🧮 Debugging

Trace Tables

A trace table lets you manually track how every variable changes as code executes, line by line — the single most reliable way to find a logical error or predict a program's output.

The method

List every variable that changes as a column heading.
Add a row for the starting (initial) values, before the loop/code runs.
Work through the code one line/iteration at a time, adding a new row each time a variable's value changes.
Check the loop's exit condition carefully at each iteration — this is where most errors happen.
The last row shows the final state — this is your answer.

Worked example

Click through the steps below to see exactly how a trace table is built for a simple summing loop.

Worked example — click through step by step:

int sum = 0;
for (int i = 1; i <= 4; i++) {
    sum = sum + i;
}
System.out.println(sum);

i	sum
—	0

Before the loop: sum is initialised to 0.

💡 Exam Tip

The most common trace-table mistake is getting the loop's exit condition wrong — always double-check whether the loop is a pre-check (condition tested BEFORE the body runs, e.g. for/while) or a post-check (tested AFTER, e.g. do…while) — this changes whether the loop body can ever run zero times.

Test yourself

0/14 answered

Question 1

What value is printed after this loop finishes?

int sum = 0;
for (int i = 1; i <= 4; i++) {
    sum = sum + i;
}
System.out.println(sum);

Question 2

How many times does the loop body execute (what is the final value of count)?

int count = 0;
int n = 20;
while (n > 1) {
    n = n / 2;
    count++;
}
System.out.println(count);

Question 3

What is printed after this code runs?

int a = 5, b = 8;
int temp = a;
a = b;
b = temp;
System.out.println(a + " " + b);

Question 4

What is the value of flag after this code runs?

int x = 7;
boolean flag = false;
if (x % 2 == 0) {
    flag = true;
} else {
    flag = false;
}
System.out.println(flag);

Question 5

What is the final value of result?

int result = 100;
int num = 246;
while (num > 0) {
    result = num % 10;
    num = num / 10;
}
System.out.println(result);

Question 6

What is printed? (Remember: continue skips the rest of that iteration only.)

int total = 0;
for (int i = 0; i < 5; i++) {
    if (i == 3) {
        continue;
    }
    total += i;
}
System.out.println(total);

Question 7

What is printed after these nested loops finish?

int count = 0;
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 2; j++) {
        count++;
    }
}
System.out.println(count);

Question 8

What is the final value of highest?

int[] marks = {60, 75, 40, 90};
int highest = marks[0];
for (int i = 1; i < marks.length; i++) {
    if (marks[i] > highest) {
        highest = marks[i];
    }
}
System.out.println(highest);

Question 9

What is printed? (Remember: a do-while always runs its body at least once, checking the condition afterwards.)

int n = 3;
int total = 0;
do {
    total += n;
    n--;
} while (n > 0);
System.out.println(total);

Question 10

What is printed after this code runs?

int x = 4;
int y = 9;
if (x > y) {
    x = x + y;
} else if (x == y) {
    x = 0;
} else {
    y = y - x;
}
System.out.println(x + " " + y);

Question 11

How many even numbers are counted in the array?

int[] nums = {3, 1, 4, 1, 5};
int even = 0;
for (int i = 0; i < nums.length; i++) {
    if (nums[i] % 2 == 0) {
        even++;
    }
}
System.out.println(even);

Question 12

What is printed after this loop finishes?

String word = "exam";
String result = "";
for (int i = word.length() - 1; i >= 0; i--) {
    result = result + word.charAt(i);
}
System.out.println(result);

Question 13

What is printed after this code runs? (Careful — trace each assignment in order.)

int a = 2;
int b = 3;
int c = a;
a = b;
b = c + a;
System.out.println(a + " " + b);

Question 14

What is the final value of total (sum of multiples of 3 from 1 to 10)?

int total = 0;
for (int i = 1; i <= 10; i++) {
    if (i % 3 == 0) {
        total += i;
    }
}
System.out.println(total);