Selection (Cambridge (CIE) O Level Computer Science)

Revision Note

Author

Robert Hampton

Expertise

Computer Science Content Creator

What is Selection?

Selection is when the flow of a program is changed, depending on a set of conditions
The outcome of this condition will then determine which lines or block of code is run next
Selection is used for validation, calculation and making sense of a user's choices
There are two ways to write selection statements:
- if... then... else...
- case...

If Statements

What is an If statement?

As If statements allow you to execute a set of instructions if a condition is true
They have the following syntax:

IF <condition>
THEN
<statement>
ENDIF
A detailed look at a Python IF statement:

if-else-if-else-statement-pseudocode-computer-science-revision-notes

Example

Concept	Pseudocode	Python
IF-THEN-ELSE	`IF Answer ← "Yes"` `THEN` `OUTPUT "Correct"` `ELSE` `OUTPUT "Wrong"` `ENDIF`	`if answer == "Yes":` `print("Correct")` `elif answer == "No":` `print("Wrong")` `else:` `print("Error")`

Concept

Pseudocode

Python

IF-THEN-ELSE

IF Answer ← "Yes"

THEN

OUTPUT "Correct"

ELSE

OUTPUT "Wrong"

ENDIF

if answer == "Yes":

print("Correct")

elif answer == "No":

print("Wrong")

else:

print("Error")

Nested Selection

What is nested selection?

Nested selection is a selection statement within a selection statement, e.g. an If inside of another If
Nested means to be 'stored inside the other'

Example

Pseudocode
`IF Player2Score > Player1Score` `THEN` `IF Player2Score > HighScore` `THEN` `OUTPUT Player2, " is champion and highest scorer"` `ELSE` `OUTPUT Player2, " is the new champion"` `ENDIF` `ELSE` `OUTPUT Player1, " is still the champion"` `IF Player1Score > HighScore` `THEN` `OUTPUT Player1, " is also the highest scorer"` `ENDIF` `ENDIF`

Pseudocode

IF Player2Score > Player1Score

THEN

IF Player2Score > HighScore

THEN

OUTPUT Player2, " is champion and highest scorer"

ELSE

OUTPUT Player2, " is the new champion"

ENDIF

ELSE

OUTPUT Player1, " is still the champion"

IF Player1Score > HighScore

THEN

OUTPUT Player1, " is also the highest scorer"

ENDIF

Python
// Prompt the user to enter a number `print("Enter a number: ")` `test_score = int(input())` // Outer statement to check the test_score is above 40 `if test_score > 40:` // Inner statement to assign the result from the test `if test_score > 70:` `result = "Distinction"` `elif test_score > 55:` `result = "Merit"` `elif test_score > 40:` `result = "Pass` `else:` `result = "Fail"` // Output the result `print(result)`

Python

// Prompt the user to enter a number
print("Enter a number: ")
test_score = int(input())

// Outer statement to check the test_score is above 40

if test_score > 40:

// Inner statement to assign the result from the test

if test_score > 70:

result = "Distinction"

elif test_score > 55:

result = "Merit"

elif test_score > 40:

result = "Pass

else:

result = "Fail"

// Output the result

print(result)

Case Statements

What is a case statement?

A case statement can mean less code but it only useful when comparing multiple values of the same variable
If statements are more flexible and are generally used more in languages such as Python
The format of a CASE statement is:
CASE OF <identifier>
<value 1> : <statement>
<value 2>: <statement>
....
OTHERWISE <statement>
ENDCASE

Concept	Pseudocode	Python
CASE	`INPUT Move` `CASE OF Move` `'W' : Posistion ← Posistion - 10` `'E' : Posistion ← Posistion + 10` `'A' : Posistion ← Posistion - 1` `'D' : Posistion ← Posistion + 10` `OTHERWISE OUTPUT "Beep"` `ENDCASE`	`match day:` `case "Sat":` `print("Saturday")` `case "Sun":` `print("Sunday")` `case _:` `print("Weekday")`

Concept

Pseudocode

Python

CASE

INPUT Move

CASE OF Move

'W' : Posistion ← Posistion - 10

'E' : Posistion ← Posistion + 10

'A' : Posistion ← Posistion - 1

'D' : Posistion ← Posistion + 10

OTHERWISE OUTPUT "Beep"

ENDCASE

match day:

case "Sat":

print("Saturday")

case "Sun":

print("Sunday")

case _:

print("Weekday")

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
- 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

Worked Example

Write an algorithm using pseudocode that:

Inputs 3 numbers
Outputs the largest of the three numbers

[3]

Exemplar answer

Psuedocode

INPUT A

INPUT B

INPUT C

IF A > B

THEN

IF A > C

THEN

OUTPUT A

ELSE

OUTPUT C

ENDIF

ELSE

OUTPUT B

ENDIF

Python

if A > B AND A > C:

print(A)
elif B > C:

print(B)
else:

print(C)

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Selection (Cambridge (CIE) O Level Computer Science)

Revision Note

What is Selection?

If Statements

What is an If statement?

Example

Nested Selection

What is nested selection?

Example

Case Statements

What is a case statement?

Exam Tip

Worked Example

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Author: Robert Hampton