Write list of data.frames to separate CSV files with lapply

Question

The question says it all - I want to take a list object full of data.frames and write each data.frame to a separate .csv file where the name of the .csv file corresponds to the name of the list object.

Here's a reproducible example and the code I've written thus far.

df <- data.frame(
    var1 = sample(1:10, 6, replace = TRUE)
    , var2 = sample(LETTERS[1:2], 6, replace = TRUE)
    , theday = c(1,1,2,2,3,3)
)

df.daily <- split(df, df$theday) #Split into separate days

lapply(df.daily, function(x){write.table(x, file = paste(names(x), ".csv", sep = ""), row.names = FALSE, sep = ",")})

And here is the top of the error message that R spits out

Error: Results must have one or more dimensions.
In addition: Warning messages:
1: In if (file == "") file <- stdout() else if (is.character(file)) { :
  the condition has length > 1 and only the first element will be used

What am I missing here?

@Joshua - dar, Google search fail. I should go home now. Feel free to close this one. — Chase
– Chase, Commented Nov 17, 2010 at 22:03
It's not exactly a duplicate. Your error is telling that you are passing more than one string to write.table as a file name. names(x) returns all colnames of a data frame. — Brandon Bertelsen
– Brandon Bertelsen, Commented Nov 17, 2010 at 22:09

IRTFM · Accepted Answer · 2010-12-19 22:04:53Z

33

Try this:

sapply(names(df.daily), 
 function (x) write.table(df.daily[[x]], file=paste(x, "txt", sep=".") )   )

You should see the names ("1", "2", "3") spit out one by one, but the NULLs are the evidence that the side-effect of writing to disk files was done. (Edit: changed [] to [[]].)

edited Dec 19, 2010 at 22:04

answered Nov 17, 2010 at 22:07

IRTFM

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5 Comments

Chase Over a year ago

Thanks for base R solution and the more succinct way to use paste.

Marek Over a year ago

I think there should be df.daily[[x]].

IRTFM Over a year ago

Both df.daily[x] and df.daily[[x]] give the same result on my machine. I guess I can see the logic of your suspicion, though.

IRTFM Over a year ago

I think the reason that both forms succeed is that write.table coerces to data.frame class with as.data.frame() if the object is not one to start with and that will succeed if the object is a list with one dataframe.

Marek Over a year ago

For me result is not the same. Columns names in [] version have prefix (e.g. X1.var1), in [[]] version don't.

Marek · Accepted Answer · 2010-11-18 08:52:51Z

11

You could use mapply:

mapply(
  write.table,
  x=df.daily, file=paste(names(df.daily), "txt", sep="."),
  MoreArgs=list(row.names=FALSE, sep=",")
)

There is thread about similar problem on plyr mailing list.

edited Nov 18, 2010 at 8:52

answered Nov 17, 2010 at 22:29

Marek

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Comments

Brandon Bertelsen · Accepted Answer · 2010-11-17 22:06:03Z

7

A couple of things:

laply performs operations on a list. What you're looking for is d_ply. And you don't have to break it up by day, you can let plyr do that for you. Also, I would not use names(x) as that returns all of the column names of a data.frame.

d_ply(df, .(theday), function(x) write.csv(x, file=paste(x$theday,".csv",sep=""),row.names=F))

answered Nov 17, 2010 at 22:06

Brandon Bertelsen

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2 Comments

Chase Over a year ago

I always forget about the "_" plyr functions for some reason. This will do the trick. Thanks!

zach Over a year ago

this solution doesn;t work for me. I get an error message as follows: Error in file(file, ifelse(append, "a", "w")) : invalid 'description' argument In addition: Warning message: In if (file == "") file <- stdout() else if (is.character(file)) { : the condition has length > 1 and only the first element will be used

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