How to set a specific attribute of an object if the object name is in a variable?

Question

Normally you would set the attribute of an object as

attributes(x) <- list(dummy = 123)

But I have the variable names stored in a character vector. The following code throws an error:

var <- "x"
attributes(eval(as.name(var))) <- list(dummy = 123)

Error in attributes(eval(as.name(var))) <- list(dummy = 123) : could not find function "eval<-"

If eval(as.name()) is not the right way could someone suggest a way to solve this problem?

class(eval(as.name(var))) is character, whereas argument to attribute(...) should be an object, not a character — parth
– parth, Commented Sep 14, 2017 at 8:48

sebastian-c · Accepted Answer · 2017-09-14 12:05:56Z

1

You can use a function to apply the attributes and the assign function to apply them:

add_dummy <- function(obj, name, attribute){
  attr(obj, name) <- attribute
  return(obj)
}

assign(var, add_dummy(get(var), "attr_name", list(dummy = 123)))

edited Sep 14, 2017 at 12:05

answered Sep 14, 2017 at 9:13

sebastian-c

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4 Comments

Lazarus Thurston Over a year ago

your solution works fine for an atomic variable but converts a dataframe object to a list. Could you please modify the answer such that the obj returned does not change to a list; just the new attribute must be added to the object,

sebastian-c Over a year ago

Ah, so you don't want to replace all attributes, you want to append one?

Lazarus Thurston Over a year ago

yes, and I managed to do it by assigning a specific attribute; I used obj$attr_name<-attr_value instead of attributes(obj) <- attribute and my job is done. Thanks. Maybe you can add this part also in your answer so it becomes complete and useful in all situations.

sebastian-c Over a year ago

I've modified my answer to allow modification rather than replacement of attributes

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