How do I create a loop to generate a list of random samples in R?
I am trying to create a loop that creates a series of objects containing a random sample, for example:
sample <- ceiling(runif(9, min=0, max=20))
(This is an example of a rounded uniform, but can be replaced with a regular, Poisson, or whatever you want.)
So, I built a loop to automatically generate different of these generators for inclusion in the data frame. Then I constructed this loop:
N=50 dep=as.vector(N) count=1 for (i in 1:N){ dep[count] <- ceiling(runif(9, min=0, max=20)) count=count+1 }
But it didn't work! For each dep [i], I only have a number, not a list of nine.
How am I supposed to do this? And if I want to include every dep [i] in a data frame?
Thanks a lot, I hope you understand what I want.
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This is because you made a dep
vector (which is 1D by default), but you are trying to store a 2D object in it.
You can dep
disable like NULL
and rbind
(row-bind) to it in a loop. Also note that instead of using count
in your loop, you can simply use i
:
dep <- NULL
for (i in 1:N){
dep <- rbind(dep, ceiling(runif(9, min=0, max=20)))
}
# if you look at dep now it a 2D matrix.
# We'll convert to data frame
dep <- as.data.frame(dep)
However , there is an easier way to do this. You don't need to generate dep
row-by-row rows , you can generate it in front by creating a vector containing 9*N
your rounded uniform spread numbers:
dep <- ceiling(runif(9*N,min=0,max=20))
Now it dep
is currently a vector of length 9 * N. Let's turn into an Nx9 matrix:
dep <- matrix(dep,nrow=N)
Done!
So, you can do all your code above in one line:
dep <- matrix( ceiling(runif(9*N,min=0,max=20)), nrow=N )
If you want, you can call data.frame
on dep
(after it has been placed in its 2D matrix form) to get the dataframe.
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As @ Mathematical.coffee explained. But also it seems in your case for runif
, you can use sample
instead. And actually sample.int
more reliable ... and about 3x faster than using runif here):
N <- 1000000
system.time( dep <- matrix(sample.int(20, 9*N, replace=TRUE), N) ) # 0.16 secs
range(dep) # 1 20
system.time( dep <- matrix(ceiling(runif(9*N, min=0, max=20)), N) ) # 0.45 secs
range(dep) # 1 20
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