Selection
What is selection?
Selection is a programming concept that allows you to execute different sets of instructions based on certain conditions. There are two main types of selection statements: IF statements and CASE statements.
Selection (CIE IGCSE Computer Science)
Revision Note
Author
Becci PetersExpertise
Computer Science
If Statements
IF statements allow you to execute a set of instructions if a condition is true. They have the following syntax:
IF condition
THEN
instructions
ENDIF
Pseudocode example:
x← 5
IF x > 0
THEN
PRINT "x is positive"
ENDIF
Python example:
x = 5
if x > 0:
print("x is positive")
Java example:
int x = 5;
if (x > 0) {
System.out.println("x is positive");
}
Visual Basic example:
Dim x As Integer = 5
If x > 0 Then
Console.WriteLine("x is positive")
End If
IF ELSE Statements
If else statements are used to execute one set of statements if a condition is true and a different set of statements if the condition is false. They have the following syntax:
IF condition
THEN
Instructions
ELSE
Instructions
ENDIF
Pseudocode example:
x ← 5
IF x > 0
THEN
PRINT "x is positive"
ELSE
PRINT “x is negative”
ENDIF
Python example:
x = 5
if x > 0:
print("x is positive")
else:
print("x is negative")
Java example:
int x = 5;
if (x > 0) {
System.out.println("x is positive");
} else {
System.out.println("x is negative");
}
Visual Basic example:
Dim x As Integer = 5
If x > 0 Then
Console.WriteLine("x is positive")
Else
Console.WriteLine("x is negative")
End If
IF ELSE IF Statements
If else if statements are used to test multiple conditions and execute different statements for each condition. They have the following syntax:
IF condition
THEN
Instructions
ELSE IF condition
THEN
Instructions
ELSE
Instructions
ENDIF
Pseudocode example:
x ← 5
IF x > 0
THEN
PRINT "x is positive"
ELSE IF x < 0
THEN
PRINT “x is negative”
ELSE
PRINT “x is 0”
ENDIF
Python example:
x = 5
if x > 0:
print("x is positive")
elif x < 0:
print("x is negative")
else:
print("x is 0")
Java example:
int x = 5;
if (x > 0) {
System.out.println("x is positive");
} else if (x < 0) {
System.out.println("x is negative");
} else {
System.out.println("x is 0");
}
Visual Basic example:
Dim x As Integer = 5
If x > 0 Then
Console.WriteLine("x is positive")
ElseIf x < 0 Then
Console.WriteLine("x is negative")
Else
Console.WriteLine("x is 0")
End If
Worked example
Write an algorithm using pseudocode that:
- Inputs 3 numbers
- Outputs the largest of the three numbers
[3]
Case Statements
CASE statements allow you to execute different sets of instructions based on the value of a variable. They have the following syntax:
CASE OF variable
value1: instructions
value2: instructions
...
OTHERWISE instructions
END CASE
Pseudocode example:
CASE OF number
1: PRINT "Monday"
2: PRINT "Tuesday"
3: PRINT “Wednesday”
4: PRINT “Thursday”
5: PRINT “Friday”
6: PRINT “Saturday”
7: PRINT “Sunday”
OTHERWISE PRINT "Invalid number"
END CASE
Python example:
match (number)
case 1:
print "Monday";
case 2:
print "Tuesday";
case 3:
print "Wednesday";
case 4:
print "Thursday";
case 5:
print "Friday";
case 6:
print "Saturday";
case 7:
print "Sunday";
case _:
print "Invalid number";
Java example:
switch (number) {
case 1:
return "Monday";
case 2:
return "Tuesday";
case 3:
return "Wednesday";
case 4:
return "Thursday";
case 5:
return "Friday";
case 6:
return "Saturday";
case 7:
return "Sunday";
default:
return "Invalid number";
}
Visual Basic example:
Select Case number
Case 1
Return "Monday"
Case 2
Return "Tuesday"
Case 3
Return "Wednesday"
Case 4
Return "Thursday"
Case 5
Return "Friday"
Case 6
Return "Saturday"
Case 7
Return "Sunday"
Case Else
Return "Invalid number"
End Select
Exam Tip
- Make sure to include all necessary components in the selection statement: the condition, the statement(s) to execute when the condition is true, and any optional statements to execute when the condition is false
- Use proper indentation to make the code easier to read and understand
- Be careful with the syntax of the programming language being used, as it may differ slightly between languages
- Make sure to test the selection statement with various input values to ensure that it works as expected
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