[SciPy-user] Scilab to Scipy

[Warren Weckesser]

Peter,

The problem is in your original scilab code.  You say
    x = linspace(0,100);
and in the calculation of the mode, you have the term sin(Kx*x). Since Kx =
2*pi, and your x ranges from 0 to 100, you should see 100 oscillations in
the x direction.  You don't see it, because your grid is too coarse.  The
default number of samples for the linspace function is 100.

What you really want is 0 <= x <= Lx.  In your scilab code, x should be set
like this:
    x = linspace(0,Lx,grid_size)
The third argument, as its name suggests, is the number of samples to use.

Here's a modified version of your scilab code (I also changed e^(%i*w*t) to
cos(w*t), but that was not the source of the problem):

####################
Lx = 1;
Ly = 1;
n = 2;
m = 2;
f = 100;
w = 2*%pi*f;
t = 1;
A = 2;
Kx = n*%pi/Lx;
Ky = m*%pi/Ly;

grid_size = 501;
x = linspace(0,Lx,grid_size);
y = linspace(0,Ly,grid_size);
z = zeros(grid_size,grid_size);

for i = 1:grid_size
    for j = 1:grid_size
        z(i,j) = A * sin(Kx*x(i)) * sin(Ky*y(j)) * cos(w*t);
    end
end

contour(x,y,z,20)

####################

Cheers,

Warren

On Fri, Aug 22, 2008 at 10:01 AM, peter websdell <
flyingdeckchair at googlemail.com> wrote:

> Hello all,
>
> I'm converting from using sclab/matlab to python. I'm finding it great so
> far, but I've discovered a problem that I don't understand.
>
> The following code displays a contour plot of the natural modes of a plate
> in scilab:
>
> ###################
> Lx=1;
> Ly=1;
> n=2;
> m=2;
> f=100;
> w=2*%pi*f;
> t=1;
> A=2;
> Kx=n*%pi/Lx;
> Ky=m*%pi/Ly;
>
> x=linspace(0,100);
> y=linspace(0,100);
> z=zeros(100,100);
>
> for i = 1:100
>     for j = 1:100
>         z(i,j) = A * sin(Kx*x(i)) * sin(Ky*y(j)) * %e^(%i*w*t);
>     end
> end
>
> contour(x,y,z,20)
> ##################
>
> Now here's how I've done it in python:
>
> ##################
> from pylab import *
>
> Lx=1
> Ly=1
> n=2
> m=2
> f=100
> w=2*pi*f
> t=1
> A=2
>
> Kx=n*pi/Lx
> Ky=m*pi/Ly
>
> x,y =mgrid[0:100,0:100]
> z=empty((100,100))
> z=A * sin(Kx*x) * sin(Ky*y) * e**(1j*w*t)
>
> contour(x,y,z)
> show()
> ###################
>
> The result does plot a contour, but it is garbage. I have tried replecating
> the silly for loop approach, and also using the real of abs value of the
> result, but it is still garbage.
>
> Can anyone offer some advice as to why the two scripts produce different
> results?
>
> Thanks a lot,
> Pete.
>
>
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>
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