R - how to evaluate a text string

Question

I'm quite new to R and failed to google the answer. The question is, how can I tell R to treat a character string value as a part of code, in a manner SAS resolve() function does?

Say, I have a data frame containing numeric columns V1, ..., Vn and exactly n rows. I wish to sum up all the 'diagonal' elements V = V1[1] + V2[2]... + Vn[n] but n is large enough for manual summation (below, n=2 for simplicity).

I'm trying to put the strings "dat$V1[1]", "dat$V2[2]" in a character C and then extract the corresponding numerical value (all in a loop step):

> dat <- data.frame(V1 = c(2,3), V2 = c(7,11))
> dat
    V1 V2
  1  2  7
  2  3 11

V = 0
for(i in 1:nrow(dat))
{
  C = paste('dat$V',format(i,trim=TRUE),'[',format(i,trim=TRUE),']', 
            sep="" )
  f = Xfun(C) 
  V = V + f
}

What should be used instead of Xfun? I've tried as.formula(), asOneSidedFormula(), get("...") and some other, but it's essential that dat$V1 is not an object:

> exists("dat")
[1] TRUE
> exists("dat$V1")
[1] FALSE

Your help is much appreciated.

Answer 1

If you just want to sum the diagonal elements of a square matrix, just do

dat <- data.frame(V1 = c(2,3), V2 = c(7,11))

sum(diag(as.matrix(dat)))

If you want to evaluate a text string in R, read up a bit on eval or do ? eavl ? eavl in R.

For your problem, you can do this:

dat <- data.frame(V1 = c(2,3), V2 = c(7,11))
dat

V = 0
for(i in 1:nrow(dat))
{
  C = paste('dat$V',format(i,trim=TRUE),'[',format(i,trim=TRUE),']', 
            sep="" )
  f = eval(parse(text=C)) 
  V = V + f
}

Answer 2

To finalize:

a) correct answer using text parsing and evaluation

V = 0
for(i in 1:nrow(dat))
{ C = paste('dat$V',as.character(i),'[',as.character(i),']', 
            sep='' )
  V = V + eval(parse(text=C))
}

b) correct action ESPECIALLY for a data frame - without requiring text evaluation but addressing data frame columns directly by name

V = 0
for(i in 1:nrow(dat))
{ col = paste('V',as.character(i),
              sep='')
  V = V + dat[i,col]
}