[R] Odd results with %% and conserving memory

[Patrick Burns]

You seem to be in Circle 1 of 'The R Inferno'.

Your technique does work, just not the
way that you expect.  Try doing:

range( (mat * 100) %% 1)

The 'zapsmall' function might be of interest
as well.

On 26/03/2010 21:05, JustinNabble wrote:
>
> Can anyone explain this?
>
> I have a matrix with double components. It's taking up a lot of memory, so I
> want to multiply then turn it to integers. I'm pretty certain that there are
> only 2 decimal places, but I wanted to check by using modulo. E.g.
>
> mat = matrix(11:50/100, ncol=4,nrow=10) #Matrix with values out to the
> hundredths
> any((mat * 100)%%1!=0)
>
> But oddly enough it doesn't work. Even in this simple example the result I
> get is this:
>               [,1]         [,2] [,3] [,4]
>   [1,] 0.000000e+00 0.000000e+00    0    0
>   [2,] 0.000000e+00 0.000000e+00    0    0
>   [3,] 0.000000e+00 0.000000e+00    0    0
>   [4,] 1.776357e-15 0.000000e+00    0    0
>   [5,] 0.000000e+00 0.000000e+00    0    0
>   [6,] 0.000000e+00 0.000000e+00    0    0
>   [7,] 0.000000e+00 0.000000e+00    0    0
>   [8,] 0.000000e+00 3.552714e-15    0    0
>   [9,] 0.000000e+00 1.000000e+00    0    0
> [10,] 0.000000e+00 0.000000e+00    0    0
>
> Two non-zero values are just very small, but one is value is actually 1. Can
> someone explain this?
>
> If you pick just a single number you can see some odd results too.
>
>> (4.1*100)%/%1
> [1] 409
>> (4.1*10*10)%/%1
> [1] 410
>
> Shouldn't the result be 410 each time?
>
>
> I think in this case it should have returned all 0s, and I could have done
> something like
>
> newmat = as.integer(mat*100)
> dim(newmat) = dim(mat)
> rm(mat)
>
> Is there a better way to convert my double matrix to an integer matrix
> without losing precision? Or are there better ways to conserve memory? I'm
> at the limit.
>
> Thanks,
> Justin

-- 
Patrick Burns
pburns at pburns.seanet.com
http://www.burns-stat.com
(home of 'Some hints for the R beginner'
and 'The R Inferno')