Debugging

Remember to

• download debugging-example-unfixed.R and debugging-example-fixed.R.
• send debugging-example-unfixed.R to students so they can follow along.
• set up a new Git project that includes debugging-example-unfixed.R.

Basic Manual Debugging Strategy

• How do we figure out what’s wrong with a program?
  • Be a scientist.
    • Hypothesize about what is wrong.
    • Make one change that is expected to fix error.
    • Check if change worked/fixed error.
  • Read the error message.
  • Talk through the code.
    • Observe what the code is doing.
    • Rubber duck programming
  • Do not change something without a reason.

Observe

Run debugging-example-unfixed.R.

• Read the error message and try to fix it.
• Where did code fail?
  • Traceback
  • options(error = tamper::tamper) helps with pipes (%>%)
• Read the code.
• Look at the current state of the environment.
  • This gives us a snapshot of what’s going on in the code.
  • Can’t see variables inside the function

Run line by line

• Run the code line by line checking each step.
• More difficult once functions are involved
  • Use print() to verify input and output.
  • Run function with simplified input.
  • Write test script that separates function from other code chunks.

print() statements

• Shows us things that we can’t see after error
  • Dynamic changes in code (e.g., for loop objects)
  • Things inside functions

print(head(df))

Fix error by passing data_size_class instead of data. Run debugging-example-unfixed.R.

• Something seems weird about these results.
  • Difficult to tell with manual strategy

Debugger

• Like a print() statement, but better
  • Contains a complete snapshot of the status of the program
  • Let’s you walk through the code dynamically
  • Doesn’t accidentally get left in the code

Break points

1. Insert breakpoint.
2. Source the code.
3. Program runs until breakpoint and stops (before executing the line of code).
4. Environment gives the current values for all variables.
   • Local and global
   • Can switch scopes with dropdown

Stepping

• Next
  • Runs next line of code
  • Does not enter functions
• Step Into
  • Goes line by line through the code including entering functions
• Step Out
  • Leaves the current function or loop
  • Returns to whatever called it
• Continue
  • Continues to the next breakpoint

Bugs to fix

• Passes data instead of data_size_class
• Not using threshold
• Not passing threshold