Conditionals: Study Guide (Computer Science Principles 3.6)

So far every program in this unit has run its statements in a fixed order from the first line to the last. Real programs must respond: charge a child fare to a child, refuse a wrong password, award a bonus only if the score is high enough. The tool for this is selection, written in code as a conditional statement (an “if-statement”). A conditional uses a Boolean expression, the topic you have just studied, to decide which statements run. This topic shows how IF and IF ... ELSE work, how to trace them, and how to write one from a description.

Notation. The exam uses its own pseudocode, not a real programming language. The code in this pack is Marlbridge’s rendering of that reference notation, not a copy of the official sheet: ← is assignment, = tests equality, lists start at index 1, DISPLAY shows a value, and braces mark blocks. Use the Exam Reference Sheet from your teacher for the official wording.

Selection and flow of control

The order in which statements run is the flow of control. By default it is sequential: top to bottom. Selection changes that: it determines which parts of an algorithm are executed based on a condition being true or false. Every algorithm can be built from just three ideas: sequencing, selection and iteration (Topic 3.8). Selection is the second.

An algorithm can be described without any programming language. A recipe that says “if the sauce is too thick, add water” is a selection written in plain English, and a flowchart with a diamond is the same idea drawn as a diagram. Programs are just these descriptions written in a language.

IF and IF … ELSE

IF alone. The block runs only when the condition is true; if it is false, nothing happens and the program continues after the block.

IF(condition)
{
  <block of statements>
}

IF with ELSE. Exactly one of the two blocks runs: the first if the condition is true, otherwise the second.

IF(condition)
{
  <first block of statements>
}
ELSE
{
  <second block of statements>
}

Notice how the block is chosen by the value of a Boolean expression at the moment the IF is reached. The expression can be a comparison (age < 13), a logical combination (score ≥ 500 AND lives > 0) or simply a Boolean variable (IF(isOpen)).

Worked example 1: tracing an IF/ELSE, and the trap of two IFs

A cinema charges 8 for under-13s and 12 otherwise. Trace the segment for age equal to 9, then for age equal to 30.

age ← 9
IF(age < 13)
{
  price ← 8
}
ELSE
{
  price ← 12
}
DISPLAY(price)

Output: 8

Age 9. 9 < 13 is true, so the first block runs and price becomes 8. The ELSE block is skipped completely. Output 8.

age ← 30
IF(age < 13)
{
  price ← 8
}
ELSE
{
  price ← 12
}
DISPLAY(price)

Output: 12

Age 30. 30 < 13 is false, so the ELSE block runs and price becomes 12. Output 12. Exactly one price is ever assigned.

The trap. Now compare two separate IF statements with an IF … ELSE, using x equal to 15:

Two separate conditionals

x ← 15
IF(x > 10)
{
  DISPLAY("A")
}
IF(x > 5)
{
  DISPLAY("B")
}
ELSE
{
  DISPLAY("C")
}

Output: A B

The first IF stands alone and is true, so A is displayed. The second IF is a new test: 15 > 5 is true, so B is displayed and the ELSE (C) is skipped. Both A and B appear because independent IF statements do not cancel one another. In contrast, if the second test were attached to the first as an ELSE branch, only one of A or B could be shown.

Check. Count the blocks that can run. After an IF ... ELSE exactly one block runs; after two separate IF statements zero, one or both can run.

Worked example 2: writing a conditional from a description

A game awards a bonus of 50 points if the player’s score is at least 500 and they still have lives left. Otherwise the bonus is 0. The program then displays the score plus the bonus.

Step 1. Say it in words (an algorithm without a language). Look at the score and the lives. If both tests pass, the bonus is 50. If not, the bonus is 0. Add the bonus to the score and display the total.

Step 2. Identify the condition. Two conditions joined by AND: score ≥ 500 AND lives > 0. Use ≥ because exactly 500 qualifies.

Step 3. Write the code.

score ← 600
lives ← 2
IF(score ≥ 500 AND lives > 0)
{
  bonus ← 50
}
ELSE
{
  bonus ← 0
}
DISPLAY(score + bonus)

Output: 650

Step 4. Test every branch. One test is never enough, because a conditional has more than one path:

score	lives	Condition	bonus	Displayed
600	2	true AND true = true	50	650
600	0	true AND false = false	0	600
400	2	false AND true = false	0	400
500	1	true AND true = true	50	550

The last row tests the boundary: a score of exactly 500 must earn the bonus. If you wrote score > 500 instead, this row would reveal the error.

Check. The four rows cover both outcomes of each part of the AND, plus the boundary.

IF without ELSE

An IF without an ELSE is useful when you only need to change something in a special case. The variable keeps its earlier value when the condition is false:

discount ← 0
IF(total ≥ 100)
{
  discount ← 10
}
DISPLAY(total - discount)

With total equal to 120 the block runs and the display is 110. With total equal to 80 the block is skipped, discount stays 0 and the display is 80. Setting a default value before the IF is a common, safe pattern.

Choosing the right structure

When you design a decision, ask two questions before you type anything.

1. Are the outcomes mutually exclusive? If exactly one thing should happen (child price or adult price), use IF … ELSE, because it guarantees one block runs. If several things may be true together (add a bonus and show a warning), use separate IF statements.
2. Is there a sensible default? If most cases do nothing special, set a default value first and use an IF without ELSE to override it in the unusual case.

A useful habit is to read the condition aloud as a sentence: “if the score is at least 500 and the player has lives left, then award the bonus”. If the sentence does not match the requirement, the condition is wrong. Finally, think about the inputs that sit exactly on the boundary of the condition. Programs fail at the edges far more often than in the middle, so those are the values to choose when you test.

This way of thinking is also what the Create performance task expects: your program must include a selection that uses a condition, and you must be able to say what the selection does and what happens when the condition is true and when it is false.

Common misconceptions

• “Both the IF block and the ELSE block run.” Exactly one runs in an IF … ELSE.
• “Two IF statements in a row are the same as IF … ELSE.” They are independent: both, one or neither block can run (see A B above).
• “If the condition is false the program stops.” It simply skips the block and carries on with the next statement.
• “One test proves the code works.” Each path needs a test, plus boundary values.
• “The condition has to be a comparison.” A Boolean variable or any Boolean expression is fine.
• “IF changes the order of the lines in the file.” The lines stay where they are; the flow of control skips some of them.

Where this leads

Next, Topic 3.7 puts conditionals inside conditionals so a program can ask a second question after the first. Conditions also drive loops in Topic 3.8, and equivalent conditionals and Boolean expressions return in Topic 3.9.

Next steps: try the practice questions, skim the revision notes the night before, and tick off the topic checklist. The next topic is Topic 3.7, Nested Conditionals.