Removing elements from a list based on condition

I am breaking my head on how to remove certain elements form a list (zeros in this case). I tried to do a zl.iter, into a select 0, and then catching all non-matching values in to a zl.group. The thing is that the zl.group needs to know beforehand how many elements to group. What's the best way to approach this?

Related question 1: if i want to add a value to every value-item in a list, is there a better way than to do a zl.iter, followed by a +, followed by a zl.group? It seems overly complicated.

Related question 2: I'm dealing with incoming midi-data, how bad is it (timingwise) to handle things like the above with a simple js function (with immediate set to 1)? I could solve stuff like this easily by code, but i am wondering if it's a good idea to step outside of the native objects when timing is critical.