CS4Data Types, Variables, Constants and Input/Output

Each variable has a data type. The data type determines what kind of value the variable will hold and the set of operations that can be performed on it.

• The type decides what you can do with a value. You can divide two integers, but not two strings. You can join two strings with +.
• A string that looks like a number is still a string: "7" + "1" gives "71", but 7 + 1 gives 8.
• Exams use AQA’s general names (integer, real, Boolean, character, string). When a question asks for the Python data type, give Python’s name.
• A real number can be called a float. A whole-number value written with a decimal point, such as 2.0, is real, not integer.

Exam questions use AQA’s general names. In Python code, use Python’s names

Data type

Determines what kind of value a variable will hold and the set of operations that can be performed on it.

Integer

A whole number.

Real (float)

A number that can include a fraction (values after the decimal point).

Boolean

A data type that can represent the two values True and False.

Character

A single character from a character set, such as ASCII.

String

A sequence of characters.

Pick the type that fits the values and what the program will do with them.

• Numbers you never calculate with (phone numbers, bank sort codes, ID numbers that start with 0) are strings. An integer would lose the leading 0.
• Money with pence is real. Storing £3.85 as an integer would lose the 85p.
• A comparison such as score > 50 gives a Boolean, so its result can only take two values: True or False.
• Every expression has a type, worked out from the value it gives.

A variable is an identifier that represents a memory location that can store a value. This value can change during execution of a program.

• Assignment: an assignment statement sets the value of a variable or constant. Pseudo-code uses ←; Python uses =.
• Declaration creates a variable. In Python (and in AQA’s pseudo-code) a variable is declared the first time a value is assigned to it.
• Read an assignment right to left: work out the right-hand side, then store the answer in the variable on the left.
• Assigning a new value replaces the old one. The old value is lost.
• For exam purposes, the first value assigned fixes the type: totalCost = 2.45 makes totalCost real, and it should hold real values from then on.
• In Python, = assigns and == compares. In pseudo-code, ← assigns and = compares.

A variable’s value can be replaced while the program runs. A constant’s can’t

a ← 4
b ← a * 3
a ← b - 2
b ← b + a

Variable

An identifier that represents a memory location that can store a value. This value can change during execution of a program.

Assignment

An assignment statement sets the value of a variable or constant.

Variable declaration

Creating a variable. In Python, a variable is declared when it is first given a value, e.g. age = 12.

A constant is an identifier that represents a memory location that can store a value. This value cannot change during execution of a program.

• AQA pseudo-code: CONSTANT VAT_RATE ← 0.2. Constant names are always in capitals.
• Python has no true constants. By convention a name in capitals with underscores marks a constant: VAT_RATE = 0.2. Never change it later in the program.
• Why use a named constant?
• 1. The value is set once and can’t be changed by accident while the program runs.
• 2. If the value ever changes (a new tax rate), you edit one line, not every place the number appears, so you can’t miss one.
• 3. The name explains the number: VAT_RATE says what 0.2 is for, so the code is easier to read and maintain.
• Why use a variable? For any value that changes as the program runs: inputs, totals, counters.

if mark >= 40:
    print("Pass")

PASS_MARK = 40
mark = int(input("Mark: "))
if mark >= PASS_MARK:
    print("Pass")

Constant

An identifier that represents a memory location that can store a value. This value cannot change during execution of a program.

Named constant

A constant given a meaningful name, such as MAX_PLAYERS, used instead of typing the value each time.

An identifier is a name for a variable, a constant or a subroutine.

• A meaningful identifier says what it holds: ticketPrice, not t or x.
• Why? The code is easier to read and understand, for you later and for other programmers, so it is easier to debug and maintain, and mistakes are less likely.
• AQA’s Python style: variables in camelCase (numberOfTickets); constants in CAPITALS with underscores (MAX_SPEED).
• Python’s rules: no spaces; only letters, digits and underscores; can’t start with a digit; can’t be a keyword such as if, for or while. Capitals matter: total and Total are different names.
• Keep names short enough to write accurately: seatsLeft beats numberOfSeatsLeftInTheTheatre. Examiners see long names misspelt.
• Single letters are fine where they are meaningful: i as a loop counter, x and y for coordinates.

Identifier

A name given to a variable, constant or subroutine.

Meaningful identifier

A name that describes what the variable, constant or subroutine holds or does.

camelCase

Writing a name with no spaces, starting each new word with a capital: numberOfItemsSold.

Programs read data typed at the keyboard. In Python, input always gives a string.

• input() returns a string even if the user types 25. To do arithmetic, convert it (this is called casting): int() for whole numbers, float() for numbers with a decimal point.
• The text inside input("…") is a prompt for the user. It is optional, and examiners ignore it unless a question asks for it. Keep prompts short.
• Without a cast, age + 1 on a string is a type error, and "3" * 2 gives "33", not 6.
• Checking that the input is sensible (a range or a length) is validation, covered in the topic on robust programs.

first = input()
second = input()
print(first + second)

Casting

Converting a value from one data type to another, e.g. int("25") gives the integer 25.

USERINPUT

AQA pseudo-code for reading a value typed at the keyboard.

Programs show data and messages on the screen with OUTPUT (pseudo-code) or print() (Python).

• print(a, b) prints each item with a space between them, then moves to a new line. Pseudo-code: OUTPUT a, b.
• Concatenation with + joins strings with no space. Numbers must be converted first: "Total: " + str(total).
• f-strings put values inside text: f"{name} scored {score}". The f before the quotes is needed.
• end="" stops print moving to a new line.
• A comma in an assignment is not concatenation: msg = "Hi", name makes a pair of values, not one string. Use +.
• Never write OUTPUT in a Python answer. It is pseudo-code, and it can’t gain the mark.

name = "Sam"
score = 18
print("Well done", name)
print("Score: " + str(score))
print(f"{name} scored {score} out of 20")
print("Done", end="")
print("!")

Well done Sam
Score: 18
Sam scored 18 out of 20
Done!

Concatenation

Joining two strings into one with +.

f-string

A Python string starting with f in which {identifier} is replaced by the value, e.g. f"Hi {name}".

OUTPUT

AQA pseudo-code for showing a value on the screen. In Python, use print().

Programs are built from a few statement types: variable declaration, constant declaration, assignment, iteration, selection and subroutine (procedure or function). They are combined in three ways.

• Sequence, selection and iteration are basic to all high-level imperative programming languages.
• Selection is covered in Operators and selection; iteration in Iteration; subroutines in Subroutines.
• In a numbered program, the line that uses selection is the IF line, and the line that uses iteration is the FOR, WHILE or REPEAT line.

The three combining principles as flowchart fragments

1  CONSTANT LIMIT ← 3
2  tries ← 0
3  WHILE tries < LIMIT
4     tries ← tries + 1
5     IF tries = LIMIT THEN
6        OUTPUT 'Last try'
7     ENDIF
8  ENDWHILE

Sequence

Statements carried out one after another, in the order written.

Selection

A selection statement allows a block of code to be executed only when a certain condition is satisfied.

Iteration

The repetition of a block of code.

Short code questions (2–6 marks) usually want input → process → output. Marks go to each correct piece.

• 1. Read the bullets in the question and tick each one off as you write it.
• 2. Input each value into a meaningful variable, cast to int or float if it is a number.
• 3. Use a named constant for any fixed value you’re given, if the question asks for one.
• 4. Process: the calculation, with brackets where the order matters.
• 5. Output exactly what is asked, with print.
• 6. Write in the form asked for: ‘in Python’ means Python, not pseudo-code.

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