why doesn't this ReplaceAll work in mathematica

Question

I have

Table[{x1, 1, 2, 3}^i, {i, 0, 3}] /. x1 -> 1/2

But the following does not work, since x1 is not replaced with 1/2

Table[{x1, 1, 2, 3}^i, {i, 0, 3}] // Inverse /. x1 -> 1/2

Could anybody let me know why and how to fix it? Many thanks!

Answer 1

Look at TreeForm to see how your expression is parsed.

TreeForm@Hold[Table[{x1, 1, 2, 3}^i, {i, 0, 3}] // Inverse /. x1 -> 1/2]
_{(source: yaroslavvb.com)}

Everything after // is taken to be function head that is applied with Postfix notation. So you need some parentheses

(Table[{x1, 1, 2, 3}^i, {i, 0, 3}] // Inverse) /. x1 -> 1/2