What does [i,:] mean in Python?

Question

So I'm finished one part of this assignment I have to do. There's only one part of the assignment that doesn't make any sense to me.

I'm doing a LinearRegression model and according to others I need to apply ans[i,:] = y_poly at the very end, but I never got an answer as to why.

Can someone please explain to me what [i,:] means? I haven't found any explanations online.

Answer 1

It's specific to numpy module, used in most data science modules.

ans[i,:] = y_poly

this is assigning a vector to a slice of numpy 2D array (slice assignment). Self-contained example:

>>> import numpy
>>> a = numpy.array([[0,0,0],[1,1,1]])
>>> a[0,:] = [3,4,5]
>>> a
array([[3, 4, 5],
       [1, 1, 1]])

There is also slice assignment in base python, using only one dimension (a[:] = [1,2,3])

Answer 2

I guess you are also using numpy to manipulate data (as matrix) ?

If based on numpy, ans[i,:] means to pick the ith 'row' of ans with all of its 'columns'. Note,when dealing with numpy arrays, we should (almost) always use [i, j] instead of [i][j]. This might be counter-intuitive if you used Python or Java to manipulate matrix before.