Setup

Send debugging-example-unfixed.R to students so they can follow along.

• Start with a new project including debugging-example-unfixed.R

Basic Strategy

• How do we figure out what’s wrong with a program
  • Be a scientist
    • Hypothesis about what is wrong
    • Make one change that is expected to fix it
    • Check if it worked
    • Do not change something without a reason
  • Read the error message
  • Talk through the code (rubber duck programming)
  • Observe what the code is doing

Observe

Run

• Read the error message and try to fix it
• Where did code fail? Show traceback
• 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 inside the function

Run line by line

• Run the code line by line checking each step
• Trick once functions are involved

Print statements

• Show us things that we can’t see after error
  • Dynamic changes in code
  • Things inside functions

print(head(df)), fix by passing data_size_class
Run
Something seems weird about these results

Debugger

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

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 vs. 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 return to whatever called it
• Continue: continues to the next breakpoint

Bugs

• passes data instead of data_size_class
• not using threshold
• not passing threshold