Choosing a random entry from a list?

I’ve got a bunch of constant lists from which I would like to choose random entries. Is there a nice way to do this? This doesn’t compile:

Every turn: say a random entry in {1, 2, 3, 4, 5}.

I suppose I could define a random entry from a list by getting the length of the list, choosing N randomly from 1 to the length, and choosing the Nth entry, but is there a quicker way to do it?

I do not think there is a one-line syntax for this, no. The method you describe works, as does:

sort X in random order; now Y is entry 1 of X;
(assuming that you do not need to preserve the order of X).

So I did this:

To decide what thing is a random member of (L - a list of things): let N be the number of entries in L; let K be a random number between 1 and N; decide on entry K of L.

Which doesn’t seem totally satisfactory, because it’d require a new definition if you wanted to choose from a list of numbers – but I only have lists of one kind of thing, so it works for me.

You could make it more general with:

To decide what thing is a random member of (L - a list of values):
   let N be the number of entries in L;
   let K be a random number between 1 and N;
   decide on entry K of L.

Thanks Victor and George! Victor, somehow I missed your post when I posted the second time; I’d thought of that, too, but this is something I’ll be calling several times a turn, and resorting the lists seems like it’d be too computationally intensive.

George, when I do that I get a compiler error:

It’s a pretty quick operation for anything reasonable-sized. But for your purpose, choosing a random number is certainly more elegant.

Perhaps this should be a feature request? Choosing a random element from a list sounds like a pretty basic operation.

You want something much like that:

To decide what K is a random member of (list - list of values of kind K): let the count be the number of entries in the list; let the index be a random number between one and the count; decide on entry index of the list.
See WI 21.7.