MATLAB: For loop not running properly

beginnerfor loopMATLABwhile loop

I am still a beginner at MATLAB, and have written a simple function that pays a speficied bill using certain denominations.
Here's my code:
function [] = bill_payment ()
bill = input('Please enter the bill amount: ');
p = [50, 20, 10, 5, 2, 1, 0.5, 0.2, 0.1, 0.05, 0.02, 0.01]; 
i = 1; 
for j = 1:length(p)
while bill > 0 
    if p(i) <= bill 
        bill = bill - p(i); 
        disp(p(i))
    else i = i+1;
    end
end
end
The issue I have is that when I set bill to be a number ending in 0.09 (e.g. 17.99), I get the following output: 
10
     5
     2
    0.5000
    0.2000
    0.2000
    0.0500
    0.0200
    0.0100
And the following error message: 
Index exceeds the number of array elements (12).
Error in bill_payment (line 16)
    if p(i) <= bill
What I don't understand is why my code moves onto the 0.01 option when p(11) = 0.02 and therefore p(11) = bill at that point in the code. Surely the output should be:
   10
     5
     2
    0.5000
    0.2000
    0.2000
    0.0500
    0.0200
    0.0200
As that summed = 17.99 and the loop should stop when bill = 0?

Best Answer

17.99 does not have an exact binary floating point representation
>> format long
>> 17.99
ans =
  17.989999999999998
So, you need to apply tolerances
function [] = bill_payment ()
bill = input('Please enter the bill amount: ');
p = [50, 20, 10, 5, 2, 1, 0.5, 0.2, 0.1, 0.05, 0.02, 0.01]; 
i = 1; 
for j = 1:length(p)
while bill+0.001 >= p(end)       %<---- modify

    if p(i) <= bill +.001        %<---- modify
        bill = bill - p(i); 
        disp(p(i))
    else i = i+1;
    end
end
end
Related Question