Designing Programs

Approach Until Now Pick smallest meaningful step you can do Goto 1 Do it Test it Goto 1

New Approach Can I do it in ~10 lines? Yes: Do it No: Break the problem down into smaller tasks Define each in terms of input and output With each task, go to 1

Reasons For Functions Reuse code:

Reasons For Functions Create abstractions: Detailed Abstracted

When To Function Reusing same code in multiple locations Want to provide abstraction Existing code is too complex

How To Function A good function Does exactly one job

How To Function A good function Does exactly one job Has clear inputs/outputs

How To Function A good function Does exactly one job Has clear inputs/outputs Is the right length Probably Not Maybe Sure

Comparison Monolithic vs Modular…

Stepwise Refinement Take problem Break into subproblems With each subproblem: Can you implement it easily? Yes : Done No : Goto 1 with this problem

Building a Program Goal: Display month calendar given year, month

Break Down

Break Down

Break Down

Break Down

Break Down

Break Down

Break Down

Bottom Up Implement low level functions, test individually

Top Down Implement top level functions using stubs for lower level ones

Real World Development is cyclical Make plan Code to plan Learn stuff Goto 1