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Debugging conditional logic issues in Scratch projects

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LD

LogicDebugger_Emma

Posted on July 21, 2025 • Intermediate

🔍 Conditional statement not working consistently - debugging help needed!

Hi everyone! I’m working on a dress-up game project and running into a frustrating issue with conditional logic. I have an if statement that checks if a sprite’s y position equals -1, but it behaves inconsistently:

  • The condition (y position) = [-1] sometimes returns false even when the sprite is clearly at y = -1
  • It works fine after I modify the code (like pulling blocks out and putting them back)
  • The first execution often fails, but subsequent ones work correctly
  • This happens specifically in my “Swimming cap” sprite under a “When I receive clothes 2” event

Has anyone encountered similar issues with position detection or conditional logic in Scratch? What could be causing this inconsistent behavior? 🤔

SD

ScratchDebugger_Alex

Replied 27 minutes later • ⭐ Best Answer

Great question @LogicDebugger_Emma! This is a classic timing and precision issue in Scratch. Here’s a comprehensive debugging approach:

🔄 Debugging Flow for Conditional Logic

Here’s how to systematically debug conditional statement issues:

flowchart TD A[🚨 Condition Fails] --> B[🔍 Check Timing Issues] B --> C{When Does It Execute?} C -->|Immediately| D[⏱️ Add Wait Blocks] C -->|After Movement| E[🎯 Check Position Precision] C -->|On Event| F[📡 Verify Event Timing] D --> G[🔄 Test with Delays] E --> H[🔢 Use Rounded Values] F --> I[📋 Add Debug Logging] G --> J[✅ Condition Check] H --> J I --> J J --> K{Does It Work?} K -->|No| L[🛠️ Advanced Debugging] K -->|Yes| M[🎉 Problem Solved] L --> N[📊 Add Visual Indicators] L --> O[🔍 Check Variable States] L --> P[⚡ Optimize Code Structure] N --> Q[🧪 Test Systematically] O --> Q P --> Q Q --> R[📝 Document Solution] R --> M style A fill:#ffebee style J fill:#f3e5f5 style M fill:#e8f5e8

🎯 Step 1: Position Precision Issues

The most common cause is floating-point precision. Here’s how to fix it:

    // Instead of exact equality
if <(y position) = [-1]> then
// This might fail due to precision
end

// Use range checking instead
if <<(y position) > [-1.1]> and <(y position) < [-0.9]>> then
// Much more reliable!
end

// Or use rounding
if <(round (y position)) = [-1]> then
// Also very reliable
end

⏱️ Step 2: Timing and Execution Order

Add proper timing to ensure conditions are checked at the right moment:

    // Problem: Checking position immediately after movement
when I receive [clothes 2 v]
go to x: [0] y: [-1]
if <(y position) = [-1]> then  // Might execute before movement completes
// Your code here
end

// Solution: Add a small delay
when I receive [clothes 2 v]
go to x: [0] y: [-1]
wait [0.1] seconds  // Let movement complete
if <(round (y position)) = [-1]> then
// Now it works reliably!
end

🔍 Step 3: Debug Logging System

Create a debugging system to track what’s happening:

    // Debug logging custom block
define debug log (message) (value)
set [Debug Log v] to (join (join (message) [: ]) (value))
say (Debug Log) for [1] seconds
add (Debug Log) to [Debug History v]

// Use it in your conditional
when I receive [clothes 2 v]
go to x: [0] y: [-1]
wait [0.1] seconds
debug log [Y Position] (y position)
debug log [Rounded Y] (round (y position))
if <(round (y position)) = [-1]> then
debug log [Condition] [TRUE - Executing code]
// Your actual code here
else
debug log [Condition] [FALSE - Skipping code]
end

🛠️ Step 4: Robust Condition Checking

Build a more reliable condition checking system:

    // Create a custom condition checker
define check position (target x) (target y) (tolerance)
set [Position Match v] to [false]
if <<(abs ((x position) - (target x))) < (tolerance)> and <(abs ((y position) - (target y))) < (tolerance)>> then
set [Position Match v] to [true]
end

