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so we've now got our calculator working

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with the view model to separate the

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logic from the user interface that's

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given us a few advantages over the first

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version we don't have to worry about

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saving State for one thing the view

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model persists across configuration

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changes and doesn't lose its state when

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the activity is recreated it subscribes

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to the view model again and receives the

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latest values to display but our

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calculator works like a scientific

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calculator it's not quite as useful for

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financial calculations to see what I

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mean run the app and let's work out tax

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on a sale when the tax rate is 10% so

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I'm going to start the emulator as fast

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for this bit as always

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all right so there's our calculator on

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the screen so let's workout that tax to

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work out the tax on $56 we multiply by

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0.1 so let's try that in the calculator

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I'm going to type 56 multiplied by 0.1

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Eagles now for most scientific purposes

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that's accurate enough but it's not very

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good for financial calculations though

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accountants don't really like it when

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they see values like that but it can get

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more obscure than that so if we enter

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equals 56 equals to reset the value back

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to 56 we'll do that first and we're

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going to try it another calculation now

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this time we're going to multiply by

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0.01 so multiplied here Oh point zero

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one equals and you can see we let the

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value there of zero point five six

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so that looks right now we should be

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able to multiply that by a hundred to

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get our original value back of a

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multiplied by 100 equals so once again

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we've got a strange rounding error there

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the reason is that the decimal fraction

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0.01 is a recurring value in binary now

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I'm not going to go into the inner

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details of storing decimal numbers you

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can find out about I Triple E 754 to

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learn more now there is actually a

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Wikipedia article on I Triple E 75 for

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that describes the standard quite well

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I'll quickly bring this up on the screen

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and you can check this out at your

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leisure what we will do though is have a

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look at how we can make our calculations

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more suitable for financial work so

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we're go back to Android studio and Java

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has a big decimal class that solves

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these strange rounding errors so what we

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can do is create a version of our view

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model that uses big decimal instead of

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double and use that on our weapons did

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so I'm going to go ahead and do that and

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we'll start off by actually copying the

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calculator view model class and pasting

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it back into the cot the project so I'm

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gonna copy and paste you can see that

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IntelliJ has come up asked is what the

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new name is I'm going to call this one

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instead of calculator view model we'll

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call this one big decimal view model

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percent to that we've never got a big

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decimal view model class I think that's

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quite a descriptive name for what

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ultimately it's going to be now there

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aren't many changes to make it's quite a

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simple change in fact we're just going

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to be using the big decimal class

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instead of the double class and if

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you're offered a choice import from Java

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math well so what we need to do is make

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some changes firstly on line 15 we need

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to change the type from double there to

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a big decimal and you can see I'm

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selecting there the java dot math import

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i'll just take a copy of that because it

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going to be pasting that in various

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places we also need to make a change to

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line 19 the result should be I'll type

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big decimal now and no longer a type

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double and we have got some errors

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specifically we need to have a look in

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the operand pressed function first see

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at the moment we're using to double

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their course that should be value safe

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called dot to big decimal I'm gonna

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change that to big decimal I've got an

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error here which will fix shortly and

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then scrolling down to the neck press

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function we need to do a similar thing

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here we've got to double we in use here

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we need to change that to big decimal

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I'm gonna make that change to DBT small

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so we need to change this line here line

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59 because we can't multiply a bit a big

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decimal by double so that should be

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double values star equals and it should

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be big decimal dot value of then in

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parenthesis we're going to put them on

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as one so that's fixed up that fire

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function now I'm moving down now to our

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perform operation function will change

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the declaration from value from a double

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to a big decimal we need to change this

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test here on line 79 currently if a

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value equals equals is zero point zero

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need to change that to big decimal dot

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value of zero point zero in parentheses

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which fixes the error then on the next

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line need to handle the attempt to

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divide by zero it shouldn't be WLAN it

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should now be big decimal dot value of

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so in parentheses double 13a

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well that should be all the changes we

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need to make here there's no longer any

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errors in there so we now need to do is

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go back to our main activity class we

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need to change this line this view model

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which is currently using the calculator

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view model to use our bigdecimal view

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model so we're going to take a copy of

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that line and comment the first one out

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and we're going to change this now to

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use the incentive calculated view model

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bigdecimal view model and that's

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literally it

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it should now use the bigdecimal view

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model and be able to perform accurate

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financial calculations let's try that

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all right so we do the same resort we've

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done before we'll start off by typing 56

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multiplied by 0.1 equals this time we're

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getting the answer 5.6 without all the

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extra decimals so that's good the other

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example we used was was first to see the

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value back 256 we could do that with

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equals 56 equals and then we multiplied

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by 0.01 so multiplied by 0.01 equals to

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0.5 6 so that looks right multiplying

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that by a hundred we should get our

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original value back so times 100 equals

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this time it works we're getting fifty

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six point two zero zero with no rounding

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errors so switching back to main

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activity we could have a menu option to

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allow the user to choose between

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scientific and financial modes all we'd

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have to do is subscribe to the

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appropriate view model in the code and

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the calculators behavior would change

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you may wonder why we don't just leave

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it using big decimals and we certainly

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could do that for this simple app

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remember that we're learning about

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general techniques here you may develop

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this app further to include things like

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sine and cosines these wouldn't be much

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easier to an accountant and the

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bigdecimal class doesn't support them

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you can't find the cosine of a big

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decimal in other words so with this

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approach you can create a calculator

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that offers the best of both worlds just

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by switching the view model now before I

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finish this really is a simple

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and it may be overkill to use a view

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model class in it

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we did get some benefits though even

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with an app as simple as this one in the

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next video we'll start work on

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converting the top 10 downloader app to

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use a view model to manage its data see

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you in the next video