0
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myfunn[x_, y_] := Module[{},
If[x < 5, Return[0]];
   If[x < 10, Return[2]];
   x*y];

I'm unable to find the maximum value of this function over the following range of x: 0<=x<=20 when y=0 using the NMaxValue, NMaximize or FindMaximum commands

NMaximize[{myfunn[x, 0], 0 <= x <= 20}, x]

NMaxValue[{myfunn[x, 0], 0 <= x <= 20}, x]

All of them return 0 while the answer I'm expecting is 2. Is there anyway to get Mathematica to compute the expected answer? Thanks

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5
  • $\begingroup$ Does it work if you do Clear[myfunn] followed by the definition myfunn[x_?NumericQ, y_?NumericQ] := ...? Alternatively, use Piecewise instead of If. $\endgroup$
    – Roman
    Commented Apr 6, 2019 at 16:01
  • $\begingroup$ Thank you. myfunn[x_?NumericQ, y_?NumericQ] seems to work although I'm not yet sure why (I'm a light Mathematica user) $\endgroup$
    – piotrekp2000
    Commented Apr 6, 2019 at 16:22
  • 1
    $\begingroup$ That's because when you call your function with symbolic arguments, the If statements never get activated. Check: myfunn[x,y] returns x y. $\endgroup$
    – Roman
    Commented Apr 6, 2019 at 17:09
  • $\begingroup$ What I meant to say is that in your definition, the argument myfunn[x,0] is immediately simplified to 0 before ever putting a numerical value in for x. The If statements thus never get to see any numerical values for x. $\endgroup$
    – Roman
    Commented Apr 6, 2019 at 19:08
  • $\begingroup$ Makes sense, thank you $\endgroup$
    – piotrekp2000
    Commented Apr 9, 2019 at 12:07

2 Answers 2

Reset to default
1
$\begingroup$ 
myfunn[x_, y_] = Piecewise[{{0, x < 5}, {2, x < 10}}, x y];

NMaximize[{myfunn[x, 0], 0 <= x <= 20}, x]

{2., {x -> 6.91445}}

NMaxValue[{myfunn[x, 0], 0 <= x <= 20}, x]

2.

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0
$\begingroup$

try 

myfunn[x_, y_] := If[x<5,0,If[x<10,2,x*y]];

NMaxValue[{myfunn[x, 0], 0<=x<=20}, x]

2

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