[R] ifelse help?

[Charles C. Berry]

On Mon, 19 Jan 2009, rkevinburton at charter.net wrote:

> I am having a hard time understanding what is happening with ifelse.
>
> Let me illustrate:
>
> h <- numeric(5)
> p <- 1:5
> j <- floor(j)

And j is 0:4 + epsilon , where 0 <= epsilon < 1, evidently.

> x <- 1:1000
> ifelse(h == 0, x[j+2], 1:5)
> [1] 2 3 4 5 6
>
> My question is, "shouldn't this be retruning 25 numbers?"


No. It should be

 	A vector of the same length and attributes (including class) as
 	test and data values from the values of yes or no

according to ?ifelse, and 'test' is what you have as 'h'

Consider

> z <- as.data.frame( diag(2 ) )
>  ifelse(z == 0, letters , 1:5)
      V1  V2
[1,] "1" "c"
[2,] "b" "4"
>

all args have different lengths and classes. But the result has those of 
the 'test' arg.


> It seems that the ifelse should check 5 values of h for zero. For each 
> of the 5 values I am thinking it should return an array of 5 (x[j+2] if 
> h[index] == 0). Since the dimension of h is 5 that would mean 25 values.

h has no attributes, therefore no 'dimension'

HTH,

Chuck

> But that isn't what is being returned.Something about this that I don't 
> understand. Please help my ignorance.
>
> Thank you.
>
> Kevin
>
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Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901