How to assign object to output of a function?

Question

I'm trying to assign something to the output of Sys.Date(). For example, say I want to name the object with today's date:

format(Sys.Date(), "%b%d") <- mtcars

I get the error:
invalid (NULL) left side of assignment.

I tried:
eval(parse(text = format(Sys.Date(), "%b%d"))) <- mtcars

Gets the same error msg. What am I missing / is there a solution?

Why do you want to do this? Having data stored in the variable name itself is usually a bad idea in R. It would be better if you need to generate names dynamically to use a named list and avoid the eval() and assign() functions because they create unnecessary overhead. — MrFlick
– MrFlick, Commented Sep 11, 2020 at 21:59
I'm basically timestamping dataframes that I'm working on - I name them manually now which gets tedious the more what-ifs I want to run. I can see the value of holding them in a list, less clutter for one, but how would I add a new element to a dynamically named slot in the list? I have list x with elements Sep03 and Sep09 - how would I add a dynamically named element Sep11 to x? — armipunk
– armipunk, Commented Sep 12, 2020 at 3:08
You can assign with any string value like x[["Sep11"]] <- mtcars or a function that returns a string value x[[format(Sys.Date(), "%b%d")]] <- mtcars. Then you'll have all the values nicely stored in x and you can get at them with x[["Sep11"]] or x$Sep11 or even apply a function to each element in the list. — MrFlick
– MrFlick, Commented Sep 12, 2020 at 3:15
@MrFlick Assigning to function output in a named list worked and I see myself using it. My question was about assigning in the global environment however, so I will leave akrun's as the accepted answer. Hive it off as a separate answer for other people to find? Guessing SO will weight answers over comments. — armipunk
– armipunk, Commented Sep 12, 2020 at 18:42

akrun · Accepted Answer · 2020-09-11 21:22:22Z

1

We can use assign

assign(format(Sys.Date(), "%b%d"), mtcars)

head(Sep11)
#                    mpg cyl disp  hp drat    wt  qsec vs am gear carb
#Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
#Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
#Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
#Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
#Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
#Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

answered Sep 11, 2020 at 21:22

akrun

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1 Comment

armipunk Over a year ago

Thank you! I did try assign first but simply wrapped format() in it and got the same error.

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