[R] Extract values from a data frame

[Ista Zahn]

On Fri, Aug 12, 2011 at 3:54 AM, Lali <laurafe at gmail.com> wrote:
> Hi Ista,
> Thanks for your suggestion, I am still trying to wrap my head around the
> functions you used, as I am not familiar with any of them, but it works
> perfectly!
> I do want to understand the code, if you don't mind I would like to ask a
> few questions
> In this line:
> dfm <- melt(df[c(1, 2, 5, 8)], id = 1)
> What does the id=1 do? The variables are already specified in df[c(1, 2, 5,
> 8)], right?

It retains the first variable in df in dfm. The remaining variables
are collapsed into a single variable named "value".

> What does this line do:
> dfm.r <- ddply(dfm.r, .(name), transform, index = paste("S",
> 1:length(name), sep = ""))

It adds a variable named "index" with values of Si for each level of
"name" where i= 1:length(name)

HTH,
Ista

> Thanks again for the help.
> Laura
>
>
>
> On Thu, Aug 11, 2011 at 5:57 PM, Ista Zahn <izahn at psych.rochester.edu>
> wrote:
>>
>> Hi Laura,
>>
>> On Thu, Aug 11, 2011 at 7:01 AM, Lali <laurafe at gmail.com> wrote:
>> > Hi everyone,
>> > I have a data frame that looks *sort of* like this:
>> >
>> > name <- letters[1:5]
>> > signal.1 <- c("12", "bad signal", "noise", "10", "X")
>> > length.signal.1 <- 5:9
>> > intensity.signal.1 <- 3:7
>> > signal.2 <- c("13", "noise", "19.2", "X", "V")
>> > length.signal.2 <- 2:6
>> > intensity.signal.2 <- 1:5
>> > signal.3 <- c("NA", "15.4", "error", "NA", "17")
>> > length.signal.3 <- c("NA", 2, 3, "NA", 4)
>> > intensity.signal.3 <- c("NA",4, 5, "NA", 5)
>> >
>> > #(there are actually up to 16 signals and 50 names, but I made this
>> > short
>> > for the example)
>> >
>> > df <- data.frame(cbind(name, signal.1, length.signal.1,
>> > intensity.signal.1,
>> > signal.2,
>> >                       length.signal.2, intensity.signal.2, signal.3,
>> > length.signal.3,
>> >                       intensity.signal.3))
>> >
>> >
>> >
>> > I need to "fish out" some values and have them in a new data frame.
>> >
>> > I am only interested in values in columns 2, 5 and 8 (actually seq(2,
>> > 50, 3)
>> > in my real df)
>> > I want the values that are not:
>> > "bad signal"
>> > "noise"
>> > "error"
>> > "NA"
>> > "V"
>> >
>> > This is the output I want (the name column is unimportant for my
>> > purposes,
>> > its just there as a reference for the example).
>> >
>> > (name)  S1       S2
>> > A        12        13
>> > B        15.4     (another value found in the other signals >3 not shown
>> > on
>> > example)
>> > C        19.2     (another value found in the other signals >3 not shown
>> > on
>> > example)
>> > D        10        X
>> > E        X         17
>> >
>> > I do know that there will always be 2 values exactly that do not match
>> > the
>> > exclusions named above, or none at all
>> >
>> > I have tried different approaches, grep, matching,%nin%... But as I am
>> > not
>> > an advanced used, I am very likely doing something wrong, because I
>> > either
>> > get a vector, or I get a matrix with TRUE FALSE, and usually I get the
>> > whole
>> > rows, and I don't want that :(
>> > I have also being searching the list for answers without avail.
>> > Any suggestions? Examples including syntax are appreciated (syntax is a
>> > major weak point for me).
>>
>> Here is a solution using the reshape and plyr packages
>>
>> library(reshape)
>> dfm <- melt(df[c(1, 2, 5, 8)], id = 1)
>> dfm.r <- dfm[!dfm$value %in% c("bad signal", "noise", "error", "NA", "V"),
>> ]
>> dfm.r <- ddply(dfm.r, .(name), transform, index = paste("S",
>> 1:length(name), sep = ""))
>> cast(dfm.r, name ~ index)
>>
>> Best,
>> Ista
>> >
>> >
>> > Laura
>> >
>> >        [[alternative HTML version deleted]]
>> >
>> > ______________________________________________
>> > R-help at r-project.org mailing list
>> > https://stat.ethz.ch/mailman/listinfo/r-help
>> > PLEASE do read the posting guide
>> > http://www.R-project.org/posting-guide.html
>> > and provide commented, minimal, self-contained, reproducible code.
>> >
>>
>>
>>
>> --
>> Ista Zahn
>> Graduate student
>> University of Rochester
>> Department of Clinical and Social Psychology
>> http://yourpsyche.org
>
>



-- 
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org