[R] Accumulating results from "for" loop in a list/array

[Johannes Hüsing]

Steven Kang schrieb:
> Dear R users,
>
>
> I would like to accumulate objects generated from 'for' loop to a list or
> array.
>
> To illustrate the problem, arbitrary data set and script is shown below,
>
>
> x <- data.frame(a = c(rep("n",3),rep("y",2),rep("n",3),rep("y",2)), b =
> c(rep("y",2),rep("n",4),rep("y",3),"n"), c = c(rep("n",7),rep("y",3)), d =
> c("y", rep("n",4), rep("y",2), rep("n",3)))
>
> for (i in 1:(dim(x)[2]))  {
>          assign(paste("ind", i, sep = ""), which(x[ , i] == "y"))
>           accum <- c(ind1, ind2, ind3, ind4)
> }
>
>   

Using "for" loops is not idiomatic; I tend to use them only if I really 
need intermediate results.

res <- apply(x, 2, function(answer) which(answer == "y"))

names(res) <- paste("ind", 1:ncol(x), sep="")

>> ind1
>>     
> [1]  4  5  9 10
>   
>> ind2
>>     
> [1] 1 2 7 8 9
>   
>> ind3
>>     
> [1]  8  9 10
>   
>> ind4
>>     
> [1] 1 6 7
>   
>> accum
>>     
>  [1]  4  5  9 10  1  2  7  8  9  8  9 10  1  6  7
>
> Are there any alternative method where the highlighted statement above can
> be represented without typing individual objects manually? (as it can be
> very tedious with large number of objects; i.e ind1, ind2, ....., ind100)
>
>   
/I think you are looking for unlist()./

> Also, is there any way to extract individual objects ('ind1' etc) from
> 'accum'?
>
>   
Well, I think they are not objects by themselves anymore. You may want 
to give the vector elements individual names based on their origin and 
then extract appropriate elements by string matching.