Variables, Types and Strings

We will already be familiar with the concept of a variable (see: Variables in Python). It is It is a container, or a label, used to store data in our programs.

Examples of variables might include:

• age = 42
• answer = 'Y'
• full_name = 'Harry Jones'
• is_over_age = True
• area_of_circle = 3.142 * radius^2

A variable consists of an identifier, it's name and can be assigned a value. This value might change during the course of the program.

AQA Pseudocode

a <- 3
b <- 'Hello'

Note

For assignment AQA uses the backward pointing arrow - it's a sensible choice

OCR Pseudocode

a = '3'
b = 'Hello'

Data Types

All programming languages need to handle data, and that data can take different forms. Consider the examples above. What type of data is being assigned to each of these variables? Is it numeric? Is it text? If it's a number, is it a whole number of a number with a fractional part?

Every item of data needs to be stored in the computer's memory. How much memory is going to allocated to each of these variables? How does the operating system determine this and set aside the right amount of memory?

The answers here may depend on whether we are programming using a statically-typed language or dynamically-typed language. The different between these:

• Statically-typed: The amount of memory will be known when the code is compiled into an executable
• Dynamically-typed: The amount of memory will be allocated while the program is running

For statically typed languages the amount of memory is effectively built-in and we, the programmer, cannot change it. A typical summary is shown in the following table:

Data Type	Expected Type of Data	Typical amount of memory	Example
integer	Whole number, positive or negative	4 bytes	42, -89, 0
float/real	Number with a fractional part	8 bytes	3.142, -89.7
char	Individual character	1 byte	'a', '9', 'Z', ';'
string	Zero or more characters	1 byte per character in the string	'John', '1234'
boolean	True or False	1 byte	True, False

Note

Different languages might be set up to use different amounts of memory

For dynamically typed languages the amount of memory allocated to a variable is determined at run-time. So, if the program encounters the assignment: age = 42 a 4-byte block of memory will be allocated and labelled as age. If later in the program the user decides (and I'm not sure why they might do this) to change the value assigned to age to be age = "forty two" then the operating system would need to allocate 10 bytes of memory.

Whichever type of language we are dealing with the type of data being handled by a variable is an important consideration. It can lead to some issues:

• An integer occupies 4 bytes, 32 bits
• These 32 bits provide an allowable range of -2,147,483,648 through to 2,147,483,647
• What if you want to store a value higher or lower than this?

You can't! Well, if you know in advance this might be possible than you will have to use another integer data type provided by the language (not listed in the table above).

• Also, if we're only dealing with values between, say, 1 and 100, we're wasting three bytes of memory for each of these values!

A valid consideration. Again use an alternative data type. For example C# provides both Int16 and Int32, the first uses 16 bits, the second 32 bits.

The point is, data is important, the data type of that data is also important to us programmers. Select the correct data type wisely.

Constants

As well as variables we can declare and assign constants. As the name suggests, the value assigned to a constant can not be changed.

In this regard Python misbehaves. It does not endorse constants in the same way other languages do. it is perfectly possible in Python to declare a value for PI = 31,42, and then later in the program change that value to something completely different. This is not good practice.

Other languages may expect, when you declare a constant, to preface it with the keyword const:

• const PI = 3.142
• const VAT_RATE = 0.2

Any attempt then to change these values will lead to a syntax error. This is good practice.

Note

It is good practice to use upper case letters for the names of constants, whether in Python or not. It serves as a reminder that the value we're dealing can not (should not) be changed.

The benefits of a constant are:

• The value cannot be changed accidentally during the course of a program
• Defining the constant once, usually at the start of a program, means any subsequent updates e.g. the chancellor changes the rate of VAT, are only done in one place

AQA Pseudocode

CONSTANT PI <- 3.142 
CONSTANT CLASS_SIZE <- 30

OCR Pseudocode

OCR make no distinction in their pseudocode between variables and constants

Tip

For OCR always upper case the identifier to make it clear it is a constant

Input/Output

All languages need some way of communication with their user, either by getting the user to enter data at the keyboard or to display results to the screen or to a file or to a printer etc..

Each will have their own way of doing this, classically:

name = input("What is your name")
print(name)

• The function input waits for the user to type something, then assigns that to the variable name.
• The function print() sends whatever is included in the brackets to the screen

Note

If the brackets are empty, as in print(), a new line character is sent and the cursor will move down by one line on the screen

AQA Pseudocode

a <- USERINPUT  // get a value from the user and assign it to `a`
OUTPUT a        // output value of `a` to the screen

Somewhat unusual perhaps ...

OCR Pseudocode

name = input("Enter your name: ")       // get value from user, assign it to `name`
print(name)                             // display `name` to the screen

Nothing to surprise us here.

