eval and as.name

I'm sure there is a more general way to ask this question but how do  you 
use the elements of a character vector as names of objects in an 
expression?
For example, say you have:

a = c(1,3,5,7)
b = c(2,4,6,8)

n=c("a","b")

and you want to use the names a and b in a function (e.g. sum)

sum(eval(as.name(n[1])),eval(as.name(n[2])))

works but

what is a simpler way to effect this level of indirection?

I'm sure there is a more general way to ask this question but how do
you use the elements of a character vector as names of objects in an
expression?
For example, say you have:

a = c(1,3,5,7)
b = c(2,4,6,8)

n=c("a","b")

and you want to use the names a and b in a function (e.g. sum)

sum(eval(as.name(n[1])),eval(as.name(n[2])))

works but

what is a simpler way to effect this level of indirection?

I am new to R, so maybe I'm missing the point of your question. But
why wouldn't you just use sum(a,b)?

Murray M Cooper, Ph.D.
Richland Statistics
9800 N 24th St
Richland, MI, USA 49083
Mail: richstat at earthlink.net
----- Original Message ----- From: "Fuchs Ira" <irafuchs at gmail.com>
To: <r-help at r-project.org>
Sent: Thursday, February 05, 2009 5:10 PM
Subject: [R] eval and as.name

I'm sure there is a more general way to ask this question but how do 
you use the elements of a character vector as names of objects in an
expression?
For example, say you have:

a = c(1,3,5,7)
b = c(2,4,6,8)

n=c("a","b")

and you want to use the names a and b in a function (e.g. sum)

sum(eval(as.name(n[1])),eval(as.name(n[2])))

works but

what is a simpler way to effect this level of indirection?

Fuchs Ira wrote:
  

I'm sure there is a more general way to ask this question but how do
you use the elements of a character vector as names of objects in an
expression?
For example, say you have:

a = c(1,3,5,7)
b = c(2,4,6,8)

n=c("a","b")

and you want to use the names a and b in a function (e.g. sum)

sum(eval(as.name(n[1])),eval(as.name(n[2])))

works but

what is a simpler way to effect this level of indirection?

    

in this particular case, the simplest i can come up with is:

sum(sapply(n, get))

you may want to avoid this sort of indirection by using lists with named
components:

d = list(a=c(1,3,5,7), b=c(2,4,6,8))
sum(unlist(d))
with(d, sum(a+b))
  

Hi,

Why do you use the equals sign for assignment instead of the arrow, is this
equal?

  

Mvh.
Marie

On Thu, Feb 5, 2009 at 11:59 PM, Wacek Kusnierczyk <
Waclaw.Marcin.Kusnierczyk at idi.ntnu.no> wrote:

  

you may want to avoid this sort of indirection by using lists with named
components:

d = list(a=c(1,3,5,7), b=c(2,4,6,8))
sum(unlist(d))
with(d, sum(a+b))
sum(d[['a']], d[['b']])
sum(sapply(n, function(v) d[[v]]))

and so on.

vQ

equal?  you mean equivalent?  mostly, yes.  briefly, this is why:

1. a copy-over from other programming languages;
2. to avoid learning yet another operator;
3. after having learned the other operator, to avoid that ugly operator;
4. after an r guru complained here about people using this instead of
that, to annoy him.