[R] Assignment of values with different indexes

[David Winsemius]

On Dec 5, 2012, at 6:47 PM, Brian Feeny wrote:

>
> I would like to take the values of observations and map them to a  
> new index.  I am not sure how to accomplish this.  The result would  
> look like so:
>
> x[1,2,3,4,5,6,7,8,9,10]
> becomes
> y[2,4,6,8,10,12,14,16,18,20]

This suggests to me that you are having difficulty transfering your  
Matlab knowldege to R.


>
> The "newindex" would not necessarily be this sequence, but a  
> sequence I have stored in a vector, so it could be all kinds of  
> values.  here is what happens:
>
>> x <- rnorm(10)
>> myindex <- seq(from = 1,to = 20, by = 2)
>> y <- numeric()
>> y[myindex] <- x
>> y
> [1] -0.03745988          NA -0.09078822          NA   
> 0.92484413          NA  0.32057426          NA
> [9]  0.01536279          NA  0.02200198          NA   
> 0.37535438          NA  1.46606535          NA
> [17]  1.44855796          NA -0.05048738
>
> So yes, it maps the values to my new indexes, but I have NA's.  The  
> result I want would look like this instead:

The only structure in R that allows NULL values is a list. Matrices  
need to have either value or NA.

>
>
> [1] -0.03745988
> [3] -0.09078822
> [5] 0.92484413
> [7] 0.32057426
> [9]  0.01536279
> [11] 0.02200198
> [13] 0.37535438
> [15] 1.46606535
> [17]  1.44855796
> [19] -0.05048738
>
>
> and remove the NA's.  I tried this with na.omit() on x, but it looks  
> like so:
>
>> x <- rnorm(10)
>> myindex <- seq(from = 1,to = 20, by = 2)
>> y <- numeric()
>> y[myindex] <- na.omit(x)
>> y
> [1]  0.87399523          NA -0.39908184          NA   
> 0.14583051          NA  0.01850755          NA
> [9] -0.47413632          NA  0.88410517          NA  
> -1.64939190          NA  0.57650807          NA
> [17]  0.44016971          NA -0.56313802

However it will not display the way you were inteniding:

 > y <- list()
 > y[seq(1,20, by=2)] <- 1:10
 > y
[[1]]
[1] 1

[[2]]
NULL

[[3]]
[1] 2

[[4]]
NULL

[[5]]
[1] 3

snipped

-- 
David.
>
> Brian
>
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David Winsemius, MD
Alameda, CA, USA