changing the decimal portion of numbers - why is my switch() statement not behaving as I expect?

Question

I have some data in the format

test <- c(10,11,12,13.1,13.2,13.3,14,15)

The integer count is equivalent to counting units of 4 subunits, and as such the 13.1 etc. do not mean 13-and-one-tenth but are instead 13-and-one-quarter (i.e. the count from 1 to 2 would be 1, 1.1, 1.2, 1.3, 2, etc...) I am attempting to prepare plots of these, and therefore to get the spacing correct I think I need to change them such that x.1 becomes x.25, x.2 becomes x.5 and x.3 becomes x.75. I could expand the integers but that will cause other issues down the line I think, so I'd prefer to do the .25/.5/.75 conversion.

I'm attempting to do this with switch() and not getting the expected result, so I guess my questions are twofold:

Why isn't this switch() working as expected?
What is the better way to do this that I have clearly missed?

I have tried to take the modulo 1 of the input, giving me just the .0, .1 etc, multiply by 10 and add 1 to get a number 1-4 and use that to select the relevant case option from the switch() statement as below.

Desired outcome: [1] 10.00 11.00 12.00 13.25 13.5 13.75 14.00 15.00

So the 13.1 and 13.2 terms seem not be evaluating as I expect.

# testing modulo approach
sapply(test, FUN = \(x) {
  (x %% 1)*10+1
}
)
[1] 1 1 1 2 3 4 1 1

and then

sapply(test, FUN = \(x) {
  switch((x %% 1)*10+1,
         x,                # option 1 
         floor(x) + 0.25,  # option 2
         floor(x) + 0.5,   # option 3
         floor(x) + 0.75)  # option 4
}
)
[1] 10.00 11.00 12.00 13.10 13.25 13.75 14.00 15.00

Possibly a case of [Why are these numbers not equal?](https://stackoverflow.com/q/9508518/17303805). — zephryl, Dec 16 '22 at 13:52
@RitchieSacramento ah, so there's a floating point error somewhere way down below such that I'm not REALLY generating integers? — Paul Stafford Allen, Dec 16 '22 at 13:56
@RitchieSacramento thanks, I found I could also get it to work by removing the decimal before modulo: `switch(((x*10) %% 10)+1,` etc — Paul Stafford Allen, Dec 16 '22 at 14:01
@PaulStaffordAllen - That was @zephryl's point, but yes indeed -see the result of `as.character((test %% 1)*10+1)`. You shouldn't need to use `switch()`, arithmetic should be sufficient. — Ritchie Sacramento, Dec 16 '22 at 14:02

score 2 · Accepted Answer · answered Dec 16 '22 at 13:57

2

Maybe try with floor

floor(test) + (test - floor(test)) * 2.5
[1] 10.00 11.00 12.00 13.25 13.50 13.75 14.00 15.00

answered Dec 16 '22 at 13:57

Andre Wildberg

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changing the decimal portion of numbers - why is my switch() statement not behaving as I expect?

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