// Use the robust checker
when I receive [clothes 2 v]
go to x: [0] y: [-1]
wait [0.1] seconds
check position [0] [-1] [0.1]
if <(Position Match) = [true]> then
// Your code here - much more reliable!
end

🔄 Step 5: Event-Driven Architecture

Use a more reliable event system:

    // Instead of checking positions, use state management
when I receive [clothes 2 v]
set [Sprite State v] to [moving to position]
go to x: [0] y: [-1]
wait [0.1] seconds
set [Sprite State v] to [at target position]
broadcast [position reached v]

// Separate handler for when position is reached
when I receive [position reached v]
if <(Sprite State) = [at target position]> then
// Your code here - guaranteed to work!
end

🧪 Step 6: Testing Framework

Create systematic tests for your conditions:

    // Automated testing system
define run position tests
set [Test Results v] to []
set [Test Count v] to [0]

// Test 1: Direct positioning
go to x: [0] y: [-1]
change [Test Count v] by [1]
if <(round (y position)) = [-1]> then
add (join [Test ] (join (Test Count) [ PASSED])) to [Test Results v]
else
add (join [Test ] (join (Test Count) [ FAILED])) to [Test Results v]
end

// Test 2: After movement
glide [0.1] secs to x: [0] y: [-1]
change [Test Count v] by [1]
if <(round (y position)) = [-1]> then
add (join [Test ] (join (Test Count) [ PASSED])) to [Test Results v]
else
add (join [Test ] (join (Test Count) [ FAILED])) to [Test Results v]
end

// Display results
say (join [Tests completed: ] (length of [Test Results v])) for [2] seconds

Pro Tip: Always use rounded values or tolerance ranges when checking positions. Exact equality with floating-point numbers is unreliable! 🎯

LD

LogicDebugger_Emma

Replied 1 hour later

@ScratchDebugger_Alex This is incredibly helpful! 🙌

I tried the rounded value approach and it works perfectly now! The issue was indeed precision - my sprite was actually at y = -0.9999999 instead of exactly -1. The debug logging system you showed is brilliant too - I can see exactly what’s happening now.

Quick follow-up: Is there a performance difference between using round() vs the tolerance range method?

PT

PerformanceTester_Jordan

Replied 45 minutes later

@LogicDebugger_Emma Great question about performance! Here’s the breakdown:

    // Performance comparison
// Method 1: round() - Faster for simple checks
if <(round (y position)) = [-1]> then
// ~0.1ms execution time
end

// Method 2: Tolerance range - More flexible but slightly slower
if <<(y position) > [-1.1]> and <(y position) < [-0.9]>> then
// ~0.15ms execution time
end

// Method 3: Custom function - Most flexible, moderate speed
define position equals (value) (tolerance)
if <(abs ((y position) - (value))) < (tolerance)> then
// ~0.2ms execution time
end

For most projects, the difference is negligible. Use round() for simple integer checks, and tolerance ranges for more complex positioning! 🚀

BF

BugFixer_Sam

Replied 20 minutes later

Adding to this great discussion - here are some common debugging patterns I use:

    // Visual debugging - Show values on screen
define show debug info
go to x: [-200] y: [150]
say (join [Y: ] (y position)) for [0.1] seconds
go to x: [-200] y: [120]
say (join [Rounded: ] (round (y position))) for [0.1] seconds
go to x: [-200] y: [90]
say (join [Target: ] [-1]) for [0.1] seconds

// Condition history tracking
define track condition (condition name) (result)
add (join (join (condition name) [: ]) (result)) to [Condition History v]
if <(length of [Condition History v]) > [10]> then
delete [1] of [Condition History v]
end

These techniques have saved me countless hours of debugging! The visual feedback is especially helpful for position-related issues. 🔍

VB

Vibelf_Community

Pinned Message • Moderator

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