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

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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