Changing the current entry in a list in Mathematica

Question

Probably very basic, sorry, but I've searched extensively without finding a solution:

list_2 = ReplacePart[list_1, _?EvenQ -> "Sin(value of the entry in this position in list_1)" ]

That is, creating list_2, for every second entry replace its value with the Sinus of the corresponding value in list_1.

So for example {1, pi/2} should return {1, 1}. What is the code I should use in "..."?

On a general note: How does one address the value of the "current position" in a list? Thanks.

Answer 1

list1 = {1, Pi/2, Pi, 3 Pi/2, 3, 10};

list2 = ReplacePart[list1, part_?EvenQ :> foo[list1[[part]]]]

{1, foo[π/2], π, foo[(3 π)/2], 3, foo[10]}

list3 = ReplacePart[list1, part_?EvenQ :> Sin[list1[[part]]]]

{1, 1, π, -1, 3, Sin[10]}