Question

I have a vector A which contains zeros and ones. I would like to randomly change n percent of the ones to zero. Is this the best way to do it in R (10% change):

for (i in 1:length(A)) 
{
    if(A[i] > 0)
    {
        if(runif(1) <= 0.1)
        {
            A[i] = 0
        }
    }
}

Thanks.

Answer 1

You can do this without using the for loops and if statements:

##Generate some data
R> A = sample(0:1, 100, replace=TRUE)
##Generate n U(0,1) random numbers
##If any of the U's are less then 0.1
##Set the corresponding value in A to 0
R> A[runif(length(A)) < 0.1] = 0

The other point to note, is that you don’t have to do anything special for values of A that actually equal 0, as the probability of change a 1 to a 0 is still 0.1.

As Hadley points out, your code doesn’t randomly change 10% of 1’s to 0. If that is really your intention, then:

##Select the rows in A equal to 1
R> rows_with_1 = (1:length(A))[A==1]
##Randomly select a % of these rows and set equal to zero
##Warning: there will likely be some rounding here
R> A[sample(rows_with_1, length(rows_with_1)*0.1)] = 0