GCSEComputer ScienceOCRRevision Notes8. Programming FundamentalsProgramming Fundamentals & Data TypesProgramming Constructs

Programming Constructs (OCR GCSE Computer Science)

Revision Note

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Robert Hampton

Expertise

Computer Science Content Creator

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")

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

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)

  Has been used Has not been used
Sequence    
Selection    
Iteration    

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

  Has been used Has not been used
Sequence    
Selection    
Iteration    

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    Robert Hampton

    Author: Robert Hampton

    Rob has over 16 years' experience teaching Computer Science and ICT at KS3 & GCSE levels. Rob has demonstrated strong leadership as Head of Department since 2012 and previously supported teacher development as a Specialist Leader of Education, empowering departments to excel in Computer Science. Beyond his tech expertise, Robert embraces the virtual world as an avid gamer, conquering digital battlefields when he's not coding.