Creating a MATLAB function that determines the amount of change a customer is owed after a purchase

Question

I have created a function in MATLAB, that displays the amount of change due (GB Currency) when the cost of an item and the amount of money given for the item are input.

function C = myChange(cost, paid) 
M = [50, 20, 10, 5, 2, 1, 0.5, 0.2, 0.1, 0.05, 0.02, 0.01]; 
C = []; 
for i = 1:size(M,2) 
    if (paid - cost) >= M(i) 
        C = [M(i); myChange(cost, paid-M(i))]; 
        break 
    elseif (paid - cost) == 0 
        break 
    end 
end 
end

The function seems to work perfectly when integers are used,

>> myChange(2, 4)

ans =

     2

>> myChange(20, 32)

ans =

    10
     2

However, sometimes, but not all the time, when decimal points are incorporated in the inputs, the amount of change comes up one penny (cent) short

 >> myChange(2, 3.25)

ans =

    1.0000
    0.2000
    0.0200
    0.0200

In this case, though, the correct change is once again displayed

>> myChange(27.57, 100) 

ans =

   50.0000
   20.0000
    2.0000
    0.2000
    0.2000
    0.0200
    0.0100

I'm not sure what could be causing this inconsistency, having played around with the function for while now to no avail. Any help is much appreciated, thanks.

Besides 0.5, none of the fractional elements of `M` have an exact binary representation. — Mad Physicist, Feb 14 '20 at 15:44
Money is fundamentally an integer quantity, so adjust your units accordingly. — Mad Physicist, Feb 14 '20 at 15:45
Does this answer your question? [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) — Mad Physicist, Feb 14 '20 at 15:46
So, to make the function work, I must let £50 be represented by 5000 and 1p be represented by 1, so that all monetary values are represented by an integer? — MGMaths, Feb 14 '20 at 16:13
Yes, that is one possible solution. An easy fix would be to add a tolerance of 0.005 — Daniel, Feb 14 '20 at 16:22
You can do it internally. Multiply all user input by 100 and round in the beginning. Problem solved. — Mad Physicist, Feb 14 '20 at 16:22

Creating a MATLAB function that determines the amount of change a customer is owed after a purchase

0 Answers0