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Data types and casting

Programs store different kinds of data in different ways. Choosing a suitable type matters because it controls the values that can be represented and the operations that make sense. A whole-number value, such as a lap count or number of goals, uses the integer data type. A name needs a text type instead.

OCR expects you to recognise integer, real, Boolean, character and string data, choose suitable types for a scenario, and convert between types when necessary. In Python, float represents real-number values and a one-character string represents character data.

By the end, you should be able to identify a value's type, justify a suitable type and use int() or float() when numerical input is required.

Multiple choice1 mark

Which data type is most suitable for the number of goals scored in a match?

ABoolean
BString
CInteger
DReal

Choosing a type

Integer: a positive or negative whole number, or zero, such as 12, 0 or -4.
Real: a number that may have a fractional part, represented by float in Python, such as 12.5.
Boolean: one of two logical values, True or False, such as whether a door is locked.
Character: one symbol, such as "A", "7" or "?". Python represents this as a string with a length of one.
String: a sequence of characters, such as "Aisha" or "AB12 3CD".

Choose a type from the meaning of the data, not just its appearance. A postcode may contain digits, but it is a string because it is an identifier and is not used in arithmetic.

Fill in the blanks5 marks

The value False is label 1. The value 27 is an label 2. The value 27.5 is a label 3. The value "Q" is a label 4. The value "OCR" is a label 5.

character
string
Boolean
real
integer
constant

Casting: changing a value's type

Python's input() function returns a string, even when the user types digits. The value "12" is text, so it must be cast before it can be used as the whole number 12 in arithmetic.

int("12") produces the integer 12.
float("12.5") produces the real value 12.5.
str(12) produces the string "12".

Casting is useful when the type you have is different from the type an operation needs. It does not automatically change the original variable: after age = int(age_text), age_text still stores a string and age stores an integer.

Choose the cast that matches the valid data. A quantity is normally an integer; a price or measured distance may need a real value.

Before running the example below, predict the type of price, the type of quantity and the value that will be displayed. Then run it to check your reasoning.

Worked example

price_text = "4.50"
quantity_text = "3"

# Convert the stored text before doing arithmetic.
price = float(price_text)
quantity = int(quantity_text)
total = price * quantity

print(total)

Multiple choice1 mark

Why does age = int(input("Age: ")) use int()?

ATo display the age
BTo make the value Boolean
CTo create a constant
DTo convert input text to a whole number

Supported practice: total distance

A running app currently stores two values as text:

distance_text stores the distance of one lap, which may contain a fractional part.
laps_text stores the number of completed laps, which must be a whole number.

Complete the program below so that it:

casts distance_text to a real value and stores it in distance;
casts laps_text to an integer and stores it in laps;
calculates distance * laps and stores the answer in total_distance;
displays the total distance.

For the supplied values, the program should display 50.0. The checks will also use another pair of valid values, so calculate from the variables rather than typing the final answer directly.

Coding task3 marks

distance_text = "12.5"
laps_text = "4"

# 1. Cast distance_text to a real value.
# 2. Cast laps_text to an integer.
# 3. Calculate the total distance.
# 4. Display the result.
# Write your statements below.

Written answer6 marks

Choose a suitable data type for each value and justify each choice: a user's email address, whether a door is locked, and the temperature measured by a weather station.

Name the type and link it to the values that must be represented.

Students type their answer here.

Independent practice: order total

A small shop needs a program that calculates the total cost of an order. The user will enter:

a price, which may include pence and therefore needs a real value;
a quantity, which is a whole number and therefore needs an integer.

Write a complete program that inputs and casts both values, calculates price * quantity, stores the result in total, and displays it.

Use the variable names price, quantity and total. You may cast each input on the same line that collects it, or store the input text first and cast it afterwards. The checks will run the program with more than one order, so the result must be calculated from the inputs.

When testing it yourself, try a price of 4.50 and a quantity of 3. The calculated total should be 13.5. A short label in the displayed message is acceptable.

Coding task3 marks

# 1. Input and cast price as a real value.
# 2. Input and cast quantity as an integer.
# 3. Calculate price multiplied by quantity and store it in total.
# 4. Display total.

Written answer2 marks

A student uses an integer for a runner's finishing time of 12.47 seconds. Explain the problem and give a better type.

State what information would be lost and name the Python type used.

Students type their answer here.

Review

You should now be able to:

distinguish integer, real, Boolean, character and string values;
choose a suitable type by considering the values and operations required;
remember that input() returns a string;
cast input with int() or float() before numerical processing;
explain why an unsuitable type can lose information or prevent the intended operation.

In an exam question that asks you to justify a data type, name the type and link it to the data. For example: “Real, because a finishing time may contain a fractional part.”