safest way to assign an integer (worried about truncation)

Question

I really just want to do something like

x <- as.integer(c(1,2,3))

But because c(1,2,3) is stored as a floating point vector I'm worried that I'll have problems with truncation, such as

> as.integer(1.99999999999)
[1] 1

How do I know I'm safe?

I worry sometimes too. See also http://stackoverflow.com/q/6155643/210673 — Aaron left Stack Overflow, Mar 31 '12 at 12:01
@Aaron great thanks! And thanks for the spelling correction. I truncated truncation :) — Xu Wang, Mar 31 '12 at 16:45
Glad you found the link helpful, though the spelling correction was @Tomas. — Aaron left Stack Overflow, Mar 31 '12 at 18:53

score 10 · Accepted Answer · answered Mar 31 '12 at 06:28

10

you can use a suffix L:

> x <- c(1L, 2L, 3L)
> is.integer(x)
[1] TRUE

> x <- 1L:3L
> x
[1] 1 2 3
> is.integer(x)
[1] TRUE

Or if you already have a numeric vector and convert it into integer, you can explicitly describe the rule:

> x <- c(-0.01, 1.999, 2, 3.000001)
> as.integer(round(x))
[1] 0 2 2 3
> as.integer(floor(x))
[1] -1  1  2  3
> as.integer(trunc(x))
[1] 0 1 2 3
> as.integer(ceiling(x))
[1] 0 2 2 4

answered Mar 31 '12 at 06:28

kohske

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As you show, `round` and company return floating point numbers so need to be coerced to integer. Is there any need to worry about that coercion doing something unexpected? – Aaron left Stack Overflow Apr 01 '12 at 01:42
What kind of unexpected behavior you think of? – kohske Apr 02 '12 at 09:51
Same as above, truncation. Could the result from `round` and the others ever be stored as 1.999999999 (or the binary equivalent) so `as.integer` would return 1 instead of 2? – Aaron left Stack Overflow Apr 02 '12 at 11:57
you can find the implementations here: http://svn.r-project.org/R/trunk/src/nmath/fround.c http://svn.r-project.org/R/trunk/src/main/arithmetic.c I think you don't need to worry about that. – kohske Apr 02 '12 at 19:42

safest way to assign an integer (worried about truncation)

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