[8.1.3] Input & Output

[8.1.3] Input & Output (I/O)

Input brings data into a program from the user or another source; output presents results back to the user or to another destination. Mastering I/O is essential because it turns isolated calculations into useful, interactive programs. In IGCSE tasks you will commonly read values from the keyboard and write results to the screen. You will also need to handle data types correctly, format messages clearly, and combine I/O with validation and control structures.

Core ideas: prompts, reading, converting, and displaying

• Prompting: a short, clear message that tells the user what to enter, including units where helpful (e.g. "Enter length in centimetres:").
• Reading: get the user's keystrokes; most languages read text initially. If you need a number, convert it.
• Converting: cast the text to the required type (INTEGER/REAL) before using it in arithmetic.
• Displaying: present outputs with labels, units, and sensible formatting (e.g. two decimal places for money).

Console I/O patterns (compare common scenarios)

Below are three frequent I/O scenarios you will meet. Use the tabs to see how each is handled and what to watch out for.

• Type Conversion
• Trimming & Case
• Labels & Decimals

// CAIE Pseudocode
DECLARE a : REAL, b : REAL, total : REAL
OUTPUT "Enter first number:"
INPUT a     // some environments read numeric directly; if not, convert
OUTPUT "Enter second number:"
INPUT b
total ← a + b
OUTPUT "Sum = " & total

# Python
a = float(input("Enter first number: "))
b = float(input("Enter second number: "))
total = a + b
print("Sum =", total)

// CAIE Pseudocode
DECLARE ans : STRING
OUTPUT "Continue? (Y/N): "
INPUT ans
ans ← UPPER(ans)           // normalise
ans ← TRIM(ans)            // remove surrounding spaces
IF ans = "Y" THEN
  OUTPUT "Continuing..."
ELSE
  OUTPUT "Stopping."
ENDIF

# Python
ans = input("Continue? (Y/N): ").strip().upper()
if ans == "Y":
    print("Continuing...")
else:
    print("Stopping.")

// CAIE Pseudocode
DECLARE price : REAL ← 12.5
OUTPUT "Total price: £" & FORMAT(price, 2)   // two decimal places

# Python
price = 12.5
print(f"Total price: £{price:.2f}")

I/O with validation (good prompts reduce errors)

Clear prompts reduce invalid entries. Even so, programs should check user input and ask again when it is not acceptable. This improves robustness and user experience.

• Presence & Type checks
• Range checks
• Menu input: Validating Menu Input options

// CAIE Pseudocode
DECLARE ageText : STRING
DECLARE age : INTEGER
REPEAT
  OUTPUT "Enter your age (whole number): "
  INPUT ageText
  IF ageText = "" THEN
    OUTPUT "You must enter something."
  ELSEIF ISINTEGER(ageText) = FALSE THEN
    OUTPUT "Please enter digits only."
  ENDIF
UNTIL ageText <> "" AND ISINTEGER(ageText) = TRUE
age ← TOINTEGER(ageText)
OUTPUT "Next year you will be " & (age + 1)

// Python (one simple approach)
while True:
    age_text = input("Enter your age (whole number): ").strip()
    if age_text == "":
        print("You must enter something.")
        continue
    if not age_text.isdigit():
        print("Please enter digits only.")
        continue
    age = int(age_text)
    break
print("Next year you will be", age + 1)

// CAIE Pseudocode
DECLARE mark : INTEGER
REPEAT
  OUTPUT "Enter mark (0–100): "
  INPUT mark
UNTIL mark >= 0 AND mark <= 100
OUTPUT "Mark recorded."

# Python
while True:
    mark = int(input("Enter mark (0–100): "))
    if 0 <= mark <= 100:
        break
    print("Out of range, try again.")
print("Mark recorded.")

// CAIE Pseudocode
DECLARE choice : STRING
REPEAT
  OUTPUT "1) Add  2) Delete  3) Quit"
  OUTPUT "Enter choice (1,2,3): "
  INPUT choice
UNTIL choice = "1" OR choice = "2" OR choice = "3"
OUTPUT "You selected option " & choice

# Python
while True:
    print("1) Add  2) Delete  3) Quit")
    choice = input("Enter choice (1,2,3): ").strip()
    if choice in ("1","2","3"):
        break
    print("Invalid option.")
print("You selected option", choice)

Designing clear I/O for user experience

Think about the human at the keyboard. Good I/O design prevents confusion and speeds up tasks:

• Be specific: include units and ranges ("Enter height in cm (50–250):").
• Be consistent: use the same wording and layout for similar prompts.
• Be forgiving: accept lower/upper case, ignore extra spaces, and handle minor mistakes kindly.
• Be informative: when rejecting input, state why and how to fix it.

Deep Dive: Sequencing I/O with control structures

I/O rarely stands alone. It is usually paired with selection and iteration to build useful dialogue with a user.

1. Input → Process → Output: the standard pattern. Read values, calculate, then display a labelled result.
2. Input → Validate (loop) → Process → Output: when bad input is likely, repeat the prompt until the data is acceptable.
3. Menu loop: repeatedly show a menu, perform actions based on the choice, until the user picks Quit.

Common I/O pitfalls and fixes