Step by step

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Step by step#

This time, the challenge is to déboguer (debug) an existing program,

that is, to identify, analyze, and correct an error hidden within it. This will be

an opportunity to learn how to observe the program’s step-by-step execution, in order to fully

understand how it works.

Note to teachers

Todo

Abnormal behaviors:

  • Breakpoint ignored in some cases

  • Code that does not update in the debugger

Fix: restart the kernel or stop / restart the debugger?

The page is long, hence a lot of back and forth. How to reduce this?

«Alternative via clic:» the learner may spend time testing when it is not necessarily relevant at this stage. And it consumes vertical space. Delete?

  1. Run the two cells below to display the maze and run the program.

from laby.global_fr import *
Laby(niveau="1b")

debut()
avance()
droite()
avance()
prend()
avance()
droite()
pose()
gauche()
avance()
avance()
gauche()
avance()
droite()
ouvre()

You will notice that in the end, the ant is still carrying its pebble and therefore cannot

get out.

  1. Use the labyrinth’s command buttons to follow the program’s execution step-by-step, forwards and backwards, etc., to fully understand what the ant is doing.

At this point, you have probably already identified the problem. However, it is not

obvious to link each of the ant’s actions to the corresponding program line.

To overcome this, we will use the debugger.

  1. Activate the debugger by clicking on the insect (bug in English) to the right of the control bar at the top of this page:

../_images/debug1.png

The insect turned red, line numbers appeared on the left in the code cells, and a palette opened to the right of this sheet.

The debugger allows you to set breakpoints in the code: the execution of the

program is paused each time it reaches a breakpoint.

  1. Add a breakpoint on the second line of the program by clicking to the left of the line avance():

../_images/debug2.png

Todo

Un utilisateur était bloqué en ne cliquant pas assez à gauche. Refaire la copie d’écran avec un curseur de souris?

  1. Run the cell containing the program.

You will notice that the ant is in the starting position and that the execution is in progress

(as indicated by the asterisk in brackets [*] to the left of the cell): the execution

is paused on the breakpoint line, just before the execution of that line.

  1. Press the F10 key (on some keyboards: fn+F10) to advance one step to the next instruction. Alternatively, you can click the arrow in the PILE DES APPELS area of the palette on the right.

../_images/debug4.png

  1. Continue pressing F10 and observe the program’s step-by-step execution. To restart the program from the beginning, execute the cell again.

  2. Analyze the error and correct it. Relaunch the step-by-step execution to verify.

  3. Click on the bug icon to deactivate the debugger.

Hint

If the debugger exhibits erratic behavior (breakpoint ignored, …): click twice

on the bug icon to disable the debugger, then resume from step 3.

Conclusion#

In this sheet, you have executed step-by-step a program to observe and

precisely understand its operation. This is an effective strategy to implement

whenever you have a doubt to predict the behavior of a program, especially

to debug errors.

You have also discovered the essential commands to use the debugger.

Available in Jupyter and any good development environment, it is a tool of

choice to help you implement this strategy.

As you tackle more complex programs, you will refine your

observation strategies and become familiar with the wealth of observation features

offered by the debugger, especially via the right panel in Jupyter.

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