Pseudocode: Assignment & Selection

🎯 Syllabus & Goals 3 min

Cambridge 7.2 · Design methods — pseudocode Paper 2 · Algorithms, Programming and Logic

By the end of this lesson you can:

1. Write assignment statements with ← and work out the values they produce.
2. Write IF … THEN … ELSE … ENDIF, including a nested IF, using comparison and Boolean operators.
3. Write a CASE OF … OTHERWISE … ENDCASE statement and say when it is the better choice.

Textbook: Chapter 7, §7.2.3 Pseudocode — assignment and conditional statements (pp. 265–268)

Recap / Warm-Up 5 min

Last lesson we drew algorithms as flowcharts. Pseudocode shows the same algorithm as text. Every rectangle becomes an assignment; every decision becomes an IF or a loop.

Quick starter

In the pool flowchart, which symbol became IF N < 5 in the pseudocode, and which became D ← 0?

🧠 Key Concept 14 min

1 · What pseudocode is

Pseudocode describes an algorithm with English keywords that look like a high-level language. Data items get meaningful names, like variables. But it has no strict syntax rules — it pretends to be code. Cambridge writes it consistently:

• a fixed-width (non-proportional) font;
• keywords in CAPITALS: INPUT, IF, ENDIF;
• names of data items start with a capital letter: StudentMark, Total;
• statements inside an IF or a loop are indented by two spaces;
• on this site, always one statement per line.

2 · Assignment

A value is given to a variable with ←. The variable on the left takes the value of the expression on the right.

3 · Selection with IF … THEN … ELSE … ENDIF

When an algorithm must choose between actions, it uses a conditional statement. The THEN path runs if the condition is true; the ELSE path runs if it is false. There may be no ELSE. ENDIF always closes the statement.

IF Speed > 50
  THEN
    OUTPUT "Slow down"
  ELSE
    OUTPUT "OK"
ENDIF

IF Paid
  THEN
    OUTPUT "Ticket printed"
  ELSE
    OUTPUT "Please pay first"
ENDIF

4 · Selection with CASE OF … OTHERWISE … ENDCASE

A CASE statement chooses a path from the value of one variable. Several values are listed. OTHERWISE catches every other value. ENDCASE closes it.

Worked Example 12 min

Worked example 1 · Values after a sequence of assignments

A decorator works out how much paint a wall needs. What does each variable hold at the end?

Width ← 4
Height ← 2.5
Area ← Width * Height
Coats ← 2
Paint ← Area * Coats / 5
Label ← "Litres: "

1. Width ← 4; Height ← 2.5.
2. Area ← 4 × 2.5 = 10.the right side uses the values Width and Height hold at that moment.
3. Coats ← 2.
4. Paint ← 10 × 2 / 5 = 20 / 5 = 4.* and / have equal priority, so work left to right.
5. Label ← Litres: (a string, stored without the quotation marks).

width = 4
height = 2.5
area = width * height
coats = 2
paint = area * coats / 5
label = "Litres: "
print(label, paint)

Litres:  4.0

Worked example 2 · A nested IF for a quiz score

A quiz is marked out of 50. Scores below 0 or above 50 are invalid. A score of 25 or more is a pass.

OUTPUT "Enter the score "
INPUT Score
IF Score < 0 OR Score > 50
  THEN
    OUTPUT "Invalid score"
  ELSE
    IF Score >= 25
      THEN
        OUTPUT "Pass"
      ELSE
        OUTPUT "Fail"
    ENDIF
ENDIF

score = int(input("Enter the score "))
if score < 0 or score > 50:
    print("Invalid score")
else:
    if score >= 25:
        print("Pass")
    else:
        print("Fail")

1. Outer IF checks validity first. OR is used because a score is invalid if either test is true.a score cannot be below 0 and above 50 at once, so AND would never be true.
2. Inner IF runs only for valid scores. It sits in the outer ELSE, indented one more level, with its own ENDIF.
3. Test it:

   Score	Outer test	Inner test	Output
   60	60 > 50 → true	not reached	Invalid score
   25	false	25 >= 25 → true	Pass
   24	false	24 >= 25 → false	Fail

   25 and 24 sit either side of the pass mark — the most likely place for a mistake.

Worked example 3 · A CASE statement for a drinks machine

Button 1 is water ($1.00), 2 is juice ($1.50), 3 is cola ($1.20). Any other button is an error.

INPUT Button
Price ← 0
CASE OF Button
  1 : Price ← 1.00
  2 : Price ← 1.50
  3 : Price ← 1.20
  OTHERWISE OUTPUT "No such drink"
ENDCASE
IF Price > 0
  THEN
    OUTPUT "Please pay $", Price
ENDIF

button = int(input("Button: "))
price = 0
if button == 1:
    price = 1.00
elif button == 2:
    price = 1.50
elif button == 3:
    price = 1.20
else:
    print("No such drink")
if price > 0:
    print("Please pay $", price)

Button: 2
Please pay $ 1.5

1. Why CASE here? One variable, several fixed values.three nested IFs would do the same job but are harder to read.
2. Why Price ← 0 first? So that after OTHERWISE the final IF knows no drink was chosen.
3. Python writes the same choice as if … elif … else; == is Python's "equal to", and else plays the part of OTHERWISE.

Try It Yourself 12 min

📝 Exam Practice 10 min

Cost ← 40
Tax ← Cost * 0.2
Cost ← Cost - 5
Total ← Cost + Tax

INPUT Order
IF Order > 100
  THEN
    Discount = TRUE
    OUTPUT Discount

🗝️ Recap & Key Terms 3 min

Pseudocode uses capital keywords, capitalised names and two-space indents. ← assigns the value on the right to the variable on the left. IF chooses between two paths; a nested IF adds more; CASE chooses one path per value of a variable.

Pseudocode

A simple way of showing an algorithm using English keywords like a high-level language, without strict syntax rules.

Assignment

Giving a variable a value with ←; the variable on the left takes the value of the expression on the right.

Conditional statement (selection)

A statement that chooses which action to take depending on a condition or a value.

Nested IF

An IF statement placed inside the THEN or ELSE path of another IF statement.

CASE statement

Selection where the value of one variable decides the path; OTHERWISE covers all other values.

Homework 1 min

Task (≤ 15 min): A parcel company charges $4 for parcels up to 2 kg, $7 for over 2 kg up to 10 kg, and $12 for over 10 kg up to 30 kg. Heavier parcels are refused. Write pseudocode that inputs a weight and outputs the charge or "Refused". [5]

INPUT Weight
IF Weight > 30
  THEN
    OUTPUT "Refused"
  ELSE
    IF Weight > 10
      THEN
        OUTPUT "$12"
      ELSE
        IF Weight > 2
          THEN
            OUTPUT "$7"
          ELSE
            OUTPUT "$4"
        ENDIF
    ENDIF
ENDIF