DSE ICT | Algorithms and flowcharts: lesson-prep guide for tutors

2026-11-02

Tutor lesson-prep guides · Secondary ICT

Students can draw a flowchart, then freeze when asked what the variable holds on the third pass. This lesson turns drawing into instructions that can actually be run step by step.

Objectives — Students can: (1) break a word problem into sequence, selection and iteration; (2) draw a flowchart with correct symbols and a condition on every branch; (3) run a trace table one step per row and spot an infinite loop before marking.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Variables	After x <- x + 1, what is x?	The old value plus one; the right side is evaluated first
Conditions	How does IF differ from WHILE?	IF tests once; WHILE repeats until the condition is false
Symbols	What do a diamond and a parallelogram mean?	Diamond is a decision; parallelogram is input or output

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0–5 min	Opening question	Write three lines of pseudocode and ask which line repeats	Answer by show of hands and point at the condition
5–15 min	Prerequisite check	Three quick checks (assignment, IF versus WHILE, symbols)	Written answers, marked live
15–37 min	Three-part script	Teach the structures, symbols and branches, then trace tables	Recap each part in one sentence
37–49 min	Question walk-through	Model four questions, filling every row of the table aloud	Copy and complete the last two rows
49–55 min	Independent practice	Circulate; ask only which variable changed at that step	Do three items and self-mark
55–60 min	Exit check	Collect answers and count misses aloud	Written, no talking

03 Teaching script (read it out)

Scene 1: Part 1: only three structures exist

Every program reduces to three structures: sequence, selection and iteration. Write pseudocode before you draw, using five words only: INPUT, SET, IF, WHILE and OUTPUT. Then ask one question: does this loop have an exit? If the condition can never become false, it is an infinite loop, and that is the most common lost mark in this topic.

When does this condition become false? What if it never does?

Scene 2: Part 2: symbols and branches

Ovals are start and end, parallelograms are input and output, rectangles are processing, diamonds are decisions. A diamond must have two exits, one labelled Yes and one No, and an unlabelled branch counts as unanswered. The loop-back arrow must return to the decision diamond, never to a rectangle in the middle, or the trace table will not line up.

How many exits does a diamond have, and have you labelled them?

Scene 3: Part 3: trace tables, one step per row

Open four columns: iteration number, each variable, output, and the condition result. One step per row, no skipping. The final row is the one where the condition turns false, and you must write it, because the examiner is checking that you reached the exit. On efficiency: for a thousand records a linear search needs up to a thousand comparisons, while a binary search halves the range each time and needs ten, since two to the tenth is 1,024, just enough to cover a thousand.

Why ten comparisons for a thousand records, and not eight?

04 Question types and worked steps

Type	Example	Steps	Answer
Flowchart completion	Two or three boxes are blank.	(1) Read context to see if the gap is process or decision; (2) a decision needs two branches; (3) walk it aloud afterwards.	Example: finding a maximum, the missing box is “x > max?”.
Trace table	Pseudocode and initial values are given.	(1) Copy the initial values; (2) one step per row; (3) include the false row; (4) match the output.	Example: x from 1 to 5 fills five rows, with a sixth where the condition fails.
Pseudocode to flowchart	Convert written steps into a diagram.	(1) Start; (2) input; (3) process; (4) decision with two branches; (5) output and end.	Example: largest of three numbers, using either three IFs or a loop.
Efficiency comparison	Compare comparison counts.	(1) State n; (2) linear is n; (3) binary is log2 n rounded up; (4) give a numeric example.	Example: n = 1000 gives 1,000 versus 10.

Sanity check (teach this): Sanity check: for a thousand records the linear search caps at 1000 comparisons, while binary search needs 10, because 2^10 = 1024 which just covers 1000; for a hundred records it needs 7. An answer of “binary is ten times faster” is wrong: the gap is 1000 against 10, so a hundred times. Also, the trace table has one more row than the number of iterations, because the false row must appear.

05 Independent practice (3 questions)

Question	Answer	What to watch
Write pseudocode and a trace table for summing 1 to n with n = 4.	SET sum <- 0; SET i <- 1; WHILE i <= 4: sum <- sum + i; i <- i + 1. Five rows, sum = 10.	Check that i increments inside the loop; without it the loop never ends.
Draw the same algorithm as a flowchart with both branches labelled.	Start, input n, set sum and i, test i <= n, Yes accumulates and loops back, No outputs sum, end.	Check the arrow returns to the diamond and both Yes and No are labelled.
How many comparisons for 1,000 records using linear and binary search?	Linear up to 1,000; binary 10, since 2^10 = 1024.	Check that the power-of-two reasoning appears, not just the number.

06 Common mistakes and what to say

Mistake	Student writes	Say this
A diamond with one exit	The decision box has a single arrow downwards.	A decision has two outcomes. Where does Yes go and where does No go? Add them or it cannot terminate.
No update inside the loop	WHILE i <= 4 has no i <- i + 1.	i stays at 1 and the condition stays true. That is an infinite loop and it always loses marks.
Skipping rows in the trace table	One row covers two iterations.	One row, one step. You want speed, but the marker needs the row where it went wrong.
Confusing comparisons with data size	Writes that binary search needs a thousand comparisons.	A thousand is the data size, not the count. Binary halves the range, so it is ten.

07 Homework (paper, section and question numbers)

Secondary ICT index: finish the three exercises, then compare while and for in the programming loops chapter — Exercises 1–3 on this page

08 Exit check (last 5 minutes)

Question	Answer	Decision
What are the three control structures?	Sequence, selection, iteration	Cannot answer: reteach part 1
How many exits does a decision have?	Two, each labelled with its condition	Cannot answer: reteach part 2
How many comparisons does binary search need for 1,000 records?	Ten, because 2^10 = 1024	Cannot answer: reteach part 3

09 Tutor FAQ

Which year is this for?

S5 core ICT; S4 classes that have covered variables can start earlier.

Do we need computers?

No. Paper flowcharts and trace tables are enough; the point is step-by-step execution.

Is there a matching paper?

The homework link goes to the secondary ICT index, which pairs with the programming loops chapter.

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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.