitly.else-if or switch.One line: if-else always picks exactly one path—true or false—brace both bodies.
Learn how to chain more than two related conditions.
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