What's wrong with my rule?

Question

Consider a simple code to form a SparseArray:

Q = 5;
a = Table[i, {i, Q}];
SparseArray[{{j_} /; j > 2 -> a[[j]]}, {Q}]

Part::pspec: Part specification j is neither a machine-sized integer nor a list of machine-sized integers. >>

What is wrong with the rule? It seems I can not address any matrix element without getting such a warning. Sorry for such a minor question.

Answer 1

As Belisarius says in a comment, usingRuleDelayed (:>) instead ofRule (->) will fix your code.

Q = 5;
a = Table[i, {i, Q}];
SparseArray[{{j_} /; j > 2 :> a[[j]]}, {Q}]

SparseArray[<3>, {5}]

Normal[%]

{0, 0, 3, 4, 5}