Part specification error only in LogPlot

Question

I've a problem. I defined a simple list and a function like below:

lst := {5, 10, 20}
f[m_] := lst[[Floor[m/10]]];

and drew a plot

Plot[f[x], {x, 10, 38}]

which works fine. But when calling LogPlot

LogPlot[f[x], {x, 10, 38}]

the following error appears before the plot

Part::pspec: "Part specification Floor[x/10] is neither a machine-sized integer nor a list of machine-sized integers"

Can anyone suggest me what's going on and how to deal with this error?

Thank you in advance!

Answer 1

LogPlot[] is trying to symbolically manipulate your f[], and the Part[] function fails at that.

You may try to specify that your function should be evaluated only for numerical input, but not symbolically:

lst := {5, 10, 20}
f[m_?NumericQ] := lst[[Floor[m/10]]];
LogPlot[f[x], {x, 10, 38}]