DSE ICT | Programming: Loops and Debugging Lesson-Prep Guide (Tutor Edition) | for Traces, while Conditions, Fault Finding

2026-11-01

Tutor lesson-prep guides · Secondary ICT

Students can read code but freeze when asked what the variable holds after the fifth pass. This lesson makes them write a trace table, then debug from the table.

Objectives — Students can: (1) complete a trace table for a for loop; (2) predict how many times a while loop runs and when it stops; (3) locate a fault by splitting a program into input, processing and output.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Variables	After x = 3 then x = 5, what is x	5, the old value is overwritten
Squares	The square of 4	16
Comparison	When is n < 100 false	When n is 100 or more

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Trace table format	Columns: pass, i, variable, test	Draw an empty table
8-24 min	for loop trace	range(1,6) runs 1 to 5, add squares	Reach a total of 55
24-36 min	while loop	Double from 3, find the exit test	6 passes, ending at 192
36-48 min	Three fault types	One fault in input, processing, output	Check each part
48-56 min	Fix and rerun	Correct the loop, trace it aloud	Oral execution
56-60 min	Exit check	Three questions marked on the spot	Independent work

03 Teaching script (read it out)

Scene 1: for loops: trust the table, not your head

t = 0 then for i in range(1, 6): t = t + i*i. range(1,6) stops at 5, so there are five passes: 1+4+9+16+25 = 55. Each row of the table records i, i*i and t. Asked for t after the third pass, read row three: 14. No mental simulation needed.

Ask the student: how many passes does range(1,6) give (five).

Scene 2: while loops: ask when it stops, not what it does

n = 3 then while n < 100: n = n * 2. The test comes first each time: 3, 6, 12, 24, 48, 96, 192. Since 96 is still below 100 the loop runs once more to 192, then the test fails, so there are 6 passes.
Warn: students stop at 96. The condition is below 100, and 96 still satisfies it.

Ask the student: is the test checked before or after the body (before).

Scene 3: Debugging: split the program, then print the variables

Input faults: wrong type, a string added to a number. Processing faults: wrong range, accumulator not reset. Output faults: printing an intermediate value. Three moves: state the expected output, print the variables inside the loop, compare with the trace table and find the first pass that disagrees. The fault is on that line.

Ask the student: what do you compare to find the faulty pass (the trace table).

04 Question types and worked steps

Type	Example	Steps	Answer
Trace	Value of t after the third pass	Fill the table row by row	14 (1+4+9)
Count	Doublings from 3 to pass 100	Test before each pass	6 passes, ending 192
Debug	Output one short of expectation	Check the range upper bound	range excludes the bound
Rewrite	Turn a while into a for	Use a for when the count is known	for k in range(6)

Sanity check (teach this): The sum of squares 1 to 5 is 55; 91 means 6 squared was included. 6 doublings end at 192; an answer of 96 means the last test was skipped.

05 Independent practice (3 questions)

Question	Answer	What to watch
Sum of squares from 1 to 5	55	Six not included
Final value doubling 3 until at least 100	192	Stopped at 96
Multiples of 3 between 1 and 20	6	18 counted once

06 Common mistakes and what to say

Mistake	Student writes	Say this
Range bound included	Adds 6 squared	range stops before the upper bound, so it ends at 5.
while stopped early	Treats 96 as the answer	96 is still below 100, so one more pass is needed.
Accumulator not reset	Starts t from the previous question	Set t back to 0 before each new total.
No intermediate printing	Edits numbers until it looks right	Print inside the loop and find the first wrong pass.

07 Homework (paper, section and question numbers)

Secondary ICT chapter index — All chapters: data, networks, databases
Secondary lesson-prep overview — All subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
Sum of squares 1 to 5	55	All three right: move to arrays
Passes doubling 3	6	One wrong: reteach test ordering
Passes for range(1,6)	five	Two or more wrong: return to act one

09 Tutor FAQ

Must we use Python?

The exam uses pseudocode, but tracing in Python builds the same habit fastest.

How long should a trace table be?

Long enough to answer the question, usually five or six rows.

How is an infinite loop answered?

Name the missing update or the always-true test, and write the corrected line.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.