First ReplaceAll, then evaluate an expression

Question

Here is a simple example using ReplaceAll:

rvec[[2]] /. rvec -> {x, y}

I think Mathematica first tries to evaluate rvec[[2]], realising this is not a list, and only afterwards uses ReplaceAll. The warning output confirms this.

If instead of rvec[[2]] we have some complex expression (e.g. inversion of a matrix), Mathematica will to do it analytically first before making the substitution. This is not what I intend, I want to delay the evaluation of rvec[[2]] (or other more complex things in its place) until the ReplaceAll substitution has been done.

How can I tell Mathematica to substitute first, then evaluate?

Answer 1

You might be able to use Unevaluated:

Unevaluated[rvec[[2]]] /. rvec->{x,y}

y

Answer 2

You can also hold then release:

Hold[rvec[[2]]] /. rvec -> {x, y} // ReleaseHold

y

Pattern replacement works through hold.

Answer 3

(rvec /. rvec -> {x, y})[[2]]