This page examines Algorithms in more detail and looks at several different ways in which we can represent an algorithm. You need to be able to understand what an algorithm is describing and many students find this extremely difficult. Think of an algorithm as a detailed list of instructions that explain how a program can solve a problem.

A structure diagram is a simple diagram that represents how the different parts of a system are (literally) structured and connect with each other. They should include the three stages shown above, input, process and output.

The input section should include all of the things that can alter what the system actually does. This may be when a program actually asks you for something "What is your name?", "How many players are there?" but it may also include items which are fixed, like the value of Pi.

During the process phase, the program will carry out operations such as calculations and more complex processes such as sorting into a particular order. The process stage may involve repeating a block of code several times and completing different operations depending on the result of calculations.

Once the system has computed a result, something will need to be done with the result. This may just be displaying an "answer", or it could be altering what the program does, how it looks or what it does next. The output may also control something such as a motor or light.

This is the "terminator" shape that should be used at the very start and end of a flowchart.

All flowcharts must have a "Start" terminator (Labelled as Start) and most systems will also have a "Stop" terminator.

This shape is used when a program requires an input or an output.

An example of an input would be when a program asks the user to enter their name.

An example of an output would be when a program prints out an answer.

This is the process shape which is used when a program does something that does not involve a decision, an input or an output.

The process should be described briefly, and complex processes should be broken down into several smaller steps.

This is the decision shape, which is used whenever a program uses a statement that has a choice of possible routes. The box should be labelled with a brief statement that summarises the two possible choices. (Decisions should always have only two choices)

A decision shape is unusual because will have two arrows from it, which should be labelled as "Yes" or "No"

Arrows should be drawn to connect each shape. the direction of the arrow represents the "flow" of the program. Arrows should be drawn horizontally or vertically and angled or non straight lines should be avoided.

Algorithm Examples

We can also use pseudocode, or even full code to describe an algorithms.

Look at this program (its pseudocode, because Python wouldnt recognise the capitalised words)

What would you do if you followed this algorithm? (try it!)

What would your teacher do?

Here are some keywords that you should understand when discussing algorithms:

• Algorithm – the series of steps to solve a problem or perform an action.

• Flowchart – a diagram that shows the inputs, outputs and processes in an algorithms.

• Process – an action that takes place.

Pseudocode – simplified programming code that is not language specific, used to design algorithms.

Flowchart Example

This is a typical problem that you may have to solve when writing a program, and you may get something similar in the unit 2 exam.

Imagine that you are the computer, and follow the steps one at a time.

This slide show uses a trace table which is a great way of keeping track of the values of any variables as your program runs.

Step through the slides one at a time, using the ">" button.

Try to answer the question at the end.

followanalgorithm.pptx