Input, Output & Sequence — Cambridge IGCSE CS (0478)

🎯 Syllabus & Goals 3 min

Cambridge 8.1.3 · Input and output 8.1.4 · Sequence Paper 2 · Algorithms, Programming and Logic

By the end of this lesson you can:

Write INPUT and OUTPUT statements with clear prompts and messages, in pseudocode and Python.
Convert keyboard input to the correct data type in Python with int() and float().
Use a trace table to show how the order of statements changes a program's results, and correct it.

Textbook: Chapter 8, §8.1.3–8.1.4(a) (pp. 304–309)

Recap / Warm-Up 5 min

Last lesson: variables and constants are named stores, and each has a data type — INTEGER, REAL, CHAR, STRING or BOOLEAN.

Quick starter

Which data type would you give to each of these inputs: 12, 12.00, X, X marks the spot, TRUE?

Reveal the answer

INTEGER, REAL, CHAR, STRING, BOOLEAN.

🧠 Key Concept 14 min

1 · Input with a prompt

For IGCSE, programs take input from a keyboard and send output to a screen. A user must know what to type, so every input needs a prompt first.

The three stages of almost every program. In Python the keyboard always delivers a string, so numbers must be converted before any maths.

Cambridge pseudocode

OUTPUT "Enter the radius: "
INPUT Radius

The prompt is a separate OUTPUT statement.

The same in Python

radius = float(input("Enter the radius: "))

Python puts the prompt inside input(), and float() converts the text.

Casting — converting a value from one data type to another. In Python, int() turns text into an INTEGER and float() turns it into a REAL. The input data type must match the variable that stores it.

2 · Output with a message

A bare number on screen means nothing to a user. Each output should include a message saying what the value is. When one output has several parts, they are separated — by a comma in pseudocode and in Python.

OUTPUT "Volume of the tank is ", Volume

print("Volume of the tank is", volume)

Key point: Python's print puts a space between comma-separated parts automatically. That is why the Python strings here do not end with a space.

3 · Sequence — the order of the steps

A computer carries out statements one after another, top to bottom. An incorrect order gives wrong results or adds steps that are not needed. Think of a recipe: icing a cake before baking it does not work.

Same three statements, different order. Only the right-hand sequence gives the correct fare.

Real world: a cash machine must check your PIN before it hands out money, and must subtract the amount from your balance after it knows how much you want. Swap either pair and the bank loses money.

Worked Example 12 min

Example 1 · A complete input–process–output program

A bus company charges $0.25 per kilometre. Write a program that asks for the passenger's name and the distance, then outputs the fare.

Name the constant and variables, with types.the rate never changes; the name is text; distance and fare have decimals.
Prompt, then input, each value.the user must know what to type.
Calculate only after both inputs.sequence — the data must exist before it is used.
Output with a message.the user must know what the number means.

Cambridge pseudocode

CONSTANT FarePerKm ← 0.25
DECLARE PassengerName : STRING
DECLARE Distance : REAL
DECLARE Fare : REAL
OUTPUT "Enter your name: "
INPUT PassengerName
OUTPUT "Enter the distance in km: "
INPUT Distance
Fare ← Distance * FarePerKm
OUTPUT "Hello ", PassengerName
OUTPUT "Your fare is $", Fare

The same in Python

# Bus fare calculator: input, process, output
FARE_PER_KM = 0.25
passenger_name = input("Enter your name: ")
distance = float(input("Enter the distance in km: "))
fare = distance * FARE_PER_KM
print("Hello", passenger_name)
print("Your fare is $", fare)

Output

Enter your name: Alex
Enter the distance in km: 12
Hello Alex
Your fare is $ 3.0

What happens if you forget to cast? Python refuses to do maths with text:

age = input("Enter your age: ")
print(age + 1)

Enter your age: 14
TypeError: can only concatenate str (not "int") to str

Example 2 · Finding and fixing a sequence error

A runner types in the length of each run, then -1 to finish. The program should output the total, the number of runs and the average. Test data: 5, 3, 4, -1.

Version A — the steps are in the wrong order

DECLARE Distance, Total, Average : REAL
DECLARE Count : INTEGER
Total ← 0
Count ← 0
OUTPUT "Enter each run in km, -1 to finish"
REPEAT
  OUTPUT "Distance: "
  INPUT Distance
  Total ← Total + Distance
  Count ← Count + 1
UNTIL Distance = -1
Average ← Total / Count
OUTPUT "Total distance: ", Total
OUTPUT "Number of runs: ", Count
OUTPUT "Average run: ", Average

Distance	Total	Count	Average	OUTPUT
	0	0		Enter each run in km, -1 to finish
5	5	1
3	8	2
4	12	3
-1	11	4
			2.75	Total distance: 11 · Number of runs: 4 · Average run: 2.75

All three outputs are wrong: the -1 that only signals "stop" was added to the total and counted as a run.

