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EECS 280 - Class 2

Procedural Abstraction

separates what a procedure does from how it is implemented

  • In cpp, we use functions to implement PA

  • Can change implementation without affecting users as long as interface remains unchanged

.h header defines what, not how, functions do

.cpp defines how functions work • functions can be used in another file

Properties of PA

Code that calls the function depends on the interface, not implementation

As long as abstraction of a function does not change, usage of the function still works

Local

The implementation of an abstraction can be understood without examining any other abstraction implementation

Substitutable

You can replace one (correct) implementation of an abstraction with another (correct) one, and no callers of that abstraction will need to be modified

Specification Comment

Signature describes inputs and outputs of function

Comments describe what function does

REQUIRES - Pre-conditions must hold

//REQUIRES: v is not empty
//EFFECTS: returns the sum of the numbers in v
double sum( std::vector<double> v );

  • functions with REQUIRES clauses are called partial

  • functions without REQUIRES clause are called complete

MODIFIES - How inputs / global vars are modified

//MODIFIES: v
//EFFECTS: sorts v
void sort( std:: vector<double> &v );
  • A function that modifies inputs or global data has side effect

EFFECTS - What procedure does

assert()

  • Usage: assert( expression );

  • Does nothing if the expression is true

  • Exits and prints error message if expression is false

#include <cassert>
int main( )
	{
	assert ( true ); // does nothing
	assert ( false ); // crash with debug message
	}

Disabling asserts

#define NDEBUG

Testing

Goals of testing

  1. Ensure that code works as expected
  2. Meet the reqs of the spec

Unit test

Test one piece, e.g., one function, at a time. Find and fix bugs early with smaller, less complex, easier to understand tests

Write small helper functions for tests

//stats_test.cpp
void test_mean( )
	{
	vector<double> data = { 1, 2, 3 };
	double result = mean( data );
	assert( result == 2 );
	}

Small Scope Hypothesis

Thorough testing with "small" test cases is sufficient to find a large proportion of bugs within a system

Can fail if...

  1. Code coverage is incomplete
  2. Cannot detect bugs that occur as the result of interactions adn race conditions that it doesn't (or cannot) test

System test

Test the entire system after all the individually-tested pieces have been integrated

Regression test

Automatically run all unit and system tests after a code change

make test for running regression tests

Test-driven development

  1. Code tests
  2. Code implementation with the goal of passing the tests
  3. Debug and add more tests