[R] create a vector from several data frames

David L Carlson dcarlson at tamu.edu
Thu May 14 17:18:57 CEST 2015

If you combine all of the df's into a list, e.g.

dfn <- paste0("df", 1:20)
df <- lapply(dfn, get)
names(df) <- dfn

and if "target" is the day you want in the same date/time format as the day variable in the data frames:

sapply(df, function(x) x[x$day==target, "tmax"])

will return a named vector of the tmax values.

David L Carlson
Department of Anthropology
Texas A&M University
College Station, TX 77840-4352

-----Original Message-----
From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Stefano Sofia
Sent: Thursday, May 14, 2015 9:23 AM
To: r-help at r-project.org
Subject: [R] create a vector from several data frames

Dear r-users,
suppose that I have 20 data frames df1, df2, ..., df20 (one for each different location) with the same column names and column types (the first column contains a date, the others are numeric) like

day tmax tmin
2015-05-10 20 10
2015-05-11 21 12
2015-05-12 17 9
2015-05-13 24 13
2015-05-14 25 18

I need to create a vector "tmax_all" of length 20 with the tmax referred to a particular day (let's say 2015-05-14).
I would first build a new data frame

tmax_df <- Reduce(function(x, y) merge(x, y, by="day"), list(df1[ , c("day", "tmax")], df2[ , c("day", "tmax")], ..., df20[ , c("day", "tmax")]))

and then select the row of tmax_df where day is the day I want to.
Is there an easiest way? Is it possible to create straightforward this vector without passing through the merge of all the data frames?

Thank you for your help


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