Profiling

Introduction

Learning Objectives

Know the meaning of testing, debugging, profiling
Understand their place in the scientific workflow

This lesson serves as a brief introduction to the following topics (each well worthy of a lesson of their own):

testing: the process of making sure that a program does what it is suppposed to do. Achieved by 1) writing code (a test) that runs a part of the main code and compares the result to the expected result and 2) automatizing the process of running those tests.
debugging: the process of finding the source of errors in the code, in particular with the help of a symbolic debugger.
profiling: the first step of optimizing the execution speed of a program by measuring its runtime. These measures can be taken on various levels, e.g.: the complete runtime of the full code or a part of it; the runtime for each function in the code; the runtime of each line of code in a function.

A simplified process of code development, involving all the three elements can look like the following:

Write code that solves a specific problem
Write tests for that code, showing that it actually solves the problem (taking special care of corner and edge cases)
When tests fail, debug the code to find the root cause.
If the code runs too slowly for the purpose at hand, profile it to find out where the time is spent and optimize the code (using the test suite to verify that everything is still working correctly)

Some developers recommend to switch step 1 and 2 around, i.e. to actually write the tests before the code (therefore starting with a test suite full of failing tests). This is called test-driven development and adopting it at least partially is certainly a good idea: whenever a bug occurs, writing first a test that reliably fails can dramatically help in the later stages of debugging it.