Indexing and Slicing Arrays
Array indexing selects values by position.
As with lists, indexing starts at 0.
Index a matrix with row and column
For a two-dimensional array:
A[row, column]
Example:
This reads row 0, column 1, which is 2.
Slice rows and columns
Use : to mean “all of this axis.”
A[0, :]
means row 0, all columns.
A[:, 1]
means all rows, column 1.
Rows and columns
Ready to run.
Slices preserve array structure
A[0, 1] selects one scalar value.
A[0, :] selects a one-dimensional array.
Those are different shapes. Inspect them when unsure:
print(A[0, :].shape)
Exercise: Select a column
Which expression selects column 1 from all rows of A?
Answer it first, then check.
Hint
The row position comes first. Keep every row with :, then select column index
1.
Solution
Use:
A[:, 1]
The : keeps all rows, and 1 selects the second column because indexing
starts at zero.