Replace expression using a tuple

Question

Given an expression in x, y, z (for example), and a list with three elements, like {1,2,3}, how can I do the equivalent of expr/.{x->1,y->2,z->3}? So for example suppose

a = {1,2,3}
x^2+y^2+z^2 /.{x->1,y->2,z->3}

but obviously x^2+y^2+z^2/.{x,y,z}->a does not work.

Answer 1

You can do this:

a = {1, 2, 3}
x^2 + y^2 + z^2 /. Rule @@@ Transpose[{{x, y, z}, a}]

You might also use MapThread and the like. But anyways, make sure you understand what @@@ does.

Answer 2

This can be found in many answers, but I can't find the same question on this site or in the documentation. So:

x^2+y^2+z^2 /.Thread[{x, y, z} -> a]
  (* 14 *)