To draw a random number from a particular binomial distribution, we use this syntax like this
rbinom(1,0.5,10)
In my case, this call returned 4 but remember that the outcome is stochastic so you are likely to see a different value.
The first argument (1) tells RevBayes how many independent draws we want from this distribution, the second argument (0.5) specifies the probability of success, and the third argument (10) gives the number of trials. Remember that a single draw from a binomial involves multiple trials (10, in this case).
In RevBayes, you can (sometimes) get help for a function by typing
? rbinom
The output from rbinom(1,0.5,10) should look something like this
[4]
The brackets indicate that this is a vector, although it only contains a single value. Nonetheless, if we want to do math with that value, we have to extract it from the vector
binomVals <- rbinom(1,0.5,10)
binomVals[1]