repeat rows of matrix by number (k) in one colummatrix adding column j with values 1:k

Well,
I found a way to do it partly using rep(), and one loop that makes it 10x
or more faster however would still be good to do without the loop at all
matrix made slightly beigger (10000 rows):

M<-matrix(c(1:30000
), 10000,3)
M<-cbind(M,sample(1:5,size = 10000,replace = T))
#Print(M)
#Create matrix (Mout) in this case 8 rows with x,y,z in each row of M
#repeated k times with column j numbered from 1:k
# ! can do with nested loops but this is very slow ( example below)
#How do I acheive this quickly without loops?
print("double loop")
print(system.time({
Mout<-NULL

for(i in 1:nrow(M)){
  a=M[i,1:3]
  k=M[i,4]
  for (j in 1:k){
    b=c(a,j)
    Mout<-rbind(Mout,b)
    }
}
colnames(Mout)<-c("x","y","z","j")
}))

print("rep and single loop")
print(system.time({
  j<-NULL
  MOut<-NULL
  MOut<-M[,1:3][rep(1:nrow(M), times = M[,4]), ]
  for(i in M[,4])(j<-c(j,1:i))
  MOut<-cbind(MOut,j)
  colnames(Mout)<-c("x","y","z","j")
}))

On Wed, 1 Apr 2020 at 12:18, nevil amos <nevil.amos at gmail.com> wrote:

Hi

I can achieve this using two for loops but it is slow  I need to do this
on many matrices with tens of millions of rows of x,y,z and k

What is a faster method to achieve this, I cannot use rep as j changes in
each row of the new matrix
###############################################
M<-matrix(c(1,2,3,4,1,2,3,4,1,2,3,4, 2, 1, 3, 2
), 4,4, dimnames = list(NULL, c("x", "y", "z","k")))

Print(M)
#Create matrix (Mout) in this case 8 rows with x,y,z in each row of M
#repeated k times with column j numbered from 1:k
# ! can do with nested loops but this is very slow ( example below)
#How do I acheive this quickly without loops?
Mout<-NULL

for(i in 1:nrow(M)){
  a=M[i,c("x","y","z")]
  for (j in 1:M[i,"k"]){
    b=c(a,j)
    Mout<-rbind(Mout,b)
    }
}
colnames(Mout)[4]<-"j"
print(Mout)

#########################################################

Thanks

Nevil Amos

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