Programming Constructs (OCR A Level Computer Science) : Revision Note

Programming Constructs

What is a Programming Construct?

• A programming construct determines the order in which lines of code are executed

• There are three programming constructs. These are

  • Sequence

  • Iteration

  • Branching (also known as selection)

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

Branching (Selection)

• Branching, also known as selection is when the flow of the program is interrupted and a condition is tested

• The outcome of this condition will then determine which lines or block of code is run next

• There are two ways to write selection statements:

  • if... elseif... else... statements - this is when you test conditions sequentially 

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

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)

  • 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)

Identifying 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

01 numbers = []

02 # Ask the user to enter 5 numbers

03 for i in range(5):

04     num = int(input("Enter a number: "))

05     numbers.append(num)

06 # Assume the first number is the largest

07 largest_number = numbers[0]

08 # Identifying the largest number

09 for num in numbers:

10     if num > largest_number:

11        largest_number = num

12 print("The largest number is:", largest_number)

• In this example, iteration is used twice:

  1. Repeating code to ask the user to enter 5 numbers (indicated by the keyword for on line 03)

  2. Repeating code to identify the largest number (indicated by the keyword for on line 09)

• In this example, selection is used once to determine if a number is bigger than the currently stored biggest number (indicated by the keyword if on line 10) and if so it will update the largest number on line 11

• Sequence is any lines of code which does not use iteration or selection. In this example, the first and last lines of code are sequence