Selection: if/else Statements

Selection: if/else Statements

What is selection?

In everyday life, we constantly make decisions based on conditions: "If it's raining, take an umbrella — otherwise, leave it." Programming has the same idea. Selection allows a program to run different code depending on whether a condition is True or False. Without selection, a program would do exactly the same thing every single time, no matter what data was entered. Selection makes programs intelligent and responsive.

In N5 Computing Science, selection is implemented using if statements. In this lesson, you write selection in Python for code questions and read and trace the SQA reference language shown in question papers.

Comparison operators

A condition uses comparison operators to compare two values and produces a boolean result — either True or False. You must know all six operators:

The single equals sign = is the assignment operator — it stores a value. The double equals == is the comparison operator — it tests equality. Confusing them is one of the most common errors in all programming.

The simple if statement

The simplest form of selection only runs code when a condition is True. If the condition is False, nothing happens and the program moves on.

if condition:
    # code that runs only when condition is True

Two rules Python enforces strictly: the condition line must end with a colon (:), and the code inside the block must be indented (4 spaces or 1 Tab). Python uses indentation to know which lines belong inside the if — unlike many other languages that use curly braces.

The if/else statement

Often you want one thing to happen when a condition is True and something different when it is False. Add an else clause:

if condition:
    # runs when condition is True
else:
    # runs when condition is False

Exactly one of the two blocks always runs — you are guaranteed to get one outcome or the other. The else line also ends with a colon and its code is also indented.

The if/elif/else chain

When there are more than two possible outcomes, use elif (short for "else if") to test additional conditions:

if condition1:
    # runs when condition1 is True
elif condition2:
    # runs when condition1 is False AND condition2 is True
elif condition3:
    # runs when condition1 and condition2 are False AND condition3 is True
else:
    # runs when all conditions above are False

Python works through each condition in order from top to bottom. The moment one condition is True, that block runs and all remaining conditions are skipped entirely. You can have as many elif branches as you need. The else at the end is optional, but it is good practice to include it as a safety net for unexpected values.

Reading SQA reference language alongside Python

These examples implement the same selection logic. Read the SQA reference language on the left; write Python like the code on the right. For question-paper code answers, you may use any programming language; marks reward understanding rather than correct syntax (course specification, printed page 13).

IF score >= 50 THEN
  SEND "Pass" TO DISPLAY
ELSE
  SEND "Fail" TO DISPLAY
END IF

if score >= 50:
    print("Pass")
else:
    print("Fail")

Key differences: SQA reference language uses IF … THEN, ELSE, END IF and SEND … TO DISPLAY for output. Python uses lowercase if, else, colons, and indentation — there is no END IF. Python is also case-sensitive: If or IF will cause an error.

Tracing selection code

An exam "trace" question gives you a piece of code and an input, then asks what the output will be. For selection, the technique is:

1. Substitute the input value into any variables.
2. Evaluate the first condition — is it True or False?
3. If True, follow that branch and ignore the rest. If False, move to the next condition.
4. Continue until one branch runs or you reach else.

Work through every condition in sequence — never assume a branch runs without checking.