Goes through the list once, and reads like set notation!
(and does i+1 once, which is what I assume you were going for with the separate [i+1 for i in a])
I kept the i+1 separate because the parent noted that this was a trivial example and wanted to make a point about the more general map-filter-map operation. I wanted to make the point that you can do the map-filter-map in Python more succinctly and more efficiently without sacrificing any power or flexibility.
In this example, yes, it's easy to solve the problem with only one iteration through the list. In a more complicated example (especially when the first map step is expensive and you really only want to do it once) this kind of solution may not work.
In this example, yes, it's easy to solve the problem with only one iteration through the list. In a more complicated example (especially when the first map step is expensive and you really only want to do it once) this kind of solution may not work.