[R] a problem with rep() ?

[Philippe Grosjean]

Sam McClatchie wrote:

>I am wondering if there is a problem with the function rep(). Both the
>commands using rep() below were expected to produce 8 twos, but only
>the second did.

>x <- rep(2,40*(1-0.8))
>length(x)

>y <- rep(2,40*0.2)
>length(y)

>Have I misused the function? Should the second argument of rep() be
>limited to a single number or a vector of the same length as the first
>argument as it documents in the help? Or is this a bug?

This sounds to me like a rounding problem. Recall that a computer always has
a limited precision in its calculations (except when using infinite
precision algorithms like in Mathematica) and it could sometimes produce
unexpected results. As an evidence of that,

z <- rep(2, 40*(1-0.79999999999999))

produces 8 twos, as expected. With other value, behavior of rep() is
"correct":

rep(2, 40*0.4)

and

rep(2, 40*(1-0.6))

both give 16 twos, as expected.

Best regards,

Philippe


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