04-02. Save the change

Online banking apps include a feature called Save the Change.

Whenever a transaction is made, the amount debited or taken from the account is rounded up to the nearest pound, with the difference being transferred to a savings account.

For example, if you purchase something for £8.90, then the 10 pence to round up to the nearest pound is transferred to savings.

We're going to write the Save to Change algorithm and and show you how to format numbers to two decimal places using what is known as a method.

As always, we start with the boilerplate code.

Next, we'll write a function that will take in the pounds and pence as a number and return the nearest pound from that amount.

So nine would be returned from 8.9.

Static int nearestpound.

This time we need some data.

So open bracket double because there'll be a decimal place in the number followed by the amount to be rounded up.

In our implementation of Save the Change, something will be saved in every transaction.

If the amount was exactly £9, then we'll round up to 10 and save a whole pound.

Therefore, a simple rounding command won't do.

Instead we'll convert the number to an integer and add one to it.

Casting command convert to int32 always truncates or removes any decimal places.

In effect, this rounds down.

Programming is as much about problem solving, thinking about what you want to achieve and then finding a solution to the problem.

Don't worry if you find it difficult to come up with a solution when you're learning programming.

This is actually the hardest part that comes with experience after you've seen and written lots of programmes.

Okay, now we'll write the function that, calculates the savings.

Static double save thechange and again we'll need the amount of the transaction, so this is in brackets.

Next we'll declare a variable for our savings double savingssemicolon.

We need to know if amount is a whole number to determine the amount to save.

If the whole number of the amount does not equal the actual amount, then savings becomes the nearest pound to the amount minus the amount.

It may be worth taking a moment here to understand the maths.

So if the amount is £8.90, then 8 does not equal £8.90.

So savings is the nearest pound, that's 9 minus £8.90, that's 0.1 or 10 pence otherwise.

Else savings is 1.

Because we always save a pound if the amount is a whole number, return the savings.

In the main programme, we can now use Console Write to ask the user to enter the purchase price.

We're going to store the input in a variable with decimal places.

So double purchase price equals convert todouble and the input in brackets.

Remember, inputs from the keyboard are always string data types, so they need converting to double numbers.

Now we'll calculate what the nearest pound is and store it in a variable called debit.

So int debit, an integer, because it will be a whole number, equals the value returned from our nearest pound function, which in brackets requires the purchase price.

Close the bracket colon.

You might notice that the nearest pound function we wrote earlier is expecting a parameter called amount.

But here we pass the argument purchaseprice.

That's okay.

What happens is purchase price is copied into the amount variable when the nearest pound function is called.

This is known as passing by value because we're parsing the value of one argument into one parameter.

Now we calculate the savings using our save the change function, again passing the value of the purchase price.

Finally, let's output the amount to be debited from the account and the amount to credit to savings console, writeline, open bracket, quote followed by our message.

Now, we want to output the variable debit to two decimal places.

We can do this using string format.

This method takes a string and uses formatting codes embedded within the string identified with curly brackets to format the output.

So open a bracket and a quote the word debit followed by a pound sign and then comes the formatting bit.

Open a curly bracket which signifies the formatting code, the first argument after the string.

So that's argument zero, a colon and then two decimal places, which has the code F2.

Close the curly bracket and close the string with a quote.

Now we specify the arguments for the string format method.

So comma, debit.

Whoa, that's quite a lot to take in.

Let's do it again.

For the savings console, writeline, open bracket, string format, open bracket followed by the string we want to format.

That's the words credit to savings and a pound sign.

Then a curly bracket to indicate where the formatting starts, zero, because there will be, the first variable here followed by a colon and then F2, which means format to two decimal places.

Close the curly bracket, close the string comma followed by the argument, which is savings.

Close the bracket and a colon.

Let's give the programme a run and check it works.