[R] using for loop for data frame

[Rui Barradas]

Hello,

Inline.

Em 15-04-2014 18:02, Monaly Mistry escreveu:
> Hi,
>
> I have a data frame with 15 different years and each year has a different
> sample size.  I'm having trouble writing a for loop to have a t-test done
> for each year for two of the columns.  Thus far I have been able to loop
> over the years but the output is the same t-test 15 times.  I'm new to
> writing loops and functions, and am trying to figure out a way to use them
> together (if there are any general rules to follow, I would be great-full
> to know)
>
> for(year in 1998:2012){
>    b <-t.test(tr$adj_ind_pop_dif, tr$ind_nei_dif, paired=TRUE)
>    print(b)
> }

The problem with this loop is that you don't use the variable 'year' in 
it. It just repeats the same test 15 times.
>
> the other way I did it was to take subsets of the dataframe and then do a
> separate t-test for each
> ab<-subset(tr, BroedJaar =="1999")
> t.test(ab$ind_nei_dif, ab$adj_ind_pop_dif, paired=TRUE)

So BroedJaar is a character variable? Try

for(year in 1998:2012){
	ab <- subset(tr, BroedJaar == as.character(year))
	b <- t.test(ab$adj_ind_pop_dif, ab$ind_nei_dif, paired=TRUE)
	print(b)
}


Note however that this won't store the results of the t-tests, all but 
the last one will be lost. (The last one is stored in 'b'.)
If you want the results of the 15 tests you can do

result <- lapply(1998:2012, function(year){
		ab <- subset(tr, BroedJaar == as.character(year))
		t.test(ab$adj_ind_pop_dif, ab$ind_nei_dif, paired=TRUE)
	})
names(result) <- 1998:2012

Then you can access the results with something like

result[[1]]  # first test
result[["1998"]]  # equivalent


Hope this helps,

Rui Barradas

>
> Best,
>
> Monaly.
>
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>
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