How Decisions Affect Program Flow

Definition

The decisions made during design and development have a direct impact on program flow. Each decision shapes how the program operates, leading to different paths and outcomes.

Worked Example

Worked Example: Game Design Decisions

Scenario: designing a runner game.

Design choice	Option	Program-flow impact
Game type	Endless runner	Single continuous loop; no level-save transitions
Game type	Level-based	Level loading, progress tracking, save/load logic

pseudocode

procedure gameLoop():
    displayOptions(["Run", "Jump", "Slide"])
    choice = getUserInput()

    if choice == "Run" then
        moveCharacterForward()
    elseif choice == "Jump" then
        jumpOverObstacle()
    elseif choice == "Slide" then
        slideUnderBarrier()
    end if
end procedure

Each user choice sends control flow down a different branch.

Key Point

Planning point	Good practice
Unexpected user input	Include explicit else/validation branches
Multiple valid choices	Handle each branch deliberately
Error prevention	Provide clear fallback/error path

pseudocode

// Better planning for yes/no input
if userInput == "yes" then
    processOrder()
elseif userInput == "no" then
    cancelOrder()
else
    displayError("Please enter yes or no")
end if

Selection, Iteration and Nested Decisions

Where the decision is made	Effect on the flow of the program
if / elseif / else	One block of code is run and the others are skipped; execution then continues after the end of the if statement
switch / case	The value of one variable selects which block is run
while loop	The condition is tested before each pass; when it is false the loop is skipped or ends
do-until loop	The condition is tested after each pass, so the code inside always runs at least once
Nested if	The inner condition is only tested when the outer condition has already given a particular result

Worked Example

Tracing How Decisions Change the Flow

pseudocode

01 attempts = 0
02 password = input("Password")
03 while password != "open" AND attempts < 2:
04     attempts = attempts + 1
05     password = input("Password")
06 end while
07 if password == "open" then
08     print("Access granted")
09 else
10     print("Account locked")
11 end if

Passwords entered	Lines run, in order	Output
open	01, 02, 03, 07, 08	Access granted
cat, open	01, 02, 03, 04, 05, 03, 07, 08	Access granted
cat, dog, pig	01, 02, 03, 04, 05, 03, 04, 05, 03, 07, 09, 10	Account locked

The same code follows three different routes, because the conditions on lines 03 and 07 give different results for different inputs.

Exam Tip

Exam Tips for Thinking Logically:
• Identify every decision point in a given scenario: each place where the solution could do one of two things
• Write the condition tested at each one exactly, using comparison and Boolean operators
• Check the boundary values of each condition
• Follow each route through the code for different inputs to show how the decisions affect program flow

Selection, Iteration and Nested Decisions

Exam questions on Understanding How Decisions Affect Program Flow

Past paper questions