1. Selection Statements

2. Boolean Expressions
Are statements that evaluate to either a true or false value;
Are useful in computing and programming because computers think in terms of 1’s and 0’s, which can represent true and false.

3. Boolean Expressions
Can be a simple expression if it is a single statement involving a relation, equality, or not statement;
var isTeacherAGamer : boolean := true
var isWeatherNice : boolean := isSkyBlue
var isLazy : boolean := not(isHardWorker)
Can be a compound expression if there are several simple expressions connected using the and and or operators
willBallBounce := isBallRubber or isGroundRubber

4. Our Programs So Far...
procedure GetSomeData() var input : string put “Please enter the data” get input put “You inputted the data: “ + input end GetSomeData
Variable value and output
change with the user input

5. Our Programs So Far... Our programs so far cannot make decisions
E.g., the user can’t decide to just exit instead of printing their input
Our programs follow the typical Top-to-bottom flow, we need to give the programmer control

6. Selection Statements
We usually want our programs to run different lines of code depending on different variable values. This is where we use selection statements.
Selection statements are used to control the flow of the program, so are also called control structures.

7. Comparison Operators
Before we can use this control structure we need to understand how they give us control. They do so by comparing values. The following is a list of all comparisons in Turing
Operator
Meaning
Example
Evaluates to =
Equal to
teacherName = “Mr.Jackson”
False
> 
Greater than
6 > 3
True
< 
Less than
5 < 11
>=
Greater than or equal to
23 >= 23
<=
Less than or equal to
4 <= 21
not=
NOT equal to
3 not= 3

8. If-statements (the detour)
Syntax
if (<test-expression>) then
%Insert code between the if and end if lines
%This code is executed if and only if the Boolean %expression in the brackets evaluates to true.
end if

9. What is a test-expression
A test-expression is any expression that evaluates to true or false, usually a comparison of values
E.g.
%In this example, if the user entered a value of 15 or less the put
%command would be executed, otherwise it would be skipped completely
if (userAge < 16) then
put “Sorry, no license for you”
end if

10. Examples
var understood : boolean put “Is the topic understood?” get understood if ( understood = false) then %User did not understand put "We need more time" end if

11. if-else statements (The fork in the road)
Syntax
if (<test-expression>) then
% Code between the if and else lines that is to be executed if the Boolean % expression in the brackets is true
else
% Code inside the else and end if lines that is to be executed if the Boolean % expression in the brackets is false
end if
Only ONE of the branches (code blocks) will run depending on whether the test-expression evaluates to true or false.