Data types, input, conversion and debugging

The previous worksheet introduced named storage and assignment. This worksheet adds the type of each stored value and keyboard input.

You will learn how to:

• distinguish required primitive data types and their Python names;
• explain that input() returns text;
• convert suitable text using int() or float();
• complete input-process-output programs;
• read, locate and repair a simple conversion runtime error.

Syntax and logic errors are named for orientation, but independent debugging focuses only on the locally modelled conversion fault.

1. Primitive data types

A data type describes the kind of value and the operations that make sense for it.

Specification type	Meaning	Python type used here	Example
Integer	Whole number	int	18
Real	Number with a fractional part	float	2.5
Boolean	Logical truth value	bool	True
Character	One character	one-character str	"A"

Python uses str for both one character and longer text. A full string is a structured sequence of characters and is studied more deeply later.

2. Keyboard input arrives as text

input() displays a prompt, waits for the user and returns the entered characters as text.

name = input("Name: ")
print(name)

Text can be output directly. Arithmetic needs a numerical type. int() converts suitable whole-number text and float() converts suitable decimal text:

tickets = int(input("Tickets: "))
total = tickets * 4
print(total)

If the user enters 3, tickets stores integer 3 and the program outputs 12.

3. A complete input-process-output program

Requirement: ask for a number of packs and items per pack, calculate the total and report it.

packs = int(input("Packs: "))
items_per_pack = int(input("Items per pack: "))
total = packs * items_per_pack
print("Total: " + str(total))

IPO role	Program evidence
Input	two input() calls, each converted to int
Processing	multiplication assigned to total
Output	informative label and the calculated value

The names make the program readable. The conversion occurs before arithmetic. For the output, str(total) converts the number to text so it can be concatenated with the label.

4. Errors and the first debugging cycle

Three error categories recur throughout the course:

• Syntax error: the rules of the programming language have been broken.
• Runtime error: the program begins executing but an operation cannot be completed.
• Logic error: the program runs but produces incorrect behaviour or output.

For now, concentrate on one runtime example:

age = int(input("Age: "))
print(age)

If the entered text is fourteen, int() cannot convert it to a whole number. Python stops while the program is running. A useful debugging cycle is read → locate → explain → amend or use suitable data → rerun.

Reading the evidence

A shortened message might say:

ValueError: invalid literal for int() with base 10: 'fourteen'

Read it from the task backwards:

1. Locate: the failing operation is int().
2. Explain: the supplied text is not a valid whole-number representation.
3. Respond: supply suitable numeric text, or redesign the program later with validation when that technique has been taught.
4. Rerun: confirm the program now completes.

Do not call this a syntax error. Python understood the line and started executing it.

5. Supported input program

Complete the program so it:

1. inputs packs and converts it to an integer;
2. inputs items_per_pack and converts it to an integer;
3. multiplies them into total;
4. outputs Total: followed by the value.

It will be tested with 4 and 6, then 3 and 9. Do not add validation yet.

6. Apply the debugging cycle

A student enters 2.5 into this program:

distance = int(input("Distance: "))
print(distance)

The program stops during conversion. Your response must identify the failing operation, explain why this particular text is unsuitable for int(), and give a suitable amendment when fractional distances are allowed.

7. One final distinction

Type choice and conversion are related but different decisions:

• choose int because a count is a whole number;
• choose float because a measurement may contain a fractional part;
• convert text from input() into the chosen numerical type before arithmetic.

A good explanation names both the nature of the data and the operation that requires conversion.

Route forward

You can now choose primitive types, convert keyboard input, complete an IPO program and handle one carefully modelled runtime conversion fault. Next you will represent sequence visually using Pearson's flowchart conventions.