Debugging code

Learn

What you’ll learn

• The three kinds of bug
• A step-by-step debugging method
• Tools: error messages, prints, and testing in isolation

Three kinds of bug

Kind	What happens	Example
Syntax	The code won’t compile or run	Missing ; or }
Runtime	It runs, then crashes or freezes	A loop that never ends
Logic	It runs but does the wrong thing	< instead of >

The method

1. Reproduce it. Make the bug happen on purpose, reliably.
2. Read the error message, including the line number (and the line before it).
3. Print what’s really happening: sensor values, which if branch ran, variable values.
4. Isolate: test the smallest piece alone (just the sensor, just one motor).
5. Change one thing, test, and repeat.
6. Explain it out loud (“rubber duck debugging”): describing the code line by line often reveals the bug.

Common robot bugs

• = (assign) instead of == (compare) in an if.
• Wrong pin number in code versus wiring.
• Sensor logic upside down (HIGH vs LOW).
• Motors wired backwards, so “left” turns right.
• int used for millis().
• A delay() so long the robot can’t react.

Examples

Example: printing which branch ran

function loop() {
  const d = distance();
  if (d < 25) {
    print("TURN, d=" + d);
    motors(-40, 40);
  } else {
    motors(60, 60);
  }
}

If “TURN” never prints even when the robot hits a wall, the condition or the sensor reading is wrong.

Example: a logic bug

if (distance > 20) {   // bug: this stops when the path is CLEAR
  stopMotors();
}

Printing distance and “stopping” shows it stops at 80 cm and drives at 10 cm. The fix: <.

Practice

Try it: fix the broken robot

This code is meant to drive forward and turn away from obstacles, but it has three bugs. Use print() to find them.

Debug me

▶ Run↺ Reset

Your code (JavaScript)function loop() { let d = distance(); if (d > 25) { motors(-40, 40); // turn away } else { motors(60, 60); // drive forward } if (d = 5) print("very close!"); }Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

Find bug 1Easy

The robot spins in the open and drives into walls. Why? The condition is backwards: it should be if (d < 25).

Find bug 2Medium

“very close!” prints all the time. Why? d = 5 assigns 5 to d (which counts as true) instead of comparing. Use d < 5 (or === for an exact match).

Find bug 3Hard

After fixing both, the robot sometimes scrapes past box corners. What’s the cause, and how would you find it? Print distanceLeft() and distanceRight() while it drives past a box. Only the front sensor is checked, and corners slip past it. Also turn away when a side distance is very small, for example if (d < 25 || distanceLeft() < 8 || distanceRight() < 8).

Quick check

1. The code runs but the robot turns the wrong way. That’s a…

Syntax bugRuntime bugLogic bugHardware fault only

2. What’s a good first step when debugging?

Rewrite everythingReproduce the bug reliablyBuy new partsAdd more features

3. What’s wrong with if (x = 5)?

NothingIt assigns instead of comparesMissing semicolonx must be float

Check my answers