# Apply
#=========

ma <- matrix(c(1:4, 1, 6:8), nrow = 2)

apply(ma, 1, table)

apply(ma, 1, stats::quantile)
apply(ma, 2, mean)

apply(m, 2, function(x) length(x[x<0]))


sapply lapply rollapply
sapply(1:3, function(x) x^2)

lapply return a list:
lapply(1:3, function(x) x^2)
use unlist with lapply to get a vector

sapply(1:3, function(x, y) mean(y[,x]), y=m)



A<-matrix(1:9, 3,3)
B<-matrix(4:15, 4,3)
C<-matrix(8:10, 3,2)
MyList<-list(A,B,C)
Z=sapply(MyList,"[", 1,1 )

#==========
te=matrix(1:20,nrow=2)
sapply(te,mean)	# this is a vector, order arrange in matrix direction
matrix(sapply(te,mean),nrow=2)	# this is changed to matrix