Programming Constructs (OCR GCSE Computer Science)

Revision Note

Test yourself Flashcards

Written by: Robert Hampton

Reviewed by: James Woodhouse

What is a Programming Construct?

A programming construct determines the order in which lines of code are executed
They control logic and behaviour of code
There are three core programming constructs:
- Sequence
- Selection
- Iteration

Sequence

What is sequence?

Sequence refers to lines of code which are run one line at a time
The lines of code are run in the order that they written from the first line of code to the last line of code
Sequence is crucial to the flow of a program, any instructions out of sequence can lead to unexpected behaviour or errors

Example

A simple program to ask a user to input two numbers, number two is subtracted from number one and the result is outputted

Line	OCR reference language/Python
01	`print("Enter the first number")`
02	`Num1 = input()`
03	`print("Enter the second number")`
04	`Num2 = input()`
05	`Result = Num1 - Num2`
06	`print(Result)`

A simple swap of line 01 and line 02 would lead to an unexpected behaviour, the user would be prompted to input information without knowing what they should enter

Selection

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... statements - this is when you test conditions sequentially
- case select or switch... statements - this is when you test an expression against multiple possible constant values (known as cases)

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

Example

Concept	OCR exam reference	Python
IF-THEN-ELSE	`if answer == "Yes" then` `print("Correct")` `elseif answer == "No" then` `print("Wrong")` `else` `print("Error")` `endif`	`if answer == "Yes":` `print("Correct")` `elif answer == "No":` `print("Wrong")` `else:` `print("Error")`
CASE SELECT or SWITCH	`switch day :` `case "Sat":` `print("Saturday")` `case "Sun":` `print("Sunday")` `default:` `print("Weekday")` `endswitch`	`match day:` `case "Sat":` `print("Saturday")` `case "Sun":` `print("Sunday")` `case _:` `print("Weekday")`

Concept

OCR exam reference

Python

IF-THEN-ELSE

if answer == "Yes" then

print("Correct")

elseif answer == "No" then

print("Wrong")

else

print("Error")

endif

if answer == "Yes":

print("Correct")

elif answer == "No":

print("Wrong")

else:

print("Error")

CASE SELECT or SWITCH

switch day :

case "Sat":

print("Saturday")

case "Sun":

print("Sunday")

default:

print("Weekday")

endswitch

match day:

case "Sat":

print("Saturday")

case "Sun":

print("Sunday")

case _:

print("Weekday")

If vs select case

Select case can mean less code but it only useful when comparing multiple values of the same variable
If statements can be more flexible and are generally used more in languages such as Python

Iteration

What is iteration?

Iteration is repeating a line or a block of code using a loop
Iteration can be:
- count controlled - this is when the code is repeated a fixed number of times (e.g. using a for loop)

xp66-srn-for-loop-pseudocode-computer-science-revision-notes

condition controlled - this is when the code is repeated until a condition is met (e.g. using a while loop or a do while loop)

while-loop-password-pseudocode-computer-science-revision-notes

Examples

Iteration	OCR exam reference	Python
FOR loop (Count controlled)	`for x = 0 to 9` `print("Hello")` `next x`	`for x in range(10):` `print("Hello")`
This will print the word "Hello" 10 times (0-9 inclusive)
`for x = 2 to 10 step 2` `print(x)` `next x`	`for x in range(2,12,2):` `print(x)` `# Python range function excludes end value`
This will print the even numbers from 2 to 10 inclusive
`for x = 10 to 0 step -1` `print(x)` `next x`	`for x in range(10,-1,-1):` `print(x)` `# Python range function excludes end value`
This will print the numbers from 10 to 0 inclusive
WHILE loop (Condition controlled)	`while colour != "Red"` `colour = input("New colour")` `endwhile`	`while colour != "Red":` `colour = input("New colour")`
	This will loop until the user inputs the colour "Red". Check condition is carried out before entering loop
DO WHILE loop (Condition controlled)	`do` `colour = input("New colour")` `until answer == "Red"`	`# Not used in Python`
	This will loop until the user inputs the colour "Red". Loop iterates once before a check is carried out

Iteration

OCR exam reference

Python

FOR loop

(Count controlled)

for x = 0 to 9

print("Hello")

next x

for x in range(10):

print("Hello")

This will print the word "Hello" 10 times (0-9 inclusive)

for x = 2 to 10 step 2

print(x)

next x

for x in range(2,12,2):

print(x)

# Python range function excludes end value

This will print the even numbers from 2 to 10 inclusive

for x = 10 to 0 step -1

print(x)

next x

for x in range(10,-1,-1):

print(x)

# Python range function excludes end value

This will print the numbers from 10 to 0 inclusive

WHILE loop

(Condition controlled)

while colour != "Red"

colour = input("New colour")

endwhile

while colour != "Red":

colour = input("New colour")

This will loop until the user inputs the colour "Red". Check condition is carried out before entering loop

DO WHILE loop

(Condition controlled)

do

colour = input("New colour")

until answer == "Red"

# Not used in Python

This will loop until the user inputs the colour "Red". Loop iterates once before a check is carried out

How to identify programming constructs

You can identify which programming constructs are used by looking at certain keywords
The keywords if, elseif, else, endif, switch, case indicate that the construct is selection
The keywords for, while, do indicate that the construct is iteration
If none of these keywords are used, this is a good indication that the construct is sequence

Worked Example

Tick (✓) one box in each row to identify whether each programming construct has or has not been used in the program [3]

total = 0

for i = 1 to 5

num = input("Enter a number")

total = total + num

next i

print(total)

A table with three rows labeled Sequence, Selection, and Iteration, and two columns, Has been used and Has not been used. The cells are empty.

How to answer this question

Always look for keywords first, if you find any then that construct must have been used
Sequence must always be used if the program works

Answer

A table with three rows labeled Sequence, Selection, and Iteration, showing their usage status. Sequence and Iteration have been used; Selection has not been used.

Last updated: 29 July 2024

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Programming Constructs (OCR GCSE Computer Science)

Revision Note

What is a Programming Construct?

Sequence

What is sequence?

Example

Selection

What is selection?

Example

If vs select case

Iteration

What is iteration?

Examples

How to identify programming constructs

Worked Example

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Author: Robert Hampton

Author: James Woodhouse