Data Types and Casting

Every value stored in a program has a data type that determines what kind of data it is, how much memory it uses, and what operations are valid on it. OCR J277 requires knowledge of five data types.

Data type	What it stores	Example values
Integer	Whole numbers, positive or negative	0, 42, -7, 1000
Real	Numbers with a decimal (fractional) part	3.14, -0.5, 99.99
Boolean	A true or false value	True, False
Character	A single character	'A', '7', '!'
String	A sequence of zero or more characters	"hello", "Y10", ""

Choosing the wrong data type causes errors. Storing a price as an integer (e.g. price = 2) loses the decimal part. Storing a name as an integer is impossible.

The data type must match the nature of the data, not just the current value. An exam score of 85 is an integer; a measurement of 1.85 m is a real.

Integer — stores whole numbers with no decimal point. Use integers for things that are counted or indexed.

age = 17
num_students = 30
high_score = 9999

Real (also called float in Python) — stores numbers with a decimal part. Use reals for things that are measured or calculated with division.

temperature = 36.6
average = 78.5
pi = 3.14159

Worked example — choosing between integer and real:

Scenario	Data type	Reason
Number of lives remaining in a game	Integer	Lives are counted whole — you can't have 2.5 lives
Price of an item (£3.99)	Real	Prices have pence — a decimal is needed
Student's test score out of 100	Integer	Scores are whole marks
Distance run in kilometres (5.4 km)	Real	Distances are measured, not counted
Number of items in a shopping basket	Integer	Items are counted whole

If a value could ever be non-whole — a measurement, a price, a ratio — use real. If it will always be a whole count, use integer.

Boolean stores exactly one of two values: True or False. Booleans are the result of comparison expressions and control selection/iteration.

logged_in = False
game_over = False
is_valid = age >= 18      # True if age is at least 18, False otherwise

Booleans are used directly in conditions:

if logged_in:
    print("Welcome back")

Character stores a single symbol — a letter, digit, punctuation mark, or space. In most languages a character is written in single quotes.

grade = 'A'
initial = 'J'
separator = ','

A character is not the same as a single-character string. 'A' (character) and "A" (string of length 1) behave identically in Python, but the distinction matters when the spec or an exam question uses the term explicitly.

Type	Value example	Valid operations
Boolean	True	AND, OR, NOT; used in IF conditions
Character	'B'	Comparison ('A' < 'B'), concatenation into strings

A string is an ordered sequence of zero or more characters. Strings store text: names, messages, codes.

name = "Alice"
empty = ""            # zero characters — still a valid string
postcode = "SW1A 1AA"

String-specific operations (covered in full in the string manipulation lesson):

• Length: len("hello") returns 5
• Indexing: "hello"[0] returns 'h' (zero-indexed)
• Concatenation: "hello" + " " + "world" returns "hello world"

Type gotcha — numbers stored as strings:

a = "5"
b = "3"
print(a + b)     # prints "53" — string concatenation, NOT addition

Storing a number as a string prevents arithmetic. The + operator on strings concatenates rather than adds.

When reading input from a user with input(), the result is always a string. Convert it with int() or float() before doing arithmetic.

Casting converts a value from one data type to another. The conversion is temporary — it produces a new value of the target type without changing the original variable's type.

Common casting operations:

Function	Converts to	Example	Result
int(x)	Integer	int("42")	42
int(x)	Integer	int(3.9)	3 (truncates, does not round)
float(x)	Real	float("3.14")	3.14
str(x)	String	str(100)	"100"
bool(x)	Boolean	bool(0)	False

Worked example — casting to perform arithmetic on user input:

age_str = input("Enter your age: ")   # returns "17" as a string
age = int(age_str)                    # cast to integer
birth_year = 2026 - age              # arithmetic now works
print("Born in", str(birth_year))    # cast back to string for output message

Without int(), 2026 - age_str would cause a TypeError because you cannot subtract a string from an integer.

int(3.9) gives 3, not 4. Casting truncates toward zero — it does not round. Use round() if rounding is needed.

Exam questions frequently present a scenario and ask you to identify the most appropriate data type for each piece of data.

Worked example — a school database stores student records with these fields:

Field	Value example	Best data type	Reason
Student ID	10245	Integer	Whole number, used for identification
Full name	"Ahmed Khan"	String	Text — sequence of characters
Year group	11	Integer	Counted whole number
Average grade (%)	72.5	Real	May have a decimal
Has parental consent	True	Boolean	Only two possible values: yes/no
Form tutor initial	'S'	Character	Single letter
Date of birth	"2009-04-15"	String	Text representation of a date

Date of birth is stored as a string in simple programs because date arithmetic is complex. A more advanced system would use a dedicated date type, but this is beyond OCR J277 scope.

1. Choosing integer for prices or measurements

£3.99 cannot be stored as an integer without losing the pence. Whenever a value has a decimal, choose real.

2. Confusing character and string

A character is exactly one symbol. A string can be any length, including zero. If a field holds a name, it is a string. If it holds a single initial, it may be a character.

3. Forgetting that input() always returns a string

User input is always a string in Python. If you need to compare it to a number or use it in arithmetic, cast it first: int(input(...)).

4. Thinking int() rounds to the nearest whole number

int(3.9) returns 3, not 4. Casting to integer truncates (drops the decimal). To round, use round(3.9) which returns 4.

Mistake	Correction
Storing price £4.99 as integer	Use real — it has a decimal part
age = input(...) then 2026 - age	Cast first: age = int(input(...))
int(7.8) == 8	int(7.8) == 7 — casting truncates, it does not round