COMPSCI 101 Principles of Programming

1. COMPSCI 101Principles of Programming Lecture 04 – Creating Functions

2. Learning outcomes • At the end of this lecture, students should be able to: • Define a function that accepts arguments and returns values • Provide appropriate documentation for a function • Know how to trace code that involves functions COMPSCI 101 - Principles of Programming

3. Recap • We use the statement import <module> to gain access to the functions defined within a module • When we use a function defined in a module, we use the syntax: • <module name>.<function name> • The help() function provides the docstring for a module or function COMPSCI 101 - Principles of Programming

4. Why do we use functions? • To reuse our code as much as possible • Take the code that solves a specific problem • Generalise the solution so it can be used for all similar problems • Calculate the area of a rectangle with width of 5 and height of 10 • Better if we could do: """Calculates the area of a rectangle with width of 5 and height of 10 """ width = 5 height = 10 area = width * height print(area) >>> rectangle_area(5, 10) 50 COMPSCI 101 - Principles of Programming

5. Defining a function • Function to calculate the area of a rectangle: function name function arguments “def” defrectangle_area(width, height): area = width * height return area colon at end “body” of the functionis indented return value COMPSCI 101 - Principles of Programming

6. Example • Write a function that calculates the area of a square • Arguments (input) • Return value (output) • Docstring (explain what the function does) defsquare_area(side): """Returns the area of a square Arguments: side – length of the side (float or int) Returns: The area of the square """ return side ** 2 COMPSCI 101 - Principles of Programming

7. Lack of adequate documentation? • A NASA subcontractor developed part of the software system used to control the thrusters of the Mars Climate Orbiter. • They calculated the force in Imperial units (pound-seconds), and returned a floating point value. • Other functions expected the value returned to be calculated in metric value (Newton-seconds) • The thrusters fired at the wrong time, causing it to crash on its arrival at Mars in 1999. • Mission cost around $600 Million COMPSCI 101 - Principles of Programming

8. Exercise • Write a function that converts a distance from miles to kilometres. Assume that 1 mile is equal to 1.60934 kilometres. COMPSCI 101 - Principles of Programming

9. Example • Convert a weight from kilograms to pounds • Formula: 1 kilogram = 2.20462 pounds • Arguments: • kilograms (float) • Returns: • pounds (float) COMPSCI 101 - Principles of Programming

10. Example: Convert weight from kg to lbs defpounds_from_kilo(kilograms): """Converts a weight from kilograms to pounds Arguments: kilograms - weight in kg (float) Returns: Weight in pounds (float) Note: conversion rate is 1 kg = 2.20462 lbs """ return kilograms * 2.20462 COMPSCI 101 - Principles of Programming

11. Precision of the value displayed • Return value should be as precise as possible • Reduce precision at the last step if needed • For example, print output to 2 decimal places • Round the value after it has been returned >>> round(pounds_from_kilo(1), 2) 2.2 COMPSCI 101 - Principles of Programming

12. Software rounding • During the 1991 Gulf War, the U.S. deployed its Patriot missile system to protect its troops, allies, and civilians from Iraqi SCUD missile attacks. • A software rounding error in the system calculated time incorrectly, causing it to ignore some incoming missiles.  • A missile battery in Saudi Arabia failed to intercept an incoming SCUD that destroyed a U.S. Army barracks, killed 28 soldiers, and injured 100 others. COMPSCI 101 - Principles of Programming

13. Example • Write a function that takes a base cost for an item (in dollars, excluding GST) and returns the cost for an item including GST (in dollars, to the nearest cent). • Examples of output: >>> add_gst(6) 6.9 >>> add_gst(0.06) 0.07 >>> add_gst(7) 8.05 >>> add_gst(10) 11.5 >>> add_gst(100) 115.0 >>> add_gst(10.01) 11.51 COMPSCI 101 - Principles of Programming

14. Example: Adding GST defadd_gst(before_tax): """Returns the cost with GST added Arguments: before_tax - the cost before tax (i.e. excluding GST) Returns: The cost of the item including GST. The value is rounded to the nearest cent. """ after_tax = before_tax * 1.15 #GST is 15% return round(after_tax, 2) COMPSCI 101 - Principles of Programming

15. Tracing code within a function • Variable scope • Variables used as arguments only exist within the function • Variables created within a function only exist within the function • Tracing code • As you work through the code, when you encounter a function call, draw a box for that function and name it. • Any variables/arguments created within the function are written inside the box. When you are working through code inside a function, you can't access any variables inside another box. before_tax: 100 after_tax: 115.0 COMPSCI 101 - Principles of Programming

16. Summary Functions are defined using the key worddef • Accept arguments (parameters) • Return values • Functions are a way to generalise tasks • Defining a function extends the tasks that are available • Design functions to be reused as much as possible • Provide documentation so other programmers know how to use the function • Code tracing with functions • A box is drawn to represent the function • Variables drawn inside the box to indicate they are not visible outside the function COMPSCI 101 - Principles of Programming