Version B — the same statements, re-ordered

Set Distance ← 0 and Count ← -1 before the loop.the first pass adds 0 and brings Count to 0, so nothing false is recorded.
Move INPUT Distance to the end of the loop body.each value is added on the next pass — after it has been checked by UNTIL.
When -1 is typed, the loop ends before it can be added or counted.the rogue value never reaches Total.

DECLARE Distance, Total, Average : REAL
DECLARE Count : INTEGER
Distance ← 0
Total ← 0
Count ← -1
OUTPUT "Enter each run in km, -1 to finish"
REPEAT
  Total ← Total + Distance
  Count ← Count + 1
  OUTPUT "Distance: "
  INPUT Distance
UNTIL Distance = -1
Average ← Total / Count
OUTPUT "Total distance: ", Total
OUTPUT "Number of runs: ", Count
OUTPUT "Average run: ", Average

# Correct order: add the PREVIOUS value, then ask for the next one
distance = 0
total = 0
count = -1          # starts at -1 so the final -1 is not counted
print("Enter each run in km, -1 to finish")
while True:         # Python has no REPEAT...UNTIL
    total = total + distance
    count = count + 1
    distance = float(input("Distance: "))
    if distance == -1:
        break
average = total / count
print("Total distance:", total)
print("Number of runs:", count)
print("Average run:", average)

Output

Enter each run in km, -1 to finish
Distance: 5
Distance: 3
Distance: 4
Distance: -1
Total distance: 12.0
Number of runs: 3
Average run: 4.0

Distance	Total	Count	Average	OUTPUT
0	0	-1		Enter each run in km, -1 to finish
5	0	0
3	5	1
4	8	2
-1	12	3
			4	Total distance: 12 · Number of runs: 3 · Average run: 4

Try It Yourself 12 min

🟢 Easy

Goal: Write pseudocode and Python that ask for a user's first name and favourite number, then output both with a friendly message.

🟡 Medium

Goal: Write a program that inputs the radius and length of a cylinder and outputs its volume (π × r² × length). Use a constant for π. Test it with radius 2 and length 10.

🔴 Stretch

Goal: These statements convert minutes to hours and minutes, but they are in the wrong order. Put them in the correct sequence and dry-run with 135.

OUTPUT Hours, " h ", Minutes, " min"
Minutes ← TotalMinutes - Hours * 60
INPUT TotalMinutes
Hours ← DIV(TotalMinutes, 60)
OUTPUT "Enter a number of minutes: "

Hint

Ask: what does each line need to exist first? The prompt comes before the input, and Hours must be known before Minutes. DIV gives the whole-number part of a division (you will meet it fully in Lesson 6).

📝 Exam Practice 10 min

State[1]

State why an input statement should be preceded by a prompt.

Mark scheme

So the user knows what data they are expected to enter (1).

Describe[2]

Describe what is meant by sequence in a program, and why it matters.

Mark scheme

Sequence is the order in which the statements / steps are executed (1).
An incorrect order can give incorrect results / use data before it has been input or calculated (1).

Write[4]

Write an algorithm in pseudocode that inputs the price of an item and the number bought, then outputs the total cost with a suitable message.

Mark scheme

Prompt and input of price (1).
Prompt and input of quantity (1).
Total calculated as price × quantity, after both inputs (1).
Output of total with a message (1).

Explain[3]

A student writes the Python statement mark = input("Enter mark: ") and later total = total + mark. Explain why this causes an error and how to correct it.

Mark scheme

input() always returns a string (1)…
…and a string cannot be added to a number / used in a calculation (1).
Correct by casting: mark = int(input("Enter mark: ")) (1).

🗝️ Recap & Key Terms 3 min

Prompt, then input. Convert text to a number before doing maths. Output each result with a message. Statements run top to bottom, so the sequence must give each step the data it needs.

Input: Data entered into a program, e.g. from a keyboard, and stored in a variable.
Output: Data or messages sent from a program, e.g. to a screen.
Prompt: A message that tells the user what data to enter.
Casting: Converting a value from one data type to another, e.g. int("12").
Sequence: The order in which the steps in a program are executed.

Homework 1 min

Task (≤ 15 min): Write a program in pseudocode and Python that inputs a temperature in Celsius and outputs it in Fahrenheit (F = C × 1.8 + 32). Dry-run it with 25 and show the output.