Operators

Operator	Description	Example	Result
+	Addition	7 + 3	10
-	Subtraction	7 - 3	4
-	Multiplication	7 * 3	21
/	Division	7 / 3	2.33333..
^	Exponentiation	7 ^ 3	343

Note

When two integers are divided, the result is a real (floating point number)

There is also a pair of integer division operators:

Operator	Description	Example	Result
DIV	Floor division	7 // 3	2
MOD	Modulo (Remainder)	7 % 3	1

Note

Both AQA and OCR pseudocode uses the mnemonic MOD and DIV for integer division operations, equivalent to // and % in Python

Strings

A string is different to an integer. Perhaps obvious, but consider:

• the string '42' is represented in binary as \(0111010101110011_2\),
• the integer 42 is represented in binary as \(00101010_2\).

Everything the user enters at the keyboard will be received by the computer as a character, or string if more than one character is entered.

To convert string data into numeric data, and vice versa, we use one of several built-in functions:

AQA Pseudocode

Function	Description
STRING_TO_INT(str)	converts the string, str to an integer
STRING_TO_REAL(str)	converts the string, str to a float/real
INT_TO_STRING(num)	converts the numeric value, 'num' to a string
REAL_TO_STRING(num)	converts the numeric value, 'num' to a string
CHAR_TO_CODE(ch)	converts the character, ch to its ASCII value
CODE_TO_CHAR(num)	converts the integer, num, to its associated ASCII character

OCR Pseudocode

Function	Description
int(str)	converts the string, str to an integer
float(str)	converts the string, str to a float/real
str(num)	converts the numeric value, 'num' to a string

String concatenation

Two strings can be joined together using the concatenation operator, +:

• full_name = first_name + ' ' + last_name
• age = '4' + '2'

String functions

There are a number of standard functions for working with strings. These functions will be made available in most programming languages. If you know you want to find the length of a string using, say, C# or Java then a quick search on the Internet will yield the answer (yes, we all do that!)

Function	Description	Example	Output
str.length	returns the length of the string, str	n = 'john'.length	4
str.substring(start, end)	returns a slice of the string, str,	print('john',substring(0,2))	'jo'
str.left(3)	returns the first 3 characters of the string, str	print('john'.left(1))	'j'
str.right(3)	returns the last 3 characters of the string, str	print('john'.left(1))	'ohn'
str.upper()	Convert the string, str to upper case	print('john'.upper())	'JOHN'
str.lower()	Convert the string, str to lower cacse	print('John.lower())	'john'

AQA Pseudocode

AQA alters the syntax of some of these string handling functions:

Function	Description	Example	Output
LEN(str)	returns the length of the string, str	n = LEN('john')	4
SUBSTRING(start, stop, str)	returns a slice of the string, str,	print(SUBSTRING(0,2,'john'))	'jo'

You should not expect to see any of the others in the AQA pseudocode.

OCR Pseudocode

Both length and substring are used in OCR and the syntax is:

Function	Description	Example	Output
str.length	returns the length of the string, str	n = 'john'.length	4
str.substring(start, num)	returns a slice of the string, str, num indicates how many characters	print('john',substring(0,2))	'jo'

Comments

Any lines in your code that begin with the comment indicator are ignored when the program is compiled and/or executed.

Get into the habit of including comments in your code, they are really useful. All programs should contains comments:

• to describe the purpose of the program
• include your name and date of the author and when the program was written
• include a revision history, what changes were made and when

AQA Pseudocode

Adopts the Python comment: #

OCR Pseudocode

Uses // as in languages such as Javascript etc..

Questions

1. What does a variable in programming do?

Stores data

Changes the data type

Deletes data

Executes code

A variable is used to store data in a program.

Submit

2. Which symbol is used for assignment in AQA pseudocode?

=

<-

:=

->

In AQA pseudocode, the assignment operator is '<-'.

Submit

3. What is the typical memory size allocated for an integer data type?

1 byte

2 bytes

4 bytes

8 bytes

An integer typically occupies 4 bytes of memory.

Submit

4. How is a string represented differently from an integer in binary?

Strings use less memory

Integers are represented with fewer bits

Strings use more bits

Integers and strings are represented the same

Strings are represented with more bits compared to integers because they store multiple characters.

Submit

5. What does the STRING_TO_INT function do in AQA pseudocode?

Converts an integer to a string

Converts a string to an integer

Converts a string to a float

Converts a float to a string

STRING_TO_INT converts a string to an integer in AQA pseudocode.

Submit

6. Which operator is used for floor division in pseudocode?

+

/

DIV

%

The DIV operator is used for floor division, which gives the quotient without the remainder.

Submit

7. What is the result of 7 / 3 in most programming languages?

2

2.33

21

1

The division operator / typically results in a floating-point number when dividing two integers.

Submit

8. Which function in OCR pseudocode converts a string to a float?

float(str)

str(float)

int(str)

real(str)

In OCR pseudocode, float(str) converts a string to a float.

Submit

9. How does Python handle constants compared to other languages?

Python does not enforce constants.

Python uses the const keyword.

Python allows changing constants without restriction.

Python does not support constants.

Python does not enforce constants in the same way as other languages; constants can be changed.

Submit

10. What does the str.upper() function do?

Converts a string to lowercase

Converts a string to uppercase

Returns the length of a string

Extracts a substring from a string

str.upper() converts all characters in the string to uppercase.

Submit