R using rbind with a name that is from another variable

Question

If I make a matrix like so;

m<-matrix(1,dimnames=list('row','column'))

    column
row      1

I can add a new row like so with rbind

m<-rbind(m,row2=2)

     column 
row       1 
row2      2

But... if I have a string variable like

tag<-"row3"

Why can't I used rbind as

m<-rbind(m,tag=3)

where row3 should be and not tag as

     column
row       1
row2      2
tag       3

but want

     column
row       1
row2      2
row3      3

Answer 1

You could just use rbind without naming the new row, and then use rownames to name that row:

m <- rbind(m,3)
rownames(m)[3]<-tag

Some benchmarking between this and @agstudy's solution:

m <- matrix(1,dimnames=list('row','column'))
m <- rbind(m,row2=2)
tag <- "row3"

fn1<-function(m,tag){
  m <- rbind(m,matrix(3,dimnames=list(tag,'column')))
  m
}

fn2<-function(m,tag){
  m <- rbind(m,3)
  rownames(m)[3]<-tag
  m
}
library(microbenchmark)
microbenchmark(fn1(m,tag),fn2(m,tag))
Unit: microseconds
        expr    min     lq median     uq    max neval
 fn1(m, tag)  4.665  5.598  6.065  6.532 15.862   100
 fn2(m, tag) 11.663 13.062 13.063 13.530 48.517   100

Answer 2

you can do this , by creating a matrix with right dimnames

m <- matrix(1,dimnames=list('row','column'))
m <- rbind(m,row2=2)
tag <- "row3"
m <- rbind(m,matrix(3,dimnames=list(tag,'column')))

m
     column
row       1
row2      2
row3      3

Answer 3

Since substitute() does not process function arguments, the only way to emulate a dynamic variable name in this case would be eval(parse(...)):

> m <- eval(parse(text=paste("rbind(m,",tag,"=3)")))
> m
     column
row       1
row2      2
row3      3

It is freaky, and I would not recommend it, but I post it for completeness.