Model answer

DECLARE Celsius, Fahrenheit : REAL
OUTPUT "Enter the temperature in Celsius: "
INPUT Celsius
Fahrenheit ← Celsius * 1.8 + 32
OUTPUT "That is ", Fahrenheit, " Fahrenheit"

celsius = float(input("Enter the temperature in Celsius: "))
fahrenheit = celsius * 1.8 + 32
print("That is", fahrenheit, "Fahrenheit")

Enter the temperature in Celsius: 25
That is 77.0 Fahrenheit

🎯 Syllabus & Goals 3 min

Cambridge 8.1.3 · Input and output 8.1.4 · Sequence Paper 2 · Algorithms, Programming and Logic

By the end of this lesson you can:

Write INPUT and OUTPUT statements with clear prompts and messages, in pseudocode and Python.
Convert keyboard input to the correct data type in Python with int() and float().
Use a trace table to show how the order of statements changes a program's results, and correct it.

Textbook: Chapter 8, §8.1.3–8.1.4(a) (pp. 304–309)

Recap / Warm-Up 5 min

Last lesson: variables and constants are named stores, and each has a data type — INTEGER, REAL, CHAR, STRING or BOOLEAN.

Quick starter

Which data type would you give to each of these inputs: 12, 12.00, X, X marks the spot, TRUE?

Reveal the answer

INTEGER, REAL, CHAR, STRING, BOOLEAN.

🧠 Key Concept 14 min

1 · Input with a prompt

For IGCSE, programs take input from a keyboard and send output to a screen. A user must know what to type, so every input needs a prompt first.

The three stages of almost every program. In Python the keyboard always delivers a string, so numbers must be converted before any maths.

Cambridge pseudocode

OUTPUT "Enter the radius: "
INPUT Radius

The prompt is a separate OUTPUT statement.

The same in Python

radius = float(input("Enter the radius: "))

Python puts the prompt inside input(), and float() converts the text.

2 · Output with a message

OUTPUT "Volume of the tank is ", Volume

print("Volume of the tank is", volume)

Key point: Python's print puts a space between comma-separated parts automatically. That is why the Python strings here do not end with a space.

3 · Sequence — the order of the steps

Same three statements, different order. Only the right-hand sequence gives the correct fare.

Worked Example 12 min

Example 1 · A complete input–process–output program

A bus company charges $0.25 per kilometre. Write a program that asks for the passenger's name and the distance, then outputs the fare.

Name the constant and variables, with types.the rate never changes; the name is text; distance and fare have decimals.
Prompt, then input, each value.the user must know what to type.
Calculate only after both inputs.sequence — the data must exist before it is used.
Output with a message.the user must know what the number means.

Cambridge pseudocode

CONSTANT FarePerKm ← 0.25
DECLARE PassengerName : STRING
DECLARE Distance : REAL
DECLARE Fare : REAL
OUTPUT "Enter your name: "
INPUT PassengerName
OUTPUT "Enter the distance in km: "
INPUT Distance
Fare ← Distance * FarePerKm
OUTPUT "Hello ", PassengerName
OUTPUT "Your fare is $", Fare

The same in Python

# Bus fare calculator: input, process, output
FARE_PER_KM = 0.25
passenger_name = input("Enter your name: ")
distance = float(input("Enter the distance in km: "))
fare = distance * FARE_PER_KM
print("Hello", passenger_name)
print("Your fare is $", fare)

Output

Enter your name: Alex
Enter the distance in km: 12
Hello Alex
Your fare is $ 3.0

What happens if you forget to cast? Python refuses to do maths with text:

age = input("Enter your age: ")
print(age + 1)

Enter your age: 14
TypeError: can only concatenate str (not "int") to str

Example 2 · Finding and fixing a sequence error

A runner types in the length of each run, then -1 to finish. The program should output the total, the number of runs and the average. Test data: 5, 3, 4, -1.

Version A — the steps are in the wrong order

DECLARE Distance, Total, Average : REAL
DECLARE Count : INTEGER
Total ← 0
Count ← 0
OUTPUT "Enter each run in km, -1 to finish"
REPEAT
  OUTPUT "Distance: "
  INPUT Distance
  Total ← Total + Distance
  Count ← Count + 1
UNTIL Distance = -1
Average ← Total / Count
OUTPUT "Total distance: ", Total
OUTPUT "Number of runs: ", Count
OUTPUT "Average run: ", Average

Distance	Total	Count	Average	OUTPUT
	0	0		Enter each run in km, -1 to finish
5	5	1
3	8	2
4	12	3
-1	11	4
			2.75	Total distance: 11 · Number of runs: 4 · Average run: 2.75

All three outputs are wrong: the -1 that only signals "stop" was added to the total and counted as a run.

