来自Matlab的python中的数组运算/切片

Question

I have matlab array operations as the following :

[M,N]=size(I) ;
J = zeros(2*M,2*N) ;

J(1:2:end,1:2:end) = I ;

J(2:2:end-1,2:2:end-1) = 0.25*I(1:end-1,1:end-1) + 0.25*I(2:end,1:end-1) + 0.25*I(1:end-1,2:end) + 0.25*I(2:end,2:end) ;

J(2:2:end-1,1:2:end) = 0.5*I(1:end-1,:) + 0.5*I(2:end,:) ;
J(1:2:end,2:2:end-1) = 0.5*I(:,1:end-1) + 0.5*I(:,2:end) ;

I am trying to rewrite the same operations in python and I have come up with the following:

J=numpy.zeros((2*M,2*N))

J[::2,::2] = I ;

J[2:-1:2,2:-1:2] = 0.25*I[1::-1,1::-1] + 0.25*I[2::,1::-1] + 0.25*I[1::-1,2::] + 0.25*I[2::,2::] 

J[2:-1:2,1::2] = 0.5*I[1::-1,] + 0.5*I[2::,]

J[::2,2:-1:2] = 0.5*I[:,1::-1] + 0.5*I[:,2::]

however the python code gives me different results.

can anyone tell me what is wrong?

Thanks,

Answer 1

Going through this piece by piece shows that you have some errors in your ranges. I think that you have misunderstood a few things about arrays in python.

1. Unlike matlab where the first element of an array is array[1] , in python the first element of an array is array[0] .
2. Array slicing syntax is array[start:stop:step] , so to get every second element starting at the fifth element in the array to the end you would do array[4::2] .

Just go through this piece by piece and you will find problems. For example, the first element on the right hand side of the second equation should be:

0.25*I[0:-1, 0:-1]

Note that you don't need the second colon here since your step is 1 and in cases where you want to change the step, the step goes last.

Answer 2

so here is the correct ported code:

J[::2,::2] = I ;

J[1:-1:2,2:-1:2] = 0.25*I[0:-1,0:-1] + 0.25*I[1::,0:-1] + 0.25*I[0:-1,1::] + 0.25*I[1::,1::] 

J[1:-1:2,0::2] = 0.5*I[0:-1,] + 0.5*I[1::,]

J[0::2,1:-1:2] = 0.5*I[:,0:-1] + 0.5*I[:,1::]