Date: Nov 9, 2011 5:58 AM
Author: mailcwc@gmail.com
Subject: Question about Manipulate

I tried the following code in Mathematica 8.0

Clear["Global`*"]
f = Sin[a x + b]
Manipulate[Plot[f, {x, 0, 4 \[Pi]}], {a, 0.5, 2}, {b, 0, 2 \[Pi]}]

The Manipulate block is showed, but the Sin function is not plotted.

When I replace f in the third line with its definition,

Clear["Global`*"]
Manipulate[ Plot[Sin[a x + b], {x, 0, 4 \[Pi]}], {a, 0.5, 2}, {b, 0, 2
\[Pi]}]

The Sin function is plotted.

Is this a bug or a limitation in Mathematica?