[R] subsetting question

William Dunlap wdunlap at tibco.com
Wed May 20 17:41:52 CEST 2015

Here is a self-contained example of what you might be trying to do.
You would get better answers if you supplied this yourself.

dataset1 <-
"2015-04-19")), Weight=11:14)
datums <- as.POSIXct(c("2015-04-01", "2015-04-08", "2015-04-19"))
# note that neither dataset1$Date nor datums is a subset of the other

for (i in 1:length(datums)){
    meanweight <- mean(dataset1[dataset1$Date==datums[i],]$Weight)

You said is 'obviously' doesn't work, but didn't say in what was wrong with
what it did.  One obvious (to me) thing is that in each iteration you
overwrote the
meanweight assigned in the previous iteration so you end up with only the
last one calculated.  You can fix that by using making meanweight a vector
and assigning elements of it in the loop.

meanWeight <- numeric(length(datums))
for (i in 1:length(datums)){
    meanWeight[i] <- mean(dataset1[dataset1$Date==datums[i],]$Weight)
data.frame(datums, meanWeight)
#      datums meanWeight
#1 2015-04-01       11.5
#2 2015-04-08        NaN
#3 2015-04-19       14.0

This looks to me like the appropriate result, but your self-contained
would make me more convinced of that.

By the way, why is is important to you to use a for loop and not a function
tapply() or aggregate()?  They can cause hassles with the data I gave above
because datums does not partition dataset1$Date, but I don't know what your
problem with them is.

Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Wed, May 20, 2015 at 3:03 AM, Dieter Anseeuw <dieter.anseeuw at inagro.be>

> Dear all,
> I would like to do multiple actions on a subset of my data. Therefore, I
> want to create a for loop on the variable "Date" (actually a double for
> loop on yet another variable, but let's omit that for a moment).
> I want to run down every level of "Date" and perform multiple actions on
> the data from a certain date. Here is my code:
> for (i in 1:length(datums)){
> meanweight<-mean(dataset1[dataset1$Date==datums[i],]$Weight)
> ...
> However, this subsetting obviously doesn't work. How can I adjust my code
> so that R runs down all levels of Data in a for loop?
> (I need the for loop, not tapply(), sapply(), ...)
> Thanks in advance,
> Dieter Anseeuw
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