[SciPy-user] Scilab to Scipy

Warren Weckesser warren.weckesser@gmail....
Fri Aug 22 12:23:06 CDT 2008


Here is one way to implement the corrected scilab code with pylab:

####################
from pylab import *

Lx = 1.0
Ly = 1.0
n = 2
m = 3
f = 1.0
w = 2*pi*f
t = 1.0
A = 2.0
Kx = n*pi/Lx
Ky = m*pi/Ly

grid_size = 200
x = linspace(0,Lx,grid_size)
y = linspace(0,Ly,grid_size)
[X,Y] = meshgrid(x,y)
z = A * sin(Kx*X) * sin(Ky*Y) * cos(w*t)

contour(X,Y,z)
show()

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


On Fri, Aug 22, 2008 at 12:37 PM, Warren Weckesser <
warren.weckesser@gmail.com> wrote:

> 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@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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>> SciPy-user@scipy.org
>> http://projects.scipy.org/mailman/listinfo/scipy-user
>>
>>
>
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