GCSE Computing A451 Unit 7.1 – Algorithms www.mrfraser.org

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Unit 7.1 – Algorithms

What is an Algorithm?

An algorithm is a sequence of instructions (operations) that can be executed (carried out) to

perform a particular task and then end.

There are 2 main categories of algorithmic operations:

· Sequential Operations – instructions that are executed in order.

· Control Operations – instructions do not have to be carried out in order.

These can be divided further into:

o Conditional Operations - a control structure that checks if a condition is TRUE

or FALSE and then executes an instruction based on the answer (IF...ELSE).

o Iterative Operations - a control structure that repeats a set of instructions

until a condition is met (loops).

Representing Algorithms

To write an algorithm in everyday language would be time consuming, and possibly

inaccurate due to being open to interpretation in different ways.

To write an algorithm in a specific programming language would require specific knowledge

of the syntax of that language and would be too complicated.

There are 2 main ways to represent algorithms so that they are accurate, succinct and easy

to understand for everyone (including non-programmers!)...

Candidates should be able to:

a. Understand algorithms (written in pseudo-code or as flow diagrams), explain what

they do, and correct or complete them

b. Produce algorithms in pseudo-code or flow diagrams to solve problems.

GCSE Computing A451 Unit 7.1 – Algorithms www.mrfraser.org

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System Flow Charts / Flow Diagrams

· Visual representation of a system

· Small number of symbols used to represent individual steps

· Arrows show the flow of control within the algorithm (the order of the instructions).

· Easier to trace a sequence of steps & locate logic errors than in pseudo-code.

Symbol Name Symbol Picture Comments

Start / Stop Symbol

· Used at start and end of flowchart

· Only one start point!

· Multiple stop points possible

Process Symbol

· Instructions to be executed

· No decisions required

· Name of process written in box

Input / Output

Symbol

· IO operation to be performed

· Text inside symbol state if I/O

e.g. INPUT subTotal

Decision Symbol

· Point at which decision is needed

· One entry point

· Minimum of 2 exit points

· Text should be a question

· Yes/No or True/False

Case Symbol

· Represents Switch / Case statement

· One entry point

· Multiple exit points

· Text shows name of variable name

being used in case statement

Subroutine Symbol

· Shows when a subroutine is called

· Text shows name of subroutine

· Subroutine has its own flowchart

· Parameters should also be shown

STOP

START

Process

Name

I/O Name

Name

CASE

x > 3? Yes No

GCSE Computing A451 Unit 7.1 – Algorithms www.mrfraser.org

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Pseudo Code

Pseudo-code represents an algorithm by using adapted everyday language and common keywords

from programming languages. Instructions are easy for us to read and understand, but still look

something like the statements available in many programming languages. Pseudo-code is not a strict

list of commands since it is not meant to be understood by a computer.

There are therefore no universal standards for pseudo-code, but it should follow these guidelines:

- Sequential operations arranged on separate lines with first instruction at the top

- Indentation should be used with blocks of instructions that are part of control

operations (such as conditional and iterative operations)

- Keywords should be in UPPERCASE and the rest of the pseudo-code should be in lower

case

- Pseudo-code should not be ambiguous

- Pseudo-code must actually be computable! (possible for it to be carried out)

- Descriptions/comments can be included but should be as brief as possible

- It must be complete, with nothing is left out.

- It must have an end, so it does not continue forever!

· Pseudo-code will often end with a list of all the variables used in the algorithm.

· Once algorithms are written (in pseudo-code or flow charts) they should be tested using dry-

runs. This involves tracing through the entire algorithm from start to finish with sample test

data.

· Once a programmer is happy with the pseudo-code it can then be developed as a working

program using the source code of the programming language in use.

Keywords Description

WHILE / ENDWHILE Iterative operation (loop) where condition is tested at start of loop.

Instructions within loop are repeated until condition becomes FALSE.

REPEAT / UNTIL Iterative operation (loop) where condition is tested at end of loop.

Instructions within loop will be repeated until condition becomes FALSE.

CASE OF Multiple decision (selection) branch based on the value of an expression.

READ, OBTAIN, GET Used to input data

PRINT, DISPLAY, SHOW Used to output data

COMPUTE,

CALCULATE,

DETERMINE

Used to carry out calculations

SET, INIT Used to initialise variables

INCREMENT,

++ / +=

Used to increment a variable

DECREMENT,

-- / -=

Used to decrement a variable

IF / THEN / ELSE / ENDIF Used to select different instructions based on result of Boolean test

FOR / NEXT Iterative Operation (loop) based on a count, rather than a condition.

Instructions within loop are executed a specific number of times as

determined by count index.

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