R: merging matrices (not data.frames)

Question

merge is a very nice function: It merges matrices and data.frames, and returns a data.frame.

Having rather big character matrices, is there another good way to merge - without data.frame conversion?

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Comment 1: A small function to merge a named vector with a matrix or data.frame. Elements of the vector can link to multiple entries in the matrix:

expand <- function(v,m,by.m,v.name='v',...) {
  df <- do.call(rbind,lapply(names(v),function(x) {
    pos <- which(m[,by.m] %in% v[x])
    cbind(x,m[pos,],...)
  }))
  colnames(df)[1] <- v.name
  df
}

Example:

v <- rep(letters,each=3)[seq_along(letters)]
names(v) <- letters
m <- data.frame(a=unique(v),b=seq_along(unique(v)),stringsAsFactors=F)
expand(v,m,'a')

Answer 1

No, not without either (a) overwriting the merge function or (b) creating a new merge.matrix() S3 function (this would be the right approach to the problem).

You can see in the merge help:

Value

A data frame.

Also, the merge.default function:

> merge.default
function (x, y, ...) 
merge(as.data.frame(x), as.data.frame(y), ...)

Answer 2

You can use a combination of match and cbind to do the equivalent of merge without conversion to data frame, a simple example:

st1 <- state.x77[ sample(1:50), ]
st2 <- as.matrix( USArrests )[ sample(1:50), ]

tmp1 <- match(rownames(st1), rownames(st2) )

st3 <- cbind( st1, st2[tmp1,] )
head(st3)

Keeping track of which columns you want, and merging whith many to 1 relationships or missing rows in one group require a bit more thought but are still possible.