list manipulation - How to "ignore" an element of Map or MapIndexed

Say I have some function that I'm applying every element in a list to... if that element matches some criteria:

If[#==,{#},  ] & /@ LongList

Is there a way to do something like this? I want the result to just not have that element.

Answer

This specific behaviour can be achieved using

If[condition, something, Unevaluated@Sequence[]]& /@ list

The key is Sequence[]. Unevaluated prevents it from disappearing from inside the If.

Alternatively you can use Cases (or many other solutions shown in other answers and comments---some of these solutions may be better suited for the problem but Sequence[] has its place too).

Cases[list, element_ /; condition :> something]