Tracing a loop by hand

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Quick Questions

Shade in the A or B circle for each question.

1. Which describes how to trace a loop by hand using a trace table?

A Write one row for each repeat, showing the counter, other variables and any output

B Write one row only, showing the final value of each variable

2. for (let i = 1; i <= 3; i++) { console.log(i); } How many rows does the trace table need for the repeats?

A 4 rows: i is 0, then 1, then 2, then 3

B 3 rows: i is 1, then 2, then 3

3. When do you stop adding rows to a trace table?

A When the loop condition becomes false

B When the counter reaches 10

4. let total = 0; let n = 1; while (n <= 3) { total += n; n++; } After the second repeat, what are the values in the trace table?

A n is 2 and total is 3 (before n++ runs), then n becomes 3

B n is 2 and total is 2

5. Predicting the output before running a program is useful because...

A The computer no longer needs to run the code

B You can compare your trace with the real output to find mistakes

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1for (let i = 1; i <= 3; i++) { console.log(i); }		AThe total was not added up. It should keep a running sum: 1, then 3, then 6, because each repeat adds i to the previous total.
2let total = 0; let n = 1; while (n <= 3) { total += n; n++; }		Bfor (let i = 2; i <= 6; i += 2) { console.log(i); } // Prediction: 2, 4, 6
3Predicting the output by hand: before running, fill in a trace table, then write down what the loop will print.		CTrace table columns: n and total. Row 1: n=1, total=1. Row 2: n=2, total=3. Row 3: n=3, total=6. Then n is 4 and the loop stops.
4Comparing your trace with the real output: if the console output differs from your table, you made a tracing mistake or the code has a bug.		DThe trace includes a row for i = 4, but the condition i < 4 is false when i is 4, so the loop body never runs for it. Only 3 rows are needed.
5// Trace of: for (let i = 1; i < 4; i++) // Rows: i=1, i=2, i=3, i=4		Elet count = 3; while (count > 0) { console.log(count); count--; } // Trace: 3, 2, 1
6// Trace of: let total = 0; // for (let i = 1; i <= 3; i++) { total += i; } // Rows: total=1, total=2, total=3		FA trace table for this loop has 3 rows: i is 1, then 2, then 3, with output 1, 2, 3. It stops when i becomes 4.

Definitions

trace table

Fill in the Blanks

Complete each line of code by filling in the missing parts.

for (let i = 1; i <= 3; ___) { console.log(i); }

for (let i = 1; i __ 3; i++) { console.log(i); }

let total = 0; let n = 1; while (n <= 3) { total __ n; n++; }

let total = 0; for (let i = 1; i <= 4; i++) { total += i; } console.log(_____);

let n = 3; _____ (n > 0) { console.log(n); n--; } // trace rows: n = 3, 2, 1

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A let total = 0;

B for (let i = 1; i <= 3; i++) {

C total += i;

D console.log(i, totl);

E }

Question 2

A let count = 0;

B while (count < 3) {

C count + 1;

D console.log(count);

E }

Question 3

A let total = 0;

B for (let i = 1, i <= 4, i++) {

C total += i;

D }

E console.log(total);

Question 4

A let total;

B for (let i = 1; i <= 3; i++) {

C total += i;

D }

E console.log(total);

Question 5

A let count = 5;

B while (count < 0) {

C console.log(count);

D count -= 1;

E }

Answer Key

Tracing a loop by hand

Section A: Quick Questions

A — Write one row for each repeat, showing the counter, other variables and any output

B — 3 rows: i is 1, then 2, then 3

A — When the loop condition becomes false

A — n is 2 and total is 3 (before n++ runs), then n becomes 3

B — You can compare your trace with the real output to find mistakes

Section B: Match the Pairs

for (let i = 1; i <= 3; i++) { console.log(i); } → A trace table for this loop has 3 rows: i is 1, then 2, then 3, with output 1, 2, 3. It stops when i becomes 4.

let total = 0; let n = 1; while (n <= 3) { total += n; n++; } → Trace table columns: n and total. Row 1: n=1, total=1. Row 2: n=2, total=3. Row 3: n=3, total=6. Then n is 4 and the loop stops.

Predicting the output by hand: before running, fill in a trace table, then write down what the loop will print. → for (let i = 2; i <= 6; i += 2) { console.log(i); } // Prediction: 2, 4, 6

Comparing your trace with the real output: if the console output differs from your table, you made a tracing mistake or the code has a bug. → let count = 3; while (count > 0) { console.log(count); count--; } // Trace: 3, 2, 1

// Trace of: for (let i = 1; i < 4; i++) // Rows: i=1, i=2, i=3, i=4 → The trace includes a row for i = 4, but the condition i < 4 is false when i is 4, so the loop body never runs for it. Only 3 rows are needed.

// Trace of: let total = 0; // for (let i = 1; i <= 3; i++) { total += i; } // Rows: total=1, total=2, total=3 → The total was not added up. It should keep a running sum: 1, then 3, then 6, because each repeat adds i to the previous total.

Section C: Definitions

trace table — A table with one row per loop repeat, showing the counter, other variables and any output, filled in by hand until the condition is false.

Section D: Fill in the Blanks

for (let i = 1; i <= 3; i++) {
  console.log(i);
}

for (let i = 1; i <= 3; i++) {
  console.log(i);
}

let total = 0;
let n = 1;
while (n <= 3) { total += n; n++; }

let total = 0;
for (let i = 1; i <= 4; i++) { total += i; }
console.log(total);

let n = 3;
while (n > 0) { console.log(n); n--; }
// trace rows: n = 3, 2, 1

Section E: Spot the Error

D — The variable is misspelled as totl, so JavaScript throws a ReferenceError. It should be console.log(i, total); so the output matches the trace table columns i and total.

C — count + 1; works out a value but never stores it, so count stays 0 in every row of the trace table and the loop never ends. It should be count++; (or count += 1;).

B — The three parts of a for loop header are separated by semicolons, not commas. It should be for (let i = 1; i <= 4; i++) {.

A — Without = 0, total starts as undefined, so the first trace row would be undefined + 1 = NaN. The total must start at 0: let total = 0;.

B — count starts at 5, so count < 0 is false straight away and the trace table would have no rows. To count down the condition should be count > 0.