Session 37: Loops, Decisions & Debugging

At a glance: pupils build a sachet clean-up mini-game in Scratch that uses a loop, an IF-THEN condition and a Score variable, and they fix bugs calmly one change at a time. · Time: 90 minutes (adjust the timings if your period is shorter) · Workbook pages: 6

Learning outcomes

By the end of the session, pupils can:

Explain the difference between a counted loop (repeat (10)) and an infinite loop (forever), using the fufu pounding analogy.
Use an if <> then block with a diamond-shaped sensing block such as touching [mouse-pointer]?.
Create a Score variable, reset it with set [Score] to (0) and change it with change [Score] by (1).
Follow the 3-step debugging protocol: Spot it, Trace it, Fix one thing.
Save a working game as Cleanup_Game_[Name].sb3.

Before the session

Check Scratch Desktop (offline) on each PC, or internet for scratch.mit.edu. Keep the offline installer on USB.
Test the Code the Sprite 3D lab on the projector, including the Debug and Next step buttons.
Bring a pot and some bottle tops or cowries for the calabash demonstration.
Write the Page 6 "Be the Computer" code on the board for the review.

Running order

Time	Activity	Workbook	What the mentor does
0–10 min	Typing warm-up	–	Pupils open the typing trainer (Typing in the menu, or the warm-up banner on the session page) and do the 10-minute warm-up. Walk round checking posture and home-row fingers.
10–19 min	Fufu pounding: what is a loop?	Page 1	Clap "pound, turn". Compare `repeat (10)` and `forever`.
19–29 min	Conditions: the IF-THEN law	Page 2	"IF it rains, THEN open my umbrella." Show diamond blocks and the sensing table.
29–37 min	Variables: the storage calabash	Page 3	5 cowries, add 2, take 3. Link to `set` and `change`.
37–46 min	Debugging and Grace Hopper's moth	Page 4	Tell the moth story (her team found it in 1947). Bugs are clues; change one thing, then test.
46–58 min	3D lab: loops and debugging	–	Demonstrate, then pairs do the missions.
58–80 min	Guided lab: Sachet Clean-up Game	Page 5	Pupils follow the 6 steps. Check stacks against the Visual Target.
80–90 min	"Be the Computer", Spot the Bug, self-assessment	Page 6	Answer alone, then discuss. Sign off.

Key ideas and how to explain them

Loop (iteration): one person repeats the pounding until the fufu is smooth; you don't hire 50 people to strike once each. "Lazy programmers are smart programmers": one repeat (100) with move (1) steps inside saves 99 blocks.
Counted vs forever: repeat runs a set number of times and stops. forever keeps going until someone clicks the red Stop.
Condition: a true/false question. If true, the blocks inside the C-mouth run; if false, they are skipped. Diamond blocks fit only diamond holes.
Variable: a named calabash that holds a value that can change. set empties it to a starting value; change adds or takes away.
Bug and debugging: any mistake is a bug; debugging is calm detective work, one change at a time.

The 3D lab

Use Code the Sprite. On the projector, demonstrate:

"Load 'Square — the long way', then 'Square — with a loop'. Which is shorter?" Count the blocks with the class; the loop version does the same job with just 4 Scratch blocks, because repeat is one C-shaped block that holds the others.
"Load 'Buggy star' and run it in Debug mode, step by step." Press Debug, then Next step repeatedly, and ask pupils to watch the numbers. This is the "Trace it" step.

In pairs, pupils then:

"Fix the bug so the sprite draws a 5-pointed star." Let them try to spot it first. The fix is to change the turn from 72 to 144 degrees. Point out that they changed only one number: the debugging law in action.
"Change the clean-up route so the sprite collects ALL 7 pieces of litter." They read the x and y of each piece and can add go to x: y: blocks. This revisits Session 36 coordinates and links to the clean-up theme of the Page 5 game.

Common mistakes and quick fixes

Pupils often…	Try this
Put `set [Score] to (0)` inside the forever loop	Score never rises above 1. Move `set` above the loop, straight under the hat block.
Sachet flips upside down when it bounces	In the sprite pane, click Direction → Left/Right (step 1).
Put the `if` block outside the forever loop	It checks only once at the start. Drag it inside the C-mouth, below `if on edge, bounce`.
Can't fit `touching [mouse-pointer]?` into the `if`	Drag it by its left edge into the diamond hole until it highlights.
Score shoots up while the pointer rests on the sprite	The `go to [random position v]` block is missing. Add it inside the `if`, straight after `change [Score v] by (1)`, so the sprite jumps away and one touch scores one point.
Change lots of blocks when something breaks	Stop them. Spot it, trace it, change one thing, test.

When things go wrong

No internet: use Scratch Desktop. The calabash, fufu clapping and Page 6 need no connection; load the 3D lab in advance or use the offline pack.
Not enough PCs: pairs, with a driver and a navigator reading Page 5 aloud; swap roles after step 3.
A PC freezes or the power goes: remind pupils to save after step 3 and again at the end. Browser projects that were not saved are lost; rebuild quickly from the Visual Target on Page 5.
Mixed Scratch versions: colours may differ; look for palette names (Variables, Control, Sensing).

Answer key

1. "Be the Computer" Logic Tracing Challenge: Final Score value = 25 (10 + 5 + 5 + 5 = 25).

2. Spot the Bug Challenge: Kwesi forgot set [Score] to (0), placed under when green flag clicked at the top of his script, so the score resets every new game. Accept "set Score to 0" or "the reset block".

Support and stretch

Needs more support: build and test steps 1 to 3 before adding the if block.
Finished early: add a Timer variable that starts at 60 and goes down by 1 each second, or try change [Score v] by (10) for a bigger reward. Always test after each single change.

Evidence for sign-off

A saved Cleanup_Game_[Name].sb3 that resets Score to 0 on the Green Flag, moves and bounces continuously, and, when the pointer touches the sprite, adds 1 to Score, jumps to a random position and says "Cleaned!".
Correct answers to the two Page 6 challenges.
The pupil can explain repeat vs forever and name the 3 debugging steps.

Safety and wellbeing

No Scratch accounts or online sharing; save to the computer only. Bugs are normal: praise calm detective work and never allow mocking. Ask before deleting files on shared PCs. Keep names and speech bubbles kind and fictional.