Input and Output

This lesson covers input and output operations as required by OCR J277 Section 2.3. Input and output (often abbreviated as I/O) are fundamental to every program — without them, a program cannot receive data from the user or display results.

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What Is Input?

Input is data entered into a program by the user or received from another source (such as a file or sensor). In OCR GCSE Computer Science, input typically means data entered via the keyboard.

OCR Pseudocode:

name = input("What is your name? ")
age = int(input("How old are you? "))

Python:

name = input("What is your name? ")
age = int(input("How old are you? "))

Important Points About Input

Point	Explanation
input() always returns a string	Even if the user types 42, it is stored as the string "42"
You must cast for numbers	Use int() for integers, float() (or real() in OCR pseudocode) for decimals
The prompt message is optional	But it is good practice to tell the user what to enter
Input waits for the user	The program pauses until the user presses Enter

OCR Exam Tip: If the exam question says "the user enters a number", you must use int(input()) or real(input()). Forgetting to cast is a common mistake that means arithmetic operations would fail or give unexpected results.

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What Is Output?

Output is data displayed by the program, typically on the screen. In OCR pseudocode and Python, the print() function is used for output.

OCR Pseudocode:

print("Hello, world!")
name = "Alice"
print("Welcome, " + name)
score = 95
print("Your score is: " + str(score))

Python:

print("Hello, world!")
name = "Alice"
print("Welcome, " + name)
score = 95
print("Your score is:", score)  # Python's print() can accept multiple arguments
print("Your score is: " + str(score))  # String concatenation version

Formatting Output

When combining text and numbers in output, you often need to convert numbers to strings:

OCR Pseudocode:

price = 9.99
quantity = 3
total = price * quantity
print("Total: £" + str(total))

Python:

price = 9.99
quantity = 3
total = price * quantity
print("Total: £" + str(total))
# Or using f-strings (more modern Python):
print(f"Total: £{total}")
# Or using format to 2 decimal places:
print(f"Total: £{total:.2f}")

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Combining Input and Output

A typical program follows an Input → Process → Output pattern:

OCR Pseudocode:

// Input
length = real(input("Enter length: "))
width = real(input("Enter width: "))

// Process
area = length * width

// Output
print("The area is: " + str(area))

Python:

# Input
length = float(input("Enter length: "))
width = float(input("Enter width: "))

# Process
area = length * width

# Output
print("The area is:", area)

OCR Exam Tip: Many exam questions follow the Input-Process-Output pattern. When writing algorithms, clearly separate these three stages. This makes your code easier to read and demonstrates structured thinking.

The Input-Process-Output cycle, including the cast step that exam candidates frequently forget, is shown below:

flowchart TD
    A([Start]) --> B[Prompt user with input message]
    B --> C[Read raw value as string from input]
    C --> D{"Will value be<br/>used in arithmetic?"}
    D -->|Yes| E[Cast: int or real / float]
    D -->|No| F[Keep as string]
    E --> G[Process: calculate result]
    F --> G
    G --> H[Convert numbers to string with str]
    H --> I[print formatted output]
    I --> J([End])

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Worked Example: Temperature Converter

Question: Write a program that asks the user for a temperature in Celsius and converts it to Fahrenheit. The formula is: F = (C × 9/5) + 32.

OCR Pseudocode:

celsius = real(input("Enter temperature in Celsius: "))
fahrenheit = (celsius * 9 / 5) + 32
print(str(celsius) + "°C is " + str(fahrenheit) + "°F")

Python:

celsius = float(input("Enter temperature in Celsius: "))
fahrenheit = (celsius * 9 / 5) + 32
print(f"{celsius}°C is {fahrenheit}°F")

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Worked Example: Simple Calculator

OCR Pseudocode:

num1 = real(input("Enter first number: "))
num2 = real(input("Enter second number: "))
operator = input("Enter operator (+, -, *, /): ")

if operator == "+" then
    result = num1 + num2
elseif operator == "-" then
    result = num1 - num2
elseif operator == "*" then
    result = num1 * num2
elseif operator == "/" then
    if num2 != 0 then
        result = num1 / num2
    else
        print("Error: cannot divide by zero")
    endif
else
    print("Invalid operator")
endif

print("Result: " + str(result))

Python:

num1 = float(input("Enter first number: "))
num2 = float(input("Enter second number: "))
operator = input("Enter operator (+, -, *, /): ")

if operator == "+":
    result = num1 + num2
elif operator == "-":
    result = num1 - num2
elif operator == "*":
    result = num1 * num2
elif operator == "/":
    if num2 != 0:
        result = num1 / num2
    else:
        print("Error: cannot divide by zero")
        result = None
else:
    print("Invalid operator")
    result = None

if result is not None:
    print("Result:", result)

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Common Mistakes with Input and Output

Mistake	Problem	Fix
Not casting input	"5" + "3" gives "53" (concatenation)	Use int(input()) for numbers
Concatenating string with number	"Score: " + 42 causes error	Use str(42) to convert
No prompt message	User does not know what to enter	Always include a clear prompt
Spelling errors in variable names	scroe instead of score	Check variable names carefully

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Summary

• Input receives data from the user using input().
• Output displays data using print().
• input() always returns a string — cast to int() or real()/float() for numbers.
• Use str() to convert numbers to strings for concatenation in output.
• Programs follow the Input → Process → Output pattern.

OCR Exam Tip: When completing pseudocode in the exam, always include int() or real() around input() when the value will be used in calculations. This is one of the most commonly penalised errors in the marking scheme.

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Worked Example: Receipt Printer