Question

I have been looking over and interpreting some sample code and am having some trouble understanding what is going on with a bit of slicing syntax. For some context I am working on a computational program that uses method of finite differences approximating temperatures over a square region.

Here is some of the code:

maxIter = 500
lenX = lenY = 20 
delta = 1
Ttop = 100
Tbottom = 0
Tleft = 0
Tright = 0
Tguess = 30

T = np.empty((lenX, lenY))
T.fill(Tguess)

T[(lenY-1):, :] = Ttop
T[:1, :] = Tbottom
T[:, (lenX-1):] = Tright
T[:, :1] = Tleft

the last bit is what I am having trouble understanding. I am unsure exactly what the placement of the colons and commas are doing in the T[] boundary conditions. The next part of the code proceeds with for loops with T[i,j].

Like looking at the first one it seems to say in first index of T slice from 9 (lenY-1) to the end of the list and in the second index dont slice anything, then it sets that to 100. I understand this is making the temp along the top boundary 100, I am just not sure how that is happening from the syntax present.

Answer 1

T[a:b,c:d] references the square matrix formed by the intersection of the rows a, a+1 ....b-1 and the columns c,c+1,....d-1

If a: is given it means that it is a: end where the end is the references the next address of the last element

If: a is given it means that it is begin:a where begin is the address of the first element

Consider T[(lenY-1):,:] = T[(lenY-1):end,begin:end] = T[lenY-1,begin:end] So we are essentially accessing the entire last row

Similarly T[:1,:] accesses the entire first row

T[:,(lenX-1):] accesses the entire last column

T[:,:1] accesses the entire first column