PYTHON | SELECTION

ON THIS PAGE
SECTION 1| IF Statements
SECTION 2 | IF Statement Structure
SECTION 3 | Using Variables in IF statements

ALSO IN THIS TOPIC
 GETTING STARTED 
 VARIABLES AND DATA TYPES
OPERATORS 
YOU ARE HERE | SELECTION 
ITERATION
ARRAYS
FUNCTIONS AND PROCEDURES  
FORMATTING DATA 
ERROR CHECKING
OBJECT ORIENTED PROGRAMMING 

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WHAT IS SELECTION

Selection is the process of making a selection between options. We naturally make selections everyday in our life's, and we make selections based on information / data available to us, for example: 
' If it is sunny outside then we select to wear sun glasses
Or else if it is raining then we select to take an umbrella '
In Python we can use the same selection logic with IF Statements IF, ELIF and ELSE. The command IF holds the first criteria, ELIF holds all further criteria and  ELSE is used when no previous criteria are met.

WATCH THIS VIDEO TO GUIDE YOU THROUGH THIS SECTION

SECTION 1 | IF STATEMENTS

This video walks you through the basics of if statements. There is a coding challenge at the end of the video.

The example below shows the IF, THEN, ELIF, ELSE being used in practice.  Note that the print statements are indent (moved in by one TAB space), this tell Python that these actions are inside the IF statement and should only be run if the above criteria is met.

The code below demonstrates how to produce a simple if statement, using weather as the theme.

Why the two equals == signs in the code above? Using two equals signs is a comparator. It is the same as asking 'does it equal?'. Whereas one equals = sign, means 'make it equal to'.

WHAT IS THE WEATHER ?

Expand on the IF statement example from above by:
1: Asking the user to input the weather
2: Adding more elif sections to cater for various user responses in your IF statement

SECTION 2 | IF STATEMENT STRUCTURE

It is very tempting to write an if statement using just if and not using elif and else. Remember your if statement must start with if, then any further criteria should be check with an elif (short for else if), and then finally if the data does not meet any of the criteria tested you can use else. Remember else cannot have a criteria.  It is crucial to recognise the pivotal role of 'if', 'elif', and 'else'. Your 'if' statement should always commence with 'if', followed by the condition to check. To introduce additional checks after your initial 'if', use 'elif' (a contraction of 'else if'), ensuring your code explores alternative conditions sequentially. Finally, cap your condition-checking with 'else', a safety net catching all remaining cases where previous conditions fail — note, 'else' stands alone without a specified condition.

Indentation is Python’s way of assigning blocks of code to execute when the associated condition is true.

The example in the statement below uses only the IF part of the statement, whilst this does work in some situations, it is not good practice and is not very efficient for the computer to process.

If you write the statement with just multiple IFs then the computer will have to read them all even if the criteria has already been met. As a general rule try not to use IF statements like they have been used in the example above. Instead use a IF,ELIF, ELSE statement this is much better because when the criteria is met the computer will finish reading the IF statement block and jump to the next part of the program.  

Also note that the ELSE in the statement above will only work with the last IF, the first IF in the example ends before the second IF starts.

In some cases you might have completely separate criteria that would require to code using just IF, without ELIF and ELSE. An example of this might be monitoring a key press on the keyboard.

SECTION 3 | USING VARIABLE IN IF STATEMENTS

Using variables in IF statements is just the same as using strings, except you do not put variables in the speech-marks. See the example below.

Note it is good practice to put the content on the right hand side of the IF statement in brackets and make sure you leave spaces so you can more clearly see/read your code.

DEBUGGING

The code below shows an example of an if statement using numbers as the comparison to output a student grade.

QUESTION
1: What is wrong with the code above? What output problems could it cause?

TASK
1: Fix the code above to work as expected using better IF statement structure.

SKI TEAMS

CREATE A PROGRAM THAT DOES THE FOLLOWING:
1: Ask the user for the time it took to complete the ski course
2: IF they complete the course in less than 2 minutes then they are in team A. Output ' Congratulations - You are in Team A' 3: If they complete the course in 2 minute or more and less than 3 minutes then they are on team B. Output ' Congratulations - You are in Team B'
​4: Otherwise they are on team C. Output ' Congratulations - You are in Team C'

SKI PASS

​TASK: A Ski company sells lift passes and wants you to create a program that does the following
1: Ask the user if they want a day pass or week pass
2: If the user wants a day pass then ask the user how old they are
    2b: If the user is less that 11 years old the the day pass is $40
    2c: If the user is greater than 64 years old then the day pass is $20
    2d: Otherwise the day pass is $60
3: If the user wants a week pass then ask the user how old they are
    3b: If the user is less that 11 years old the the week pass is $100
    3c: If the user is greater than 64 years old then the day pass is $100
    3d: Otherwise the day pass is $200
4: Output the cost of the correct pass in a user friendly statement

NAVIGATION

GETTING STARTED | Downloading Python and creating Hello World
VARIABLES AND DATA TYPES | Storing data in variables
OPERATORS | Performing calculations and comparisons
SELECTION | Using IF statements
ITERATION | Using FOR Loops and WHILE Loops
ARRAYS | Getting started with Lists
FUNCTIONS AND PROCEDURES | Breaking your code into logical sections
FORMATTING DATA | Further data manipulation
ERROR CHECKING | Using Try and Except to catch user errors
OBJECT ORIENTED PROGRAMMING | Getting to grips with OOP