#mean difference 
perm.test<-function(x,y,iter=10000){ 
d<-c(x,y) 
n1<-length(x) 
n<-length(d) 
st<-rep(NA,iter) 
for (i in 1:iter){ 
v<-sample(1:n,n1) 
st[i]<-mean(d[v])-mean(d[-v])} 
obs.st<-mean(x)-mean(y) 
p1<-sum(ifelse(st>=obs.st,1,0))/iter 
p2<-sum(ifelse(st<=obs.st,1,0))/iter 
p<-min(p1,p2)*2 #two-sided p-value 
list(obs.stat=obs.st,perm.dist=st,p.value=p)} 

#median difference 
st[i]<-median(d[v])-median(d[-v])} 
obs.st<-median(x)-median(y) 

#rank sum 
st[i]<-sum(rank(d)[v])} 
obs.st<-sum(rank(d)[1:n1]) 

#t-statistic 
st[i]<-as.numeric(t.test(d[v],d[-v])$statistic)} 
obs.st<-as.numeric(t.test(x,y)$statistic)