I want to cbind matrix generated for each step in R, how to create a initial empty matrix say result=[] in matlab then cbind for each iteration?
Using cbind
in a loop is very slow. If you know the size in advance, you can preallocate the matrix and fill columns in the loop. Otherwise, use a list
. Create an empty list and add vectors to the list in the loop. Then, cbind the list into a matrix after the loop has finished.
Timings:
Preallocate matrix:
user system elapsed
1.024 0.064 1.084
Grow matrix with cbind:
user system elapsed
76.036 50.146 125.840
Preallocate list:
user system elapsed
0.788 0.040 0.823
Grow list by indexing:
user system elapsed
0.821 0.043 0.859
Code:
# Preallocate matrix.
f1 = function(x) {
set.seed(2718)
mat = matrix(ncol=x, nrow=x)
for (i in 1:x) {
mat[, i] = rnorm(x)
}
return(mat)
}
# Grow matrix with cbind.
f2 = function(x) {
set.seed(2718)
mat = c()
for (i in 1:x) {
mat = cbind(mat, rnorm(x))
}
return(mat)
}
# Preallocate list.
f3 = function(x) {
set.seed(2718)
lst = vector("list", length=x)
for (i in 1:x) {
lst[[i]] = rnorm(x)
}
res = do.call(cbind, lst)
return(res)
}
# Grow list by indexing.
f4 = function(x) {
set.seed(2718)
lst = list()
for (i in 1:x) {
lst[[i]] = rnorm(x)
}
res = do.call(cbind, lst)
return(res)
}
x = 3000
system.time(r1 <- f1(x))
system.time(r2 <- f2(x))
system.time(r3 <- f3(x))
system.time(r4 <- f4(x))
all.equal(r1, r2)
all.equal(r1, r3)
all.equal(r1, r4)