trouble using sapply to perform multiple t-test

Hi folks,

I'm having trouble with code that used to work, and I can't figure out
what's going wrong.  I'd be grateful for any help in sorting this out.


Suppose I define a matrix

mm<- matrix(1:15, 25,2)

and compare the first 15 values of column 1 of mm to the values remaining
in the same column and obtain p values as follows:

c1<- mm[,1]
out<- t.test(c1[1:15],c1[16:25]) ; out$p.value

This of course works fine, but if I try to embed this line in a call to
sapply to repeat this for each column, I get the following:

mm.pvals<- sapply(mm, function(x) {out<- t.test(x[1:15],x[16:25]) ;

out$p.value})
Error in t.test.default(x[1:15], x[16:25]) : not enough 'x' observations

What is baffling is code like this has worked for me before, and I can't
tell what's triggering the error.

Hi David,
You have here the old "s" problem. I suspect that you previously ran 
this code on a data frame, not a matrix. Try this:

mm.pvals<-apply(mm,2,
  function(x) {out <- t.test(x[1:15],x[16:25]);out$p.value})

Jim

Hi David,
Try:
Check the output of:
lapply(mm,function(x) x) #mm is matrix
#and
lapply(as.data.frame(mm),function(x) x)




? sapply(split(mm,col(mm)),function(x){out <- t.test(x[1:15],x[16:25])$p.value}) 
?# ????? 1???????? 2 
#0.1091573 1.0000000


#or 
?sapply(as.data.frame(mm), function(x)? t.test(x[1:15],x[16:25])$p.value)
#?????? V1??????? V2 
#0.1091573 1.0000000 

A.K.

Hi folks,

I'm having trouble with code that used to work, and I can't figure out
what's going wrong.? I'd be grateful for any help in sorting this out.


Suppose I define a matrix

mm <- matrix(1:15, 25,2)

and compare the first 15 values of column 1 of mm to the values remaining
in the same column and obtain p values as follows:

c1 <- mm[,1]
out <- t.test(c1[1:15],c1[16:25]) ; out$p.value

This of course works fine, but if I try to embed this line in a call to
sapply to repeat this for each column, I get the following:

mm.pvals <- sapply(mm, function(x) {out <- t.test(x[1:15],x[16:25]) ;

out$p.value})
Error in t.test.default(x[1:15], x[16:25]) : not enough 'x' observations

What is baffling is code like this has worked for me before, and I can't
tell what's triggering the error.

Thanks in advance for your help!

Best,
David

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