Why does Plot try symbolic values?

I have the following code:

Test[a_] := (Print[a]; a*2)

Plot[Test[a], {a, 0, 10}, PlotPoints -> 5, MaxRecursion -> 0]

This should plot a*2 for five points between a = 0 and a = 10, and print whatever a is every time Test[a] is evaluated. Indeed, that's exactly what it does, and this is the output:

0
0.0025025
a
2.5*10^-6
2.40451
5.01127
7.44529
10.

Most of this is pretty reasonable - I'd expect Plot to try 0, 2.5, 5, 7.5, and 10 - but why is a a value that Plot tries? It's symbolic, so I don't see why it would ever be used in a plot.

Moreover, if I change Test so that it returns Null:

Test[a_] := (Print[a]; a*2;)

Plot[Test[a], {a, 0, 10}, PlotPoints -> 5, MaxRecursion -> 0]

Plot no longer tries any symbolic values at all:

0
0.0025025
2.40451
5.01127
7.44529
10.

It also no longer tries 2.5*10^-6, though I'm not sure if that's related.

What's going on here? Why is Plot trying symbolic values (and why doesn't it do this when my function returns Null?)

edit: To clarify, this is a minimal working example. I don't actually want to print out the values as a side-effect. I have a compound expression that finds some value I care about and returns that, and when I try and plot the result for different values, it breaks due to having a symbolic variable passed in.

Another trivial example of code that does this, as requested in the answers:

Test[a_] := (
val = Sqrt[a];
Print[a];
solution = val*2;
solution)

The Print[a]; line is only here to show the values of a; this behaviour still occurs when I delete that line.

• Just use '?NumericQ' pattern test on argument(s) to prevent this causing issues w/ your function... I'm sure others will post details (I'm mobile, so terse...) – ciao Apr 11 '15 at 8:58
• I did try that, but I still get errors for some reason. It continues to evaluate anyway, and I get errors turning up with NDSolve. – Matthew Apr 11 '15 at 9:04