Error in apply(x2, 1, diff) : dim(X) must have a positive length

Anyone knows hat might be the cause of this error? Thanks for any  
help!

library(MASS)
dif.mns = function(x2,tr1=.2,tr2=.3){

+ #generates four different 'means' using
+ #difference scores from x2, an n x 2 matrix
+ #for use w/ bootstrap comparisons
+     diffs = apply(x2,1,diff)
+     mn1=mean(diffs)
+     mn2=mean(diffs,tr=.2)
+     mn3=mean(diffs,tr=.3)
+     mn4=median(diffs)
+ mns=c(mn1,mn2,mn3,mn4)
+ list(mnds=round(mns,3)) }

dif.mns(shoes)

Error in apply(x2, 1, diff) : dim(X) must have a positive length

as.matrix(data.frame(shoes),nrows=10,ncols=2,byrow=T)

y=as.matrix(data.frame(shoes),nrows=10,ncols=2,byrow=T)
class(y)

apply(y,1,diff)

dif.mns <- function(x2,tr1=.1,tr2=.2){

dif.mns(y,tr1=.1,tr2=.2)

bootstrap(y,nboot=1000,theta=dif.mns)

Thank you for your help, David. I was trying to run bootrapping on the
dataset "shoes" from MASS package. But I still have some problem here.

as.matrix(data.frame(shoes),nrows=10,ncols=2,byrow=T)

A B
[1,] 13.2 14.0
[2,] 8.2 8.8
[3,] 10.9 11.2
[4,] 14.3 14.2
[5,] 10.7 11.8
[6,] 6.6 6.4
[7,] 9.5 9.8
[8,] 10.8 11.3
[9,] 8.8 9.3
[10,] 13.3 13.6

y=as.matrix(data.frame(shoes),nrows=10,ncols=2,byrow=T)
class(y)

[1] "matrix"

apply(y,1,diff)

[1] 0.8 0.6 0.3 -0.1 1.1 -0.2 0.3 0.5 0.5 0.3

dif.mns <- function(x2,tr1=.1,tr2=.2){

+ diffs=apply(x2,1,diff)
+ mn1=mean(diffs)
+ mn2=mean(diffs,tr=.1)
+ mn3=mean(diffs,tr=.2)
+ mn4=median(diffs)
+ mns=c(mn1,mn2,mn3,mn4)
+ list(mnds=round(mns,3))}

dif.mns(y,tr1=.1,tr2=.2)

$mnds
[1] 0.410 0.400 0.417 0.400

bootstrap(y,nboot=1000,theta=dif.mns)

Error in apply(x2, 1, diff) : dim(X) must have a positive length

I am not sure what I am doing wrong. When I run apply(y,1,diff) I am able to
get output, and when I run dif.mns with just the original dataset, I am also
able to get output.



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Thanks.

I use the bootstrap() function from the bootstrap package. Let me try it
again to see if I get any luck :)

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