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We have already looked at the If Else structure

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but theres one other structure that is used for selection

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within C programming thats very very useful

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and that is the Switch structure.

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If Else is a very powerful selection tool

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selecting between two possible states

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based on whether a test is true or false

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and we've seen already that you can build 

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that up using if elses

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nested inside if elses to build up more complex structures

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but that becomes difficult to follow very easily

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and difficult to check the logic

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So there an alternative structure that we can use if we want

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to make a selection between several 

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different possible outcomes

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depending on some kind of input condition 

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and thats the switch statement.

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The switch statement can be viewed almost as 

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being like a multiple choice

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where when you enter the switch statement

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there'll be a particular variable or 

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value thats going to be evaluated

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its checked and if it meets the first criteria

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then that particular option will be executed

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and then you will come out

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of the switch statement having only executed that path

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if it doesn't meet the first criteria

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it will go on to evaluate the second criteria

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and the third criteria and if any of these are true

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then that particular path for example option three 

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will be taken

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if none of the criteria are met

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then there's a structure that allows for a default

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catch all if you like

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to be run if nothing else is found to be correct

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so only one of the optional blocks of code

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either option one in this case or option two or option three

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or the default block will be run nevermore.

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The structure of the switch statement

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is quite straightforward

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the keywords that we use are switch itself

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and then for each block of code we want to execute

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the start of it is marked by case

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and then whatever the evaluation 

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term that we're going to check

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and it ends with break

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there is one limitation though about using the switch

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the switch statement works with what are termed ordinals

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ordinals are things that take on an absolute value

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like Integers or individual characters

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not strings of characters for example.

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So heres the layout of the switch statement

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at the top we have the keyword switch

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and then in brackets

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we put the variable that we're going to evaluate

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and then between the opening brace

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and the closing brace we have the actual selection stages

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and heres the structure of each individual selection of the 

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term case is used and then what the first

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evaluation test will be

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if that conditions met in between

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that colon and the break statement

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you put all the code you want to execute if that is true

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if that is true that first option

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and when the code executes down

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through those various instructions and hits the break

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it then jumps right to the end

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the switch statement and continues on

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so only one path is taken

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if the first case isn't taken

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then you can have a second alternative a third alternative

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as many as you want basically

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until eventually if none of those alternatives are met

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we have the default option

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and that then concludes the catch all code

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what to do if none of the above is true

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and then break and continue on to the rest of the code

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lets have a quick look at a real example.

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Obviously to fit on the slide this is a very simple example

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but what we're going to do is display on screen

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whatever colour the colour sensor is currently seeing

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so we've got a complete program here and on the next slide

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we'll zoom in more closely at the switch statement

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to really look at the structure of it

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but just briefly lets look at the main program 

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we've obviously got the main program setup

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with its start and end braces

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and then inside that first of all we declare a variable

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in this particular case we're using the term short

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which simply means its an integer

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but requires less storage space because its not

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going to have a large range of values

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we call it integer currentColour

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so currentColour will store a particular numeric integer value

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and then we've got a loop setup

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which uses the trick of putting while and true

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now the while statement normally has a test built into it

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which is neither true or false

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if you make the while statement permanently true

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all that means is between the start of the while

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and the end of the while the program will run forever

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until we switch the computer system off 

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or reset the program

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so its a way of having a continuous loop

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and now we get to the switch statement itself

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now to allows us to look at it in detail

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what we'll do is move on to the next slide

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but do bare in mind that what were seeing is

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this switch statement we're reading in

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the value of a particular colour from the sensor

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and we're storing it in a variable 

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so that will contain a number

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with then going to manipulate and evaluate.

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Looking at the switch statement now itself

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by zooming in a little bit we're able to look at 

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the layout slightly more neatly

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at the top where the switch statement as I described before

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and then we have the variable

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that takes on a particular value, 

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now the particular sensor we're using

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the colour sensor reads in values

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of the current colour that it sees 

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and it assigns them a number

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between zero and seven so the colour sensor on the robot

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can only see eight colours

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the first colour it can see is in fact no colour

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so you could argue it sees seven colours 

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and an absence of colour

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but basically it returns a number between zero and seven

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thats all it can return

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so the sensor is read

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the value's stored in currentColour

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and then switch statement starts to run

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and it takes the value of the currentColour and just

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for the sake of argument lets say that 

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currentColour at this moment

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contains the value three

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as the program executes the switch statement 

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starts running

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and it takes the value of currentColour and compares it

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with the first case is it zero no

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is it one nope, is it two no

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is it three yes

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currentColour currently contains three so this case

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its true therefore this line here

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executes and displays on the screen the word green

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the next line we reach is break

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so that then breaks out of the 

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switch statement to the bottom

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if you recall the main program on the previous slide

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then add a loop so we would go back to the top

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we would evaluate currentColour again

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and it may be that the value has changed

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and its now become four

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in which case 

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we'd run down through the options once again

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and when we reached the case of four

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we then execute an instruction which says

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displays on screen

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the word yellow before breaking once more to the bottom

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so switch statement simply takes

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the variable that is included in the switch bracket

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and compares it against

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all the different cases that have been provided

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but do remember if the value thats stored

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has been tested by the switch statement

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doesn't match any of the listed ones

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there is almost the default statement at the bottom

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which allows you to catch any other condition

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that you haven't already anticipated

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in the various cases you've identified

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thats how the switch statement works

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and you'll find examples in the handbook

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and also you'll find lots of examples online.