Data Frame containing hyphens using R

Question

I have created a list (Based on items in a column) in order to subset my dataset into smaller datasets relating to a particular variable. This list contains strings with hyphens in them -.

dim.list <- c('Age_CareContactDate-Gender', 'Age_CareContactDate-Group',
         'Age_ServiceReferralReceivedDate-Gender',
         'Age_ServiceReferralReceivedDate-Gender-0-18',
         'Age_ServiceReferralReceivedDate-Group',
         'Age_ServiceReferralReceivedDate-Group-ReferralReason')

I have then written some code to loop through each item in this list subsetting my main data.

for (i in dim.list) {assign(paste("df1.",i,sep=""),df[df$Dimension==i,])}

This works fine, however when I come to aggregate this in order to get some summary statistics I can't reference the dataset as R stops reading after the hyphen (I assume that the hyphen is some special character)

If I use a different list without hyphens e.g.

dim.list.abr <- c('ACCD_Gen','ACCD_Grp',
              'ASRRD_Gen',
              'ASRRD_Gen_0_18',
              'ASRRD_Grp',
              'ASRRD_Grp_RefRsn')

When my for loop above executes I get 6 data.frames with no observations.

Why is this happening?

Hyphens aren't allowed in standard variable names. Think of a simple example: `a-b`. Is it a variable name with a hyphen or is it a minus b? The R interpreter assumes a minus b, because it doesn't require spaces for binary operations. You can force non-standard names to work using backticks, `\`a-b\` <- 5` or `\`x+y\` <- 10`, but you're better off following the rules and using standard names without special characters like `+ - * / % $ # @ ! & | ^` in them. — Gregor Thomas, Feb 15 '18 at 15:54
Gregor is exactly correct. But you likely are much better off by putting your subsetted data.frames in a list, instead of assigning them all separately. This is why I suggest using `split` (which would save you quite a lot of coding too!). — Axeman, Feb 15 '18 at 15:56
And, I completely agree with Axeman. Use `split` to divide up your data frame into a list. And keep it in a list rather than use `assign`, it will be much easier to loop over or use `lapply` with that way. You might want to read my answer at [How to make a list of data frames](https://stackoverflow.com/a/24376207/903061) for a lot of discussion and examples. — Gregor Thomas, Feb 15 '18 at 15:56
@Axeman I played around with split but it wasn't really what i wanted, i was aiming for 6 separate datasets rather than a list of datasets. Besides dim.list is not the complete set of unique entries in the Dimensions column. — JDraper, Feb 15 '18 at 15:58
@Gregor Thanks both, sorry to be a pain but can either of you explain the advantage of using a list of datasets rather then 6 individual ones? At least from a ease of use point of view it seems difficult to view the individual datasets when in list form — JDraper, Feb 15 '18 at 16:02
`for (i in seq_along(dim.list)) {assign(paste("df1.",i,sep=""),df[df$Dimension==dimlist[i],])}` should work. But I still advise using lists. Viewing is easy, with `list[[1]]` etc. Using lists reduces the number of objects, make it easy to change your list of variables later, and allows you to `lapply` over all subsets easily. — Axeman, Feb 15 '18 at 16:02
Read my [How to make a list of data frames](https://stackoverflow.com/a/24376207/903061) answer. It's the best sell I can do on the benefits of using lists. — Gregor Thomas, Feb 15 '18 at 16:04

Gregor Thomas · Accepted Answer · 2018-02-15T19:05:48.427

Comment to answer:

Hyphens aren't allowed in standard variable names. Think of a simple example: a-b. Is it a variable name with a hyphen or is it a minus b? The R interpreter assumes a minus b, because it doesn't require spaces for binary operations. You can force non-standard names to work using backticks, e.g.,

# terribly confusing names:
`a-b` <- 5
`x+y` <- 10
`mean(x^2)` <- "this is awful"

but you're better off following the rules and using standard names without special characters like + - * / % $ # @ ! & | ^ ( [ ' " in them. At ?quotes there is a section on Names and Identifiers:

Identifiers consist of a sequence of letters, digits, the period (.) and the underscore. They must not start with a digit nor underscore, nor with a period followed by a digit. Reserved words are not valid identifiers.

So that's why you're getting an error, but what you're doing isn't good practice. I completely agree with Axeman's comments. Use split to divide up your data frame into a list. And keep it in a list rather than use assign, it will be much easier to loop over or use lapply with that way. You might want to read my answer at How to make a list of data frames for a lot of discussion and examples.

Regarding your comment "dim.list is not the complete set of unique entries in the Dimensions column", that just means you need to subset before you split:

nice_list = df[df$Dimension %in% dim.list, ]
nice_list = split(nice_list, nice_list$Dimension)

Ok thanks (@Axeman as well), probably not the answer i wanted but it helps - will go back and rewrite this using split and see if i can get this to work! — JDraper, Feb 15 '18 at 16:10
If you want to continue with your method, you just need to replace all the hyphens with underscores first. But lists are not *that* hard and learning to use them well will make you a much more effective R programmer. (corrected typo! First version of this comment said "lists are *that* hard"! Completely the opposite!) — Gregor Thomas, Feb 15 '18 at 16:57

Data Frame containing hyphens using R

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