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I use GNU Octave 4.4.1 (x86_64-pc-linux-gnu on Arch Linux), and I have problem with fplot function. I would like to know why

fplot(@(x) 1/(1+25*x.^2),[-1,1])

Have a different result of

fplot(@(x) (1+25*x.^2).^-1,[-1,1])

(the latter returns the correct graphic)

Had the exact same problem with my students in a windows version of Octave 4.4.1.

I would like to know if it is an actual error or there's a fancy explanation for why the latter command is correct and the first one is wrong (so I can try to convince my students that they should use GNU Octave. OK, I'll try it anyway).

Rui F Ribeiro
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Felipo Bacani
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1 Answers1

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The problem is a lack of a ".", however octave should produce a error message that is not coming out. To demonstrate the difference just calculate the values to be plotted:

octave:16> x=linspace(-1,1,11)
x =

 Columns 1 through 8:

  -1.00000  -0.80000  -0.60000  -0.40000  -0.20000   0.00000   0.20000   0.40000

 Columns 9 through 11:

   0.60000   0.80000   1.00000

octave:17> 1/(1+25*x.^2)
error: operator /: nonconformant arguments (op1 is 1x1, op2 is 1x11)
octave:17> 1./(1+25*x.^2)
ans =

 Columns 1 through 7:

   0.038462   0.058824   0.100000   0.200000   0.500000   1.000000   0.500000

 Columns 8 through 11:

   0.200000   0.100000   0.058824   0.038462

So the correct form is 

fplot(@(x) 1./(1+25*x.^2),[-1,1])
matzeri
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  • Thanks for the answer. I also don't know why the vector division doesn't return the same error I get with the multiplication without the "." – Felipo Bacani Dec 17 '18 at 22:39