# Symbolic and numeric calculations (and plots) simultanuosly [closed]

I use Mathematica to do a bunch of symbolic calculations (integrals, ...). This is good because I found that sometimes, if I plug in numeric values, Mathematica takes much longer.

However, sometimes I am interested in the numeric result. And sometimes I just want to plot something.

Currently I perform the symbolic calculations and then use pattern substitution with numeric values when I want to plot something or get the numeric result.

Very simple example:

ymodcst =
Integrate[
u*InverseLaplaceTransform[Kint/(s - pint), s, t] /.
t -> t - \[Tau], {\[Tau], 0, t}]


At some point, when I need to plot or want numeric values:

ymodcst/.Kint -> 100*^6*2*\[Pi] /. pint -> -1*^6/3*2*\[Pi]/.t -> 1*^-6/.u->1


or

Plot[ymodcst/.Kint -> 100*^6*2*\[Pi] /. pint -> -1*^6/3*2*\[Pi]/.u->1,{t,0,1*^-6}]


However, usually the symbolic expressions contain tons of variables and substituting them every time is cumbersome and error-prone.

When I define the variables first globally, e.g.

Kint = 100*^6*2*\[Pi]
pint = -1*^6/3*2*\[Pi]


I am back at the original problem.

So what I would look for is something like a toggle between "fill in the numeric values" and "do everything symbolically". Or better: A function "apply_numeric_values"

I hope my setup is clear.

## closed as off-topic by user9660, m_goldberg, MarcoB, dr.blochwave, JensMay 11 '16 at 16:33

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• I do not see what the problem actually. Can you not write something like params={a->100,b->200}; then do the symbolic calculations, and end write symbolicStuff/.params. I also find I found that sometimes, if I plug in numeric values, Mathematica takes much longer. interesting. One would expect the other way around, i.e. normally symbolic takes longer (but I guess it depends) – Nasser Nov 6 '14 at 2:04
• First: This was exactly I was looking for! You may create an answer with that. Second: I am not saying that numerical integration is slower than symbolic, I am saying that symbolic integration with (some) symbols replaced with their numerical values is slower than doing everything without replacing the symbols and replace the symbols just in the end. – divB Nov 6 '14 at 3:44