3.2.2 Programming Concepts

Variable - A named space in memory, large enough to store a single piece of data.
Each variable is a value that can change during the running of a program, depending
on conditions or on information passed to it. Typically, a program consists of
instructions that tell the computer what to do and data that the program uses
when it is running.

Declaration - Code that causes a variable or constant to exist. Once a variable or
constant has been declared, it can be used. Trying to use a variable or constant
before it has been declared will cause an error.

Assignment - The process of putting a value into a variable or constant. Most
languages use a single equals = to do this. In the instruction i = 5, the value
5 has been assigned to the variable i. If this is the first time that i has
been mentioned, this is also where it is being declared.

• Any data that a program uses must be stored in memory locations, each given its own identifier (unique name) while the program is running.
• As the program runs, the values in these locations might change, which is why they are called variables.
• For example, a variable called total might change several times as numbers are added to it.
• A variable called surname might change as a program processes a list of customer orders.
• Each variable has an identifier (unique name) that refers to a location in memory where the data will be stored.
• Each variable also has a data type that defines the type of data that will be stored at the memory location so that operations that can be performed on it.
• For example, you can multiply two numbers together, but you cannot multiply two words.
• Each variable is given an identifier which is unique and meaningful to that variable, for example, total rather than t, surname rather than s.
• Additionally, variables must not include any keywords that will interfere with the programming language for instance and, input and print.

• Variables can be declared in one of two ways:
  • Local: Declared inside a function (subprogram), and can only be accessed inside that function.
  • Global: Declared outside any function (subprogram), usually at the start of the main program. They can be assessed throughout the whole program.

• Variable identifiers should also not:
  • Begin with a number.
  • Contain spaces.
  • Contain special characters or symbols.

Constant - A named space in memory with a value that can never change during the running
of a program. Useful for PI (3.142) which never changes, or VAT (20%) which seldom
changes. The Python language does not allow for constants, but some other languages do.

• In some programs, there are certain values that remain the same (constant) while the program runs.
• The programmer could use the actual value in the code each time, but it is good practice to give the value a unique name (identifier) and then use that name throughout the program.
• Constants are declared at the start of a program and can then be referred to as needed in the code.
• The same naming rules apply to naming constants, although a very common approach is to use CAPITAL LETTERS. This makes it easy to see where a constant is used in the code rather than a variable.

• Identifiers are defined at the start of the code, and can then be used throughout the program by the programmer to make coding easier.
• If the value of a constant needs to be changed, it must be done at the beginning of the code, where the constant is defined.

• The two main benefits of declaring a constant are:
  1. When its value changes, you only have to edit it in one place rather than looking for every place in the program where you used that value.
  2. The code is easier to read and understand because the constant's identifier will be used instead of a number. This makes your code easier to debug and maintain.

Sequence - In a sequence structure, an action, or event, leads to the next ordered
action in a predetermined order. The sequence can contain any number of actions, but
no actions can be skipped in the sequence.

Selection - Is when you only want some lines of code to be run when a condition
is met, otherwise you want the computer to ignore these lines and jump over them.
This is achieved using IF and ELSE statements. e.g. If a condition is met then lines
4, 5, 6 are executed otherwise the computer jumps to line 7 without even looking at
line 4,5 and 6.

Iteration - Is when you want the computer to execute the same lines of code several times.
This is done using a loop. There are three types of loops: FOR loops, WHILE loops and REPEAT
loops. Iteration is handy as it enables you not to have to copy the same lines of code many times.

Nesting - This involves placing one programming structure inside another one. The code
below contains an IF statement on line 8 that is NESTED within the WHILE loop. There is
no limit to the number of levels of nesting that you could use with IF statements within
either itself or another WHILE loop.

• Below is another example of having one loop nested inside another, this algorithm displays all the multiplication tables between 2 and 12:

Subroutines - In computer programming, a subroutine is a sequence of program instructions
that performs a specific task, packaged as a unit. In different programming languages, a
subroutine may be called a procedure, a function, a routine, a method, or a subprogram.
The generic term callable unit is sometimes used. (Source Wikipedia.org)

• The following code demonstrates how a function is created and called in the main program:

• Note: This topic will be covered in greater details in the 3.2.7-Subroutines (Procedures and Functions)