Extract column names of a dataset from a list of dataframes

Question

I have two lists of dataframes, artrk_dist and comprk_dist. I am inner joining the corresponding datasets in the two lists using dplyr. I need to refer to the column names inside the join call using the index of the column rather than the column name itself. I tried using colnames(dataset)[index], but this doesn't work. I have this:

 allrk_dist<- inner_join(artrk_dist[[1]],comprk_dist[[1]], 
                                 by=c(colnames(artrk_dist[[1]])[1]= 
                                        colnames(comprk_dist[[1]])[1],
                                      colnames(artrk_dist[[1]])[2]= 
                                        colnames(comprk_dist[[1]])[2]))

BUt this gives an error:

Error: unexpected '=' in:
" allrk_dist[[1]]<- inner_join(artrk_dist[[1]],comprk_dist[[1]], 
                                 by=c(colnames(artrk_dist[[1]])[1]="
>                                         colnames(comprk_dist[[1]])[1],
Error: unexpected ',' in "                                        colnames(comprk_dist[[1]])[1],"
>                                       colnames(artrk_dist[[1]])[2]= 
+                                         colnames(comprk_dist[[1]])[2]))
Error: unexpected ')' in:
"                                      colnames(artrk_dist[[1]])[2]= 
                                        colnames(comprk_dist[[1]])[2])"
>

Here is an example of what I'm doing which gives an error as above:

data1<- data.frame(a1=1:5,b1=6:10)
 data2<- data.frame(a2=2:6,b2=5:9)
 data_list<-list(data1,data2)
 y<- inner_join(data_list[[1]],data_list[[2]],by=colnames(data_list[[1]])[1]
                = colnames(data_list[[2]])[1])

I need to refer to the column names using an index and I am stuck there.

It's easier to help you if you include a simple [reproducible example](https://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example) with sample input and desired output that can be used to test and verify possible solutions. — MrFlick, Apr 25 '21 at 17:43
As pointed out in the other question, use `inner_join(data_list[[1]],data_list[[2]],by=setNames(colnames(data_list[[2]])[1], colnames(data_list[[1]])[1]))` — MrFlick, Apr 25 '21 at 18:03
HOw do I add another index - I need to match on two indices, [1] and [2], is it just a comma-separated list inside the SetNames call? — user2450223, Apr 25 '21 at 18:16
Can you make it so your example better represents what you are trying to do? Make it clear what the desired output is. — MrFlick, Apr 25 '21 at 18:27

score 0 · Accepted Answer · answered Apr 25 '21 at 18:02

0

As we are passing an object use setNames

inner_join(data_list[[1]],data_list[[2]],by=setNames(
            colnames(data_list[[2]])[1],colnames(data_list[[1]])[1]))

answered Apr 25 '21 at 18:02

akrun

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I actually also have two keys to join on - how would the syntax change in this case? I.e., I have two indexes [1] and [2] I'm joining on. – user2450223 Apr 25 '21 at 18:12
1

@user2450223 this is based on your example which is working fine for me – akrun Apr 25 '21 at 18:13
Sorry, the problem is I have TWO keys or indices to join on - I get a different error now: Error: `suffix` must be a character vector of length 2. suffix is a character vector of length 1. – user2450223 Apr 25 '21 at 18:21
1

@user2450223 if there are two columns, `setNames(colnames(data_list[[2]])[1:2], colnames(data_list[[1]])[1:2])` – akrun Apr 25 '21 at 18:24
@user2450223 your comment is not complete – akrun Apr 25 '21 at 18:24
Thank you, what is the general case like with non-contiguous indices? This really helped - seemed like such a small thing but was stuck for hours. – user2450223 Apr 25 '21 at 18:27
1

@user2450223 use `c` i.e. `[c(1, 3, 5)]` as indexes – akrun Apr 25 '21 at 18:28

Extract column names of a dataset from a list of dataframes

1 Answers1