Module 14 · Interactive lesson

else-if Ladder & Nested if

Many choices

With else if you can choose between many options — grades, prices, menu choices. And an if can sit inside another if. In the lab below you watch a ladder being checked from the top, see what happens when the conditions are in the wrong order, walk a decision tree, and meet the dangling else — an else that belongs to a different if than it seems.

Ladderthe first true condition wins
Ordercheck the strictest condition first
elsepairs with the nearest if

marks Wrong order

main.c

Ready

Statements run 0 Lines printed 0

Ready

Keyboard: ← → step · Space play / pause · R reset · Esc exit classroom mode

How it works

The ideas behind the lab

Exactly what you watched happen, in plain steps.

The else-if ladder

An else-if ladder is a chain of if … else if … else if … else. C checks the conditions from the top. The FIRST condition that is true runs its statement, and the rest of the ladder is skipped — later conditions are not even checked. The final else (optional) catches everything else.

if (marks >= 90)
    grade = 'A';
else if (marks >= 75)      // reached only if marks < 90
    grade = 'B';
else if (marks >= 50)      // reached only if marks < 75
    grade = 'C';
else                       // everything below 50
    grade = 'F';

Same conditions, different structure (marks = 95 and marks = 60)
Version	Code	marks = 95	marks = 60
else-if ladder	if (m >= 90) A else if (m >= 75) B else if (m >= 50) C else F	A	C
separate ifs	if (m >= 90) A if (m >= 75) B if (m >= 50) C	A B C	C
ladder, wrong order	if (m >= 50) C else if (m >= 75) B else if (m >= 90) A	C	C
ladder with ranges	if (m >= 50 && m < 75) C else if (m >= 75 && m < 90) B …	A	C

else-if ladder

Conditions are checked from the top, one after another.
Only the FIRST true condition runs; the rest is skipped.
Put the most demanding condition first (marks >= 90 before marks >= 75).
A final else handles every case that no condition caught.
Separate if statements are NOT a ladder: each is checked, so several can run.

Nested if

An if can be inside another if (or inside its else).
The inner condition is checked only when the outer path reaches it.
Nested ifs form a decision tree: one question, then another.
Often two levels can be combined: if (age >= 18 && hasId).
Keep nesting shallow — deep nesting is hard to read.

The dangling else

An else belongs to the NEAREST preceding if that does not have an else.
Indentation does not change this — C ignores it.
if (a) if (b) x(); else y(); → the else belongs to if (b).
Use braces to make the owner clear: if (a) { if (b) x(); } else y();
gcc -Wall warns: "suggest explicit braces to avoid ambiguous else".

Walkthrough

A ticket price: ladder + nested if

Every line, explained.

1 #include <stdio.h>

3 int main(void)

4 {

5 int age = 30;

6 char day = 'S'; 'S' stands for Sunday.

7 int price;

8 if (age < 5) { 30 < 5 is false…

9 price = 0;

10 } else if (age < 18) { 30 < 18 is false…

11 price = 50;

12 } else if (age >= 60) { 30 >= 60 is false…

13 price = 70;

14 } else { …so the final else runs (an adult under 60).

15 if (day == 'S') A nested if: is it Sunday? Yes.

16 price = 120; Weekend price: 120.

17 else This else belongs to if (day == 'S') — the braces of the outer else make that clear.

18 price = 100;

19 }

20 printf("Ticket: %d\n", price); Prints Ticket: 120

21 return 0;

22 }

Common mistakes

Spot the bug

Mistakes almost every beginner makes — and how to fix them.

Separate ifs instead of a ladder

✗ Wrong

if (m >= 90) puts("A");
if (m >= 75) puts("B");
if (m >= 50) puts("C");

✓ Right

if (m >= 90) puts("A");
else if (m >= 75) puts("B");
else if (m >= 50) puts("C");

Separate ifs are all checked: 95 prints A, B and C. else if stops after the first match.

The wrong order

✗ Wrong

if (m >= 50) grade = 'C';
else if (m >= 90) grade = 'A';

✓ Right

if (m >= 90) grade = 'A';
else if (m >= 50) grade = 'C';

With m >= 50 first, a 95 already matches it, so the A branch is never reached.

A condition after else

✗ Wrong

if (m >= 75) puts("B");
else (m >= 50) puts("C");

✓ Right

if (m >= 75) puts("B");
else if (m >= 50) puts("C");

A plain else never has a condition. The first version does not compile: expected ';' before 'puts'.

The dangling else

✗ Wrong

if (a > 0)
    if (b > 0) puts("both");
else
    puts("a <= 0");

✓ Right

if (a > 0) {
    if (b > 0) puts("both");
} else
    puts("a <= 0");

Without braces the else belongs to if (b > 0), so "a <= 0" is printed when a is positive.

No final else

✗ Wrong

int price;
if (age < 18) price = 50;
else if (age >= 60) price = 70;
printf("%d", price);

✓ Right

int price;
if (age < 18) price = 50;
else if (age >= 60) price = 70;
else price = 100;

For ages 18 to 59 no branch runs, so price is never set and prints garbage.

Practice

Check your understanding

Answer each question — you will see the explanation straight away.

0 / 5 correct

Question 1

int m = 80; if (m >= 90) puts("A"); else if (m >= 75) puts("B"); else puts("C"); — output?

Question 2

In an else-if ladder, how many branches can run?

Question 3

With three separate if statements (no else), how many can run?

Question 4

if (a) if (b) x(); else y(); — which if does the else belong to?

Question 5

Which is the same as if (a > 0) { if (b > 0) x(); } ?

Assessment

else-if Ladder & Nested if quiz

Test yourself: 10 questions, 70% to pass. Create a free account to take the quiz, save your progress and work towards the certificate.

Create a free account →

← Previous module if and if-else Next module → switch Statement