Evaluation in Manipulate [duplicate]

Question

I have an expression assigned to a variable. For example:

exp1 = a*x+b

Now I would like to plot it using something like this

Manipulate[
 Plot[Evaluate[ex99], {x, 0, 1}],
 {a, 0, 1},
 {b, 0, 1}]

I can see 'a' and 'b' being properly substituted by 'Evaluate', but Plot does not work as expected. I suspect it has something to do with 'x' substitution. How can I fix that? Thanks!

Answer 1

Another alternative is to define the function using set delayed.

ex99[a_, b_, x_] := a*x + b;
Manipulate[Plot[ex99[a, b, x], {x, 0, 1}], {a, 0, 1}, {b, 0, 1}]

Now it works fine.

Answer 2

You could try this:

Manipulate[Plot[a*x+b,{x,0,1},AxesOrigin->{0,0},PlotRange->{{0,1},{-2,2}}], {a,-1,1},{b,-1,1}]

Answer 3

Using CompoundExpression (a.k.a ";"), you can include expressions within Manipulate:

Manipulate[
     ex99 = a x + b;
     Plot[ex99, {x, 0, 1}],
     {a, 0, 1},
     {b, 0, 1}
   ]

The semicolon ";" is postfix notation for the CompoundExpression command, which has amongst other things the effect of evaluating the command in the kernel without producing output in the front end.