Programming constructs: sequence, iteration, branching

Programming constructs: sequence, iteration, branching.

2.2.1 Programming Techniques

Programming Techniques — Note 1 of 6

Candidates need to understand the constructs of sequence, iteration and branching, and use them independently and in combination to produce a solution. This includes IF (with ELSEIF and ELSE) and select case statements, and both condition-controlled (e.g. while, repeat until) and count-controlled (e.g. FOR) iteration. Candidates need to be able to read, create and trace code (for example using a trace table) that uses these constructs.

What you need to know

All programs are built from three constructs: sequence, iteration and branching (selection). You need to use them independently and in combination, and read, create and trace code that uses them.

What to know

A condition-controlled loop can be used as a count-controlled loop by updating a counter inside it.
Constructs can be nested, e.g. a selection statement inside a loop.

Key terms

Exam tipCommon mistake

Sequence: Instructions executed one after another in order.
Branchingselection: Choosing which code to run based on a condition, using if ... then ... elseif ... else ... endif or switch/case.
Iteration: Repeating code. Count-controlled loops (for) repeat a set number of times; condition-controlled loops (while, do ... until) repeat until a condition changes.
A while loop checks its condition BEFORE each iteration (it may run zero times); a do ... until loop checks AFTER (it always runs at least once).
Common mistake: Initialising a counter or total inside a loop, so it is reset every iteration.

Exam technique

Tip:Use a trace table to follow code that combines these constructs.

Code examples

Sequence, selection and iteration in OCR pseudocode

text

total = 0
for i = 1 to 5
    mark = int(input("Enter mark: "))
    if mark >= 0 AND mark <= 100 then
        total = total + mark
    else
        print("Invalid mark")
    endif
next i

do
    answer = input("Continue? (y/n) ")
until answer == "y" OR answer == "n"

switch answer:
    case "y":
        print("Continuing")
    default:
        print("Goodbye")
endswitch