AP CS Debugger

On the AP CS A exam, you’ll often see code that prints output based on if statements. When you spot multiple if statements in a row (with no else), treat it as a red flag. In that pattern, more than one branch can run, which frequently changes the output from what students expect. Slow down and trace carefully.

1) Core idea in one minute

• if – else if – else
  • Exactly one branch runs. As soon as one condition is true, the rest are skipped.
• if + if + if (back‑to‑back)
  • Each if is independent. Zero, one, or many branches can run.

int score = 95;

// Back-to-back ifs (dangerous if you only want one line)
if (score >= 90) 
   System.out.println("A");
if (score >= 80) 
   System.out.println("B"); // also true → prints B too

// Safer if you want exactly one result:
if (score >= 90) 
   System.out.println("A");
else if (score >= 80) 
   System.out.println("B");

2) Common traps the exam loves

Trap A: Overlapping conditions → multiple lines of output

int score = 95;

if (score >= 90) 
   System.out.println("A");
if (score >= 80) 
   System.out.println("B");

Output

A
B

If you intended just one letter, you needed an else if chain.

Trap B: Side effects change later conditions

int i = 9;

if (i < 10) 
   i++;                 // i becomes 10
if (i == 10) 
   System.out.println("ten"); // now true

Output

ten

With else if, the second condition wouldn’t even be checked:

int i = 9;

if (i < 10) 
   i++;
else if (i == 10) 
   System.out.println("ten"); // skipped

Trap C: Double‑counting categories

int count = 0;
int n = 30;

if (n % 2 == 0) 
   count++;   // true
if (n % 3 == 0) 
   count++;   // true
if (n % 6 == 0) 
   count++;   // true
System.out.println(count);

Output: 3

If you meant “exactly one category,” use else if or make conditions mutually exclusive.

Trap D: The “dangling else” (else attaches to the nearest if)

int x = 7;

if (x < 10) 
   System.out.print("A");
if (x < 8)  
   System.out.print("B");
else        
   System.out.print("C");

Output: AB

The else pairs with the second if, not the first. Braces make the intent clear:

if (x < 10) {
    System.out.print("A");
} else if (x < 8) {
    System.out.print("B");
} else {
    System.out.print("C");
}

3) Exam-day checklist (use when you see back‑to‑back if‘s)

Red flag triggered → trace carefully:

1. Do the conditions overlap? (e.g., >= 90 and >= 80 overlap)
2. Does any earlier if modify variables used later (++, =, method calls)?
3. Which if does an else actually attach to? (nearest if!)
4. Does the problem hint that only one line of output is expected? If yes, be skeptical of multiple ifs.
5. Step through line by line and write the output as you go. Don’t guess.

4) Safer rewrites & patterns

• If you want exactly one outcome (grading, menus, ranges): Use an if – else if – else chain.
• If multiple outcomes are allowed (adding multiple tags/flags): Back‑to‑back ifs are fine—but make sure that behavior is intentional.
• Use braces generously to avoid dangling‑else confusion.
• When in doubt, add mutually exclusive conditions (e.g., >= 90, >= 80 && < 90, etc.).

5) Mini practice

Q1

int x = 7;

if (x < 10) 
   System.out.print("A");
if (x < 8)  
   System.out.print("B");
else        
   System.out.print("C");

Output? → AB

(Second if is true, so B prints; its else is skipped.)

Q2

int n = 5;

if (n <= 5) 
   n++;
if (n <= 6) 
   n++;
System.out.println(n);

Output? → 7

(After the first if, n is 6; the second if is still true, so it increments again.)

Q3 (make it print exactly one grade)

int score = 88;

if (score >= 90) 
   System.out.println("A");
if (score >= 80) 
   System.out.println("B");
if (score >= 70) 
   System.out.println("C");

Fix

if (score >= 90) 
   System.out.println("A");
else if (score >= 80) 
   System.out.println("B");
else if (score >= 70) 
   System.out.println("C");
else 
   System.out.println("D");

On AP CS A, back‑to‑back if statements are a red flag. They can trigger multiple branches due to overlapping conditions, state changes, or a dangling else. When you see them, pause, trace line by line, and verify the exact output. If your intent is “pick exactly one,” switch to an if – else if – else chain.