How to used a variable value as a (another) variable name in a function

Question

This is probably easy to do, but I could not find a solution.

Say I have a serie of variables : variable1, variable2 ... I would like to use another variable/list like X<-c("variable1", variable2"...) to call one of the variable (variable1, variable2...) in a function.

Something like : plot(X[1], Y) where 'X[1]' is interpreted as 'variable1'.

Any suggestion ?

Thank you

Guillaume

----- Edit after the first anwsers --

Well the get() function was what I needed. I had tried it before, but could not get what I wanted. I does work now.

Thanks jlhoward and Ananda Mahto for your help.

There are some more explanations for future (potential) readers:

With myData, a dataframe with 3 columns (Value1, Value2, Y)

I can do :

plot(Value1~Y, data=myData)

or

plot(Value2~Y, data=myData)

now if I have a vector such as

myList<-c("Value1", "Value2")

I can do :

i=1
plot(get(myList[i])~Y, data=myData)

or

i=2
plot(get(myList[i])~Y, data=myData)

anymore comments / suggestions welcome

Answer 1

Here's a combination of my comment and Roland's. Hopefully, it helps get you closer to answering your question or helps you clarify what you're trying to do.

First, some variables, "A", "B", and "C"

set.seed(1)
A <- rnorm(100)
B <- rnorm(100)
C <- rnorm(100)

Next, a vector of the names of those variables, and another variable, "Y" that we want to plot against.

X <- c("A", "B", "C")
Y <- 1:100

Here's one approach:

lapply(X, function(x) plot(x = get(x), y = Y))

---

The preferred approach, though, especially if you're going to be doing similar things on your vectors, is to just put those vectors in a list first and work from there:

myList <- list(A, B, C)
lapply(myList, function(x) plot(x = x, y = Y))
# lapply(myList, sum) 
## or whatever else you want to do with each of these vectors

Answer 2

Is this what you meant?

f = function(x){return(x*x)}
X= c(1,2,3)
sapply(X,f)
[1] 1 4 9

sapply "applies" the function f for each element in the list X, and produces a vector as output. You can also do:

sapply(1:3,function(x){return(x*x)})