[R] operations on columns when data frames are in a list

[Adams, Jean]

If you write a function that takes a data frame as an argument and returns
a data frame, you can use lapply to carry out the tasks that you want.  For
example, if your list of data frames is called mydat ...

mon2date <- function(df) {
  if ("Month" %in% names(df)) {
    df$Month<- as.POSIXct(df$Month, format="%Y/%m/%d")
  }
  return(df)
}

mydat2 <- lapply(mydat, mon2date)

Jean

On Sun, Apr 12, 2015 at 5:30 PM, Steve E. <searl at vt.edu> wrote:

> Hello R folks,
>
> I have recently discovered the power of working with multiple data frames
> in
> lists. However, I am having trouble understanding how to perform operations
> on individual columns of data frames in the list. For example, I have a
> water quality data set (sample data included below) that consists of
> roughly
> a dozen data frames. Some of the data frames have a chr column called
> 'Month' that I need to to convert to a date with the proper format. I would
> like to iterate through all of the data frames in the list and format all
> of
> those that have the 'Month' column. I can accomplish this with a for-loop
> (e.g., below) but I cannot figure out how to do this with the plyr or apply
> families. This is just one example of the formatting that I have to perform
> so I would really like to avoid loops, and I would love to learn how to
> better work with lists as well.
>
> I would appreciate greatly any guidance.
>
>
> Thank you and regards,
> Stevan
>
>
> a for-loop like this works, but is not an ideal solution:
>
> for (i in 1:length(data)) {if ("Month" %in% names(data[[i]]))
> data[[i]]$Month<- as.POSIXct(data[[i]]$Month, format="%Y/%m/%d")}
>
>
>
> sample data (head of two data frames from the list of all data frames):
>
> structure(list(`3D_Fluorescence.csv` = structure(list(ID = 1:6,
>     Site_Number = c("R5", "R6a", "R8", "R9a", "R14", "R15"),
>     Month = c("2001/10/01", "2001/10/01", "2001/10/01", "2001/10/01",
>     "2001/10/01", "2001/10/01"), Exc_A = c(215L, 215L, NA, NA,
>     215L, 215L), Em_A = c(422.5, 410.5, NA, NA, 408.5, 408),
>     Fl_A = c(303, 296.86, NA, NA, 297.62, 174.75), Exc_B = c(325L,
>     325L, NA, NA, 325L, 325L), Em_B = c(416, 413, NA, NA, 418.5,
>     417.5), Fl_B = c(137.32, 116.1, NA, NA, 132.48, 77.44)), .Names =
> c("ID",
> "Site_Number", "Month", "Exc_A", "Em_A", "Fl_A", "Exc_B", "Em_B",
> "Fl_B"), row.names = c(NA, 6L), class = "data.frame"), algae.csv =
> structure(list(
>     ID = 1:6, SiteNumber = c("R1", "R2A", "R2B", "R3", "R4",
>     "R5"), SiteLocation = c("CAP canal above Waddell Canal",
>     "Lake Pleasant integrated sample", "Lake Pleasant integrated sample",
>     "Waddell Canal", "Cap Canal at 7th St.", "Verde River btwn Horseshoe
> and
> Bartlett"
>     ), ClusterName = c("cap", "cap", "cap", "cap", "cap", "verde"
>     ), SiteAcronym = c("cap-siphon", "pleasant-epi", "pleasant-hypo",
>     "waddell canal", "cap @ 7th st", "verde abv bartlett"), Date =
> c("1999/08/18",
>     "1999/08/18", "1999/08/18", "1999/08/18", "1999/08/18", "1999/08/16"
>     ), Month = c("1999/08/01", "1999/08/01", "1999/08/01", "1999/08/01",
>     "1999/08/01", "1999/08/01"), SampleType = c("", "", "", "",
>     "", ""), Conductance = c(800, 890, 850, 870, 830, 500), ChlA = c(0.3,
>     0.3, 0.6, 0.8, 1.1, 7.6), Phaeophytin = c(0, 0, 0, 0, 0.7,
>     4.7), PhaeophytinChlA = c(0.7, 0.7, 1.3, 5.3, 0.7, 4.7),
>     Chlorophyta = c(0L, 0L, 18L, 0L, 0L, 21L), Cyanophyta = c(8L,
>     0L, 0L, 0L, 7L, 79L), Bacillariophyta = c(135L, 76L, 0L,
>     18L, 54L, 195L), Total = c(147L, 76L, 18L, 18L, 61L, 302L
>     ), AlgaeComments = c("", "", "", "", "", "")), .Names = c("ID",
> "SiteNumber", "SiteLocation", "ClusterName", "SiteAcronym", "Date",
> "Month", "SampleType", "Conductance", "ChlA", "Phaeophytin",
> "PhaeophytinChlA", "Chlorophyta", "Cyanophyta", "Bacillariophyta",
> "Total", "AlgaeComments"), row.names = c(NA, 6L), class = "data.frame")),
> .Names = c("3D_Fluorescence.csv",
> "algae.csv"))
>
>
>
> --
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> Sent from the R help mailing list archive at Nabble.com.
>
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