R help - Aug 2011 - Details of subassignment (for vectors and data frames)

Hi All: 
I'm looking to find out a bit more about how subassignment actually works
and am hoping someone with knowledge of the details can fill me in (I've
looked at the source code, but my knowledge of C is lacking).
In the case of vectors, my reading of ?"[" would indicate that for a
vector, vec <- 1:25,    vec[c(1,5,25)] <- c(101,102,103)is functionally
the same as    indx <- c(1,5,25)    for (i in 1:length(indx)) vec[indx[i]]
<- c(101,102,103)[i]
And in the case of a data frame, df <- data.frame(d1=1:10,d2=11:20,d3=21:30),
df[c(1,5,10),c(1,3)] <- data.frame(a=101:103,b=104:106)is functionally the
same as    rowindx <- c(1,5,10)    colindx <- c(1,3)    for (i in
1:length(rowindx)) {	for (j in 1:length(colindx)) df[rowindx[i],colindx[j]]
<- data.frame(a=101:103,b=104:106)[i,j]    }		Obviously I've verified
that these examples work and I realize that my loops also contain
subassignments; what I'm really after is to understand the mechanics of
replacing multiple elements.  Is a for-loop the proper way to understand the
sequential nature of subassignments here (even if it is not actually implemented
using a loop)?
Cheers,HR 		 	   		  
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My mental model is that the left hand side forms a sort of "virtual
vector" where each element really points to an element in the vector being
modified. Then the right hand side scalar is extended in the usual repetitious
way (if necessary) until it is a vector just as long as the "virtual
vector" on the left. Then the right vector is assigned to the left vector,
which leaves the designated elements in the destination vector changed.

You could work out an equivalent for loop structure, but I think it would be
tricky to keep the behavior for different length source and destination
assignments straight.
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Al Roark <hrbuilder@hotmail.com> wrote:


Hi All: 
I'm looking to find out a bit more about how subassignment actually works
and am hoping someone with knowledge of the details can fill me in (I've
looked at the source code, but my knowledge of C is lacking).
In the case of vectors, my reading of ?"[" would indicate that for a
vector, vec <- 1:25, vec[c(1,5,25)] <- c(101,102,103)is functionally the
same as indx <- c(1,5,25) for (i in 1:length(indx)) vec[indx[i]] <-
c(101,102,103)[i]
And in the case of a data frame, df <- data.frame(d1=1:10,d2=11:20,d3=21:30),
df[c(1,5,10),c(1,3)] <- data.frame(a=101:103,b=104:106)is functionally the
same as rowindx <- c(1,5,10) colindx <- c(1,3) for (i in
1:length(rowindx)) {	for (j in 1:length(colindx)) df[rowindx[i],colindx[j]]
<- data.frame(a=101:103,b=104:106)[i,j] }		Obviously I've verified that
these examples work and I realize that my loops also contain subassignments;
what I'm really after is to understand the mechanics of replacing multiple
elements. Is a for-loop the proper way to understand the sequential nature of
subassignments here (even if it is not actually implemented using a loop)?
Cheers,HR 		 	 		 
	[[alternative HTML version deleted]]

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