What Does This Code Do?

Agenda

• Why read code?
• How to read code
• What does this code do?

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1.

Why Read Code?

I just want to write all the code

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Why Read Code?

• The most common activity for professional programmers is reading code
• A typical programmer writes (on average) 25-50 lines of code every day
• Fixing bugs require reading code
• With small rewrites
• Adding features and functionality requires integration with existing codebases
• Very difficult not to break existing code

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• What about this course?
• Next slide ➡️

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Why Read Code?

• In this course, there are two primary reasons for reading code:
• There is a bug in your code and you need to figure out how to fix it
• You are trying to integrate into a library and need to understand how other programmers have used the library

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2.

How to Read Code

“I’d never write it this way!”

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Fixing Bugs

• When code doesn’t do what you expect (intent vs implementation)
• Use different options:
• Try to understand what’s going on with different inputs
• Print variables
• Run the code “by hand”

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Different Inputs

• Least effective method of “reading code”
• Sometimes the only option
• If no access to original source code or documentation
• Run with many variations of inputs
• Use scientific method
• Vary one element at a time
• Record all results

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Print Variables

• Most common method
• There is a trade-off between time and understanding
• Print lots of variables
• Run the program in different ways
• When variables are different from expectation there is a possible bug (intent vs implementation)

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Run code by hand

• Time consuming but very effective
• Start at beginning of program
• Write down each variable as you come to it in the code
• Make a column for each variable
• Any time the variable changes, note the new value in the appropriate column
• Note anything that is printed to the screen
• Anytime there is a disconnect between intent and implementation, there may be a bug (in thinking or implementation, or both!)

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• The faster version of this is using the debugger that comes with the development environment
• Repl doesn’t really have a debugger 😕

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3.

What Does This Code Do?

Excellent question!

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What Does This Code Do? Part 1

• Walk through the “code-by-hand” process for the following code - not through repl!

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• Check your results in repl and with the rest of the class
• How would you use extra printing to help?

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What Does This Code Do? Part 1

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What Does This Code Do? Part 2

• Walk through the code by hand process for the following code - not through repl!

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• When completed, each student should have:
• Same number of variables
• Same values in each column
• Same printed output
• Check your results in repl
• How would you use extra printing to help?

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Credits

Special thanks to all the people who made and released these awesome resources for free:

• Presentation template by SlidesCarnival

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