Selecting only list of function output from piecewise function output

Question

Say I have a piecewise function:

V[m_] := Piecewise[{{(1 + m)^2, m >= 1}, {Log[m], m < 1}}]

And I input a large list of m values, I get three lists output: The list of V[m] values, the list of m values for which m >= 1, and the list of m values for which m < 1.

I only require an output list of the V[m] values. How do I get rid of the other two lists?

Apologies if this is an obvious question! Any help would be greatly appreciated.

I am not understanding the issue here. V /@ {1, 10, 200, 0.1, 100} outputs {4, 121, 40401, -2.30259, 10201}. Is this not what you want? — Mr.Wizard, Feb 18 '17 at 10:49
Possible duplicate of How to select a certain portion of a piecewise function? — zhk, Feb 18 '17 at 10:49
Ah - many apologies. I was incorrectly calling the function V - thank you @Mr.Wizard for solving. — emc3636, Feb 18 '17 at 10:55
You can have V evaluate lists the way you were expecting it to. See my answer below. — m_goldberg, Feb 18 '17 at 11:01

m_goldberg · Answer 1 · 2017-02-18T11:03:17.393

With the definition you have given, V isn't set up to thread over lists, which is what you seem to expect it to do. You can make that work that easy enough by giving it the Listable attribute.

SetAttributes[V, Listable]
V[m_] := Piecewise[{{(1 + m)^2, m >= 1}, {Log[m], m < 1}}]

SeedRandom[42]; data = RandomReal[4, 10]

{1.70362, 1.56409, 1.38828, 1.81496, 2.22385, 1.15668, 1.18739, 0.825631, 1.30068, 3.8933}

Now,

V[data]

gives

{7.30957, 6.57457, 5.70387, 7.92401, 10.3932, 4.65126, 4.78468, -0.191608, 5.29312, 23.9444}

It will still work as it did originally if given a single value.

V[10]

121

Selecting only list of function output from piecewise function output

1 Answers1