Version B — the same statements, re-ordered

Set Distance ← 0 and Count ← -1 before the loop.the first pass adds 0 and brings Count to 0, so nothing false is recorded.
Move INPUT Distance to the end of the loop body.each value is added on the next pass — after it has been checked by UNTIL.
When -1 is typed, the loop ends before it can be added or counted.the rogue value never reaches Total.

DECLARE Distance, Total, Average : REAL
DECLARE Count : INTEGER
Distance ← 0
Total ← 0
Count ← -1
OUTPUT "Enter each run in km, -1 to finish"
REPEAT
  Total ← Total + Distance
  Count ← Count + 1
  OUTPUT "Distance: "
  INPUT Distance
UNTIL Distance = -1
Average ← Total / Count
OUTPUT "Total distance: ", Total
OUTPUT "Number of runs: ", Count
OUTPUT "Average run: ", Average

# Correct order: add the PREVIOUS value, then ask for the next one
distance = 0
total = 0
count = -1          # starts at -1 so the final -1 is not counted
print("Enter each run in km, -1 to finish")
while True:         # Python has no REPEAT...UNTIL
    total = total + distance
    count = count + 1
    distance = float(input("Distance: "))
    if distance == -1:
        break
average = total / count
print("Total distance:", total)
print("Number of runs:", count)
print("Average run:", average)

Output

Enter each run in km, -1 to finish
Distance: 5
Distance: 3
Distance: 4
Distance: -1
Total distance: 12.0
Number of runs: 3
Average run: 4.0

Distance	Total	Count	Average	OUTPUT
0	0	-1		Enter each run in km, -1 to finish
5	0	0
3	5	1
4	8	2
-1	12	3
			4	Total distance: 12 · Number of runs: 3 · Average run: 4

Try It Yourself 12 min

🟢 Easy

Goal: Write pseudocode and Python that ask for a user's first name and favourite number, then output both with a friendly message.

🟡 Medium

Goal: Write a program that inputs the radius and length of a cylinder and outputs its volume (π × r² × length). Use a constant for π. Test it with radius 2 and length 10.

🔴 Stretch

Goal: These statements convert minutes to hours and minutes, but they are in the wrong order. Put them in the correct sequence and dry-run with 135.

OUTPUT Hours, " h ", Minutes, " min"
Minutes ← TotalMinutes - Hours * 60
INPUT TotalMinutes
Hours ← DIV(TotalMinutes, 60)
OUTPUT "Enter a number of minutes: "

Hint

📝 Exam Practice 10 min

State[1]

State why an input statement should be preceded by a prompt.

Mark scheme

So the user knows what data they are expected to enter (1).

Describe[2]

Describe what is meant by sequence in a program, and why it matters.

Mark scheme

Sequence is the order in which the statements / steps are executed (1).
An incorrect order can give incorrect results / use data before it has been input or calculated (1).

Write[4]

Write an algorithm in pseudocode that inputs the price of an item and the number bought, then outputs the total cost with a suitable message.

Mark scheme

Prompt and input of price (1).
Prompt and input of quantity (1).
Total calculated as price × quantity, after both inputs (1).
Output of total with a message (1).

Explain[3]

A student writes the Python statement mark = input("Enter mark: ") and later total = total + mark. Explain why this causes an error and how to correct it.

Mark scheme

input() always returns a string (1)…
…and a string cannot be added to a number / used in a calculation (1).
Correct by casting: mark = int(input("Enter mark: ")) (1).

🗝️ Recap & Key Terms 3 min

Prompt, then input. Convert text to a number before doing maths. Output each result with a message. Statements run top to bottom, so the sequence must give each step the data it needs.

Input: Data entered into a program, e.g. from a keyboard, and stored in a variable.
Output: Data or messages sent from a program, e.g. to a screen.
Prompt: A message that tells the user what data to enter.
Casting: Converting a value from one data type to another, e.g. int("12").
Sequence: The order in which the steps in a program are executed.

Homework 1 min

Task (≤ 15 min): Write a program in pseudocode and Python that inputs a temperature in Celsius and outputs it in Fahrenheit (F = C × 1.8 + 32). Dry-run it with 25 and show the output.

Model answer

DECLARE Celsius, Fahrenheit : REAL
OUTPUT "Enter the temperature in Celsius: "
INPUT Celsius
Fahrenheit ← Celsius * 1.8 + 32
OUTPUT "That is ", Fahrenheit, " Fahrenheit"

celsius = float(input("Enter the temperature in Celsius: "))
fahrenheit = celsius * 1.8 + 32
print("That is", fahrenheit, "Fahrenheit")

Enter the temperature in Celsius: 25
That is 77.0 Fahrenheit