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Selection, Boolean and Random

This content should also be familiar from the programming with Python topic but we need to see how this concept is described using pseudocode.

Selection

Recall that each line of code in a program will be executed one after the other in sequence unless we change the flow of control by introducing a construct such as selection or iteration.

With a selection statement the next statement to be executed will be dependent on some condition:

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// in pseudocode
age = 15
if age > 17 then
    print("You are old enough to drive")
else
    print("You are not old enough to drive")
endif

The condition, age > 17 is a boolean expression. It will evaluate to either True or False. In this example, age = 15, so the program will execute the print() statement that belongs to the else part of the selection block.

The if statement can provide a number of alternative branches:

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if age > 18 then
    print("You may be at university")
elif age > 16 then
    print("You may be doing your A Levels")
elif age > 14 then
    print("You will be doing your GCSEs")
elif age > 11 then
    print("You will be at secondary school")
else
    print("You will be at primary school")
endif

AQA pseudocode

Similar to previous example, do not forget the ENDIF:

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IF age > 18 THEN
    OUTPUT "You may be at university"
ELSE IF age > 16 THEN
    OUTPUT "You may be doing your A Levels"
ELSE IF age > 14 THEN
    OUTPUT "You will be doing your GCSEs"
ELSE IF age > 11 THEN
    OUTPUT "You will be at secondary school"
ELSE
    OUTPUT "You will be at primary school"
ENDIF

OCR Pseudocode

Similar to previous example, except uses elseif instead of endif. Again, do not forget the endif:

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if age > 18 then
    print("You may be at university")
elseif age > 16 then
    print("You may be doing your A Levels")
elseif age > 14 then
    print("You will be doing your GCSEs")
elseif age > 11 then
    print("You will be at secondary school")
else
    print("You will be at primary school")
endif

Nested if statements

Another if statement can appear inside another one:

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if age <= 16 then
    if day == 'Tuesday' then
        print("20% Discount is available")
    else
        print("10% Discount is available")
    endif
else
    print("Full price only")
endif

Switch/Case statement

Multiple if ... elif ... elif .. endif``` statements can sometimes be hard to follow and theswitch ... case`` statement block can make this easier to read and maintain:

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option = int(input("Enter an option: "))

switch option:
    case 1:
        print("You selected the first option")
    case 2:
        print("You selected the second option")
    case 3:
        print("You selected the third option")
    default:
        print("You made an invalid choice")
endswitch

Depending on the value entered by the user, the code will either take the first, second or third option. If none of these is recognised, it will default to the final alternative.

Note

AQA does not use switch/case in its pseudocode

Boolean Expressions

A number of operators are available for boolean expressions:

Operator Description Example Result
== Equal to 7 == 7 True
!= Not Equal to 7 != 6 True
> Greater than 7 > 6 True
< Less than 7 < 6 False
>= Greater than or equal to 7 >= 7 True
<= Less than or equal to 7 <= 7 True

Warning

Pay attention to the "is equal to" operator ( == ), it's two equal signs. It’s easy to mistake it for the assignment operator ( = ). It helps to read the assignment operator as e.g. "age is assigned the value 17" for age = 17; and "is age equal to 17?" for age == 17.

These are also known as the comparison operators

Complex Boolean Expressions

Boolean expressions can be joined together using AND, OR and NOT.

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if age > 16 AND age < 18 then
    print("You can claim a student discount")
endif

Random Number generation

Python has a built in library of functions for dealing with random numbers, and by using import random at the top of the program code makes the library available for use.

In the pseudocode, there is not need to add an import statement, it is assumed to be available.

AQA Pseudocode

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value = RANDOM_INT(first, second)  // returns a random integer between `first` and `second` inclusive

OCR Pseudocode

Random number generation is not included in the OCR pseudocode