Basic Programming Concepts (AQA GCSE Computer Science)

What is a programming concept?

• A programming concept, also known as a construct, determines the order in which lines of code are executed
• They control the logic and behaviour of code
• There are three core programming constructs:
  • Sequence
  • Selection
  • Iteration

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	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

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
• if... then... else... statements are used when you test conditions sequentially

Example

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

What is nested selection?

• Nested means to be 'stored inside the other'
• An example of nested selection is one IF statement inside of another
• It is worth noting that iteration can be nested as well and this can be seen in the iteration revision note

Examples of nested selection

// Prompt the user to enter a number
PRINT "Enter a number: "
test_score ← INPUT

// Outer statement to check the test_score is above 40 IF test_score > 40 THEN

// Inner statement to assign the result from the test         IF test_score > 70 THEN             result ← "Distinction"         ELSE IF test_score > 55 THEN             result ← "Merit"         ELSE IF test_score > 40 THEN             result ← "Pass"                      ENDIF

ELSE     result ← "Fail" ENDIF // Output the result OUTPUT result