Select list element programmatically using name stored as string

Question

I have a list

myList = list(a = 1, b = 2)
names(myList)
# [1] "a" "b"

I want to select element from 'myList' by name stored in as string.

for (name in names(myList)){
     print (myList$name)
}

This is not working because name = "a", "b". My selecting line actually saying myList$"a" and myList$"b". I also tried:

print(myList$get(name))
print(get(paste(myList$, name, sep = "")))

but didn't work. Thank you very much if you could tell me how to do it.

Try `myList[[name]]`. Also, `print` requires parentheses in R, like `print(x)` not `print x`. — Frank, Mar 29 '16 at 18:40
`paste(myList$, name, sep = "")` is not valid R syntax. You need to quote all non-variable elements. — Roman Luštrik, Mar 29 '16 at 18:42

fhlgood · Answer 1 · 2016-03-29T19:08:23.260

9

$ does exact and partial match, myList$name is equivalent to

`$`(myList, name)

As @Frank pointed out, the second argument name won't be evaluated, but be treated as a literal character string. Try ?`$`and see the document.

In your example. myList$name will try to look for name element in myList

That is why you need myList[[name]]

edited Mar 29 '16 at 19:08

answered Mar 29 '16 at 18:41

fhlgood

2

I think "for exact match" might not be a full explanation. In fact, `$` supports *partial matching*. The point is that `$` does not evaluate it's second argument (the part that appears on the right hand side). Btw, to escape the backquote, you can use a backslash or nest the code excerpt in some extra `\``s (though I'm not sure how many are needed). – Frank Mar 29 '16 at 18:43
1

@Frank Good to know that! Thank you sir! – fhlgood Mar 29 '16 at 18:48

score 1 · Answer 2 · answered Mar 29 '16 at 18:44

1

I think you want something like this:

for (name in names(myList)) {
   print(myList[[name]]) 
}

answered Mar 29 '16 at 18:44

darrelkj

2 Answers2