[R] Can this loop be delooped?

[Talbot Katz]

Hi.

I have the following code in a loop.  It splits a vector into subvectors of 
equal size.  But if the size of the original vector is not an exact multiple 
of the desired subvector size, then the first few subvectors have one more 
element than the last few.  I know that the cut function could be used to 
determine where to break up the vector, but it doesn't seem to provide 
control over where to put the larger and smaller subvectors.

numgp1_v=sidect_v%/%compmin
numgroup_v[small]=max(1,numgp1_v[small])
sidemin_v=sidect_v%/%numgroup_v
nummax_v=sidect_v%%sidemin_v
eix=0
smallindexlist<-list(NULL)
for(i in 1:numgroup_v[small])	{
	bix=eix+1
	eix=bix+sidemin_v[small]+(i<=nummax_v[small])-1
	smallindexlist[[i]]<-dlpo_sm_v[bix:eix]
}

The key fact is that smallindexlist is a list, each list element is a 
subvector of dlpo_sm_v of the proper size.  The sizes may be different.


I tried to see whether I could eliminate the loop, as follows.  First I 
defined a function:

intgpi	<-	function(totalength,numgroups,groupnum,place="LEFT")	{
#	function to split the integer sequence, 1:totalength, into the groupnum 
group out of numgroups groups of equal size, totalength%/%numgroups
#	there are totalength%%numgroups number of groups of length 
1+totalength%/%numgroups, with the large groups all to one side, left if 
place=LEFT
#	totalength >= numgroups >= groupnum all integers, or it won't work right
	if(charmatch(toupper(place),"RIGHT",nomatch=FALSE)==1){
		extra1_1=max((groupnum-1)+((totalength%%numgroups)-numgroups),0)
		extra1_2=(groupnum>numgroups-totalength%%numgroups)
	}
	else{
		extra1_1=min(totalength%%numgroups,groupnum-1)
		extra1_2=(groupnum<=totalength%%numgroups)
	}
	gsize=totalength%/%numgroups
	gleft=((groupnum-1)*gsize)+extra1_1+1
	gright=gleft+gsize+extra1_2-1
	gleft:gright
}


The function appears to work okay.  Then I used it as follows:

numgp1_v=sidect_v%/%compmin
numgroup_v[small]=max(1,numgp1_v[small])
smallindexlist<-list(NULL)
smallindexlist=sapply(1:numgroup_v[small],function(i){dlpo_sm_v[intgpi(sidect_v[small],numgroup_v[small],i)]})

In this case, smallindexlist will be a list like I had before if the 
subvectors are not all the same size, but if the subvectors are all the same 
size, it appears that I get an array.  Can I force this operation to give me 
a list the way I want it in all cases?  Or is there a better way to deloop 
my original code?

Thanks!

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