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Principles of Programming: Understanding Calculations and Variable Management in Python

In this lecture, students will explore fundamental principles of programming using Python. Topics covered include performing calculations with arithmetic operators, importing Python modules, and calling functions from these modules. Students will learn how to use variables to store values and identify the differences between integer and float data types. The importance of documentation through docstrings and comments in code will also be emphasized. By the end of the lecture, learners will be able to implement basic calculations, manage variables, and output results effectively.

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Principles of Programming: Understanding Calculations and Variable Management in Python

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  1. COMPSCI 101Principles of Programming Lecture 02 -- Calculations

  2. Learning outcomes • At the end of this lecture, students should be able to: • perform calculations using standard arithmetic operators • import standard Python modules • call functions from standard Python modules • use variables to store values • describe differences between int and float types • print numbers to standard output • use a docstring • use comments COMPSCI 101 - Principles of Programming

  3. Recap • Programs are a sequence of instructions • Instructions are specified using a formal language • Computer executes the instructions one after the other • Programs are deterministic • The result of the instructions is well defined • Rules govern the results of instructions • Once we learn the rules, we can model what the computer does • Then the output is completely predictable • Experimental • We don’t always know what the rules are • We need to experiment to find out COMPSCI 101 - Principles of Programming

  4. Sample problems • Convert a length from inches to centimetres • Convert a weight from kilograms to pounds • Calculate the area of a circle COMPSCI 101 - Principles of Programming

  5. Example • Convert from inches to centimetres • What value are we converting (input)? • What is the formula to convert inches to centimetres? • How do we show the result (output)? • Answers • Convert the value 100 from inches to centimetres • 1 inch is 2.54 centimetres • Print out the result COMPSCI 101 - Principles of Programming

  6. Storing information using variables • Variables are names for storage locations • Almost any name will work – there are some constraints though • A variable stores only 1 value at a time • Store a value to a variable location using = • Get a value from a variable location using the variable name name = 'Andrew' height = 167.5 age = 42 >>> name 'Andrew' >>> age 42 COMPSCI 101 - Principles of Programming

  7. Question • Use the following list of valid and invalid variable names to figure out the rules behind naming variables. • Valid • x • age • age_of_child • box1 • box_2 • _age • age_ • Invalid • 1 • age of child • age-child • 1st • 2_box COMPSCI 101 - Principles of Programming

  8. integers and floating point numbers • Information in a program is categorised into different types • integer • floating point • string, boolean, and many more • Integer values are numbers without decimal points. • Floating point numbers are numbers with decimal points • Limited precision 356 0 1.5 3.333333333333 COMPSCI 101 - Principles of Programming

  9. Arithmetic • Mathematical operators • Addition + • Subtraction - • Multiplication * • Division / • Exponentiation ** >>> 34 + 2 36 >>> 12 / 3 4.0 COMPSCI 101 - Principles of Programming

  10. Expression • An expression is part of the program that can be evaluated (i.e. when the computer follows the rules that define the instructions, an expression turns into a value). • An expression can be used anywhere that a value is used x = 3 + 4 3 + 4 is an expression COMPSCI 101 - Principles of Programming

  11. Printing to standard output • When text is printed, the text goes to “standard output” • Under normal circumstances standard output is the screen • Any type of data can be printed print(125) print(3.5) print('Hello') 125 3.5 Hello COMPSCI 101 - Principles of Programming

  12. Example • Convert a length from inches to centimetres length_in_inches = 100 length_in_cm = length_in_inches * 2.54 print(length_in_cm) COMPSCI 101 - Principles of Programming

  13. Tracing code • Keep track of the contents of variables • Write down the name of each variable • Change the value when (and only when) an assignment occurs • When you change a value, cross out the old one and write a new one length_in_inches: 100 length_in_cms: 254.0 COMPSCI 101 - Principles of Programming

  14. Documentation • docstring • A special kind of string (text) used to provide documentation • Appears at the top of a module • Appears at the top of a function (later lecture) • Uses three double-quotes to surround the documentation • All modules should include a docstring """Converts a length in inches to a length in centimetres. Author: Andrew Luxton-Reilly """ length_in_inches= 100 length_in_cm = length_in_inches * 2.54 print(length_in_cm) COMPSCI 101 - Principles of Programming

  15. Comments • Comment • A programming comment is a note to other programmers • Anything between a # and the end of the line is ignored by the computer • Add comments sparingly to explain code that is difficult, or tell other programmers something they need to know about the code. """Converts a length in inches to a length in centimetres. Author: Andrew Luxton-Reilly """ length_in_inches= 100 #Alter this value to convert a different length length_in_cm = length_in_inches * 2.54 print(length_in_cm) COMPSCI 101 - Principles of Programming

  16. Exercise • Convert a weight from kilograms to pounds • Starting information: number of kilograms • Formula: 1 kilogram = 2.20462 pounds • Print the resulting number of pounds docstring initialisation calculation output COMPSCI 101 - Principles of Programming

  17. Example • Calculate the area of a circle • Formula: area = π r2 COMPSCI 101 - Principles of Programming

  18. Importing functions • Code is stored in modules • We want to reuse code as much as possible • Build up libraries of code • Importing a module allows you to access code in that module • Python.org has a list of all the modules and functions provided >>> import math >>> math.pi 3.141592653589793 >>> math.sqrt(4) 2.0 COMPSCI 101 - Principles of Programming

  19. Example • Calculate the area of a circle """Calculates the area of a circle. Author: Andrew Luxton-Reilly """ import math radius = 10 area = math.pi * radius ** 2 print(area) COMPSCI 101 - Principles of Programming

  20. Summary • Instructions are executed in sequence • Variables hold one piece of information at a time • There are different types of information • The variable name is used to identify that variable • Assignment statements are used to store information in a variable • Expressions are things that can be evaluated and may be used anywhere that a value is expected • Variable names • Calculations involving variables and literal values • Every module should start with a docstring COMPSCI 101 - Principles of Programming

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