Accessing array entries

Array entries are accessed by indexes. In contrast to vector coefficients two indexes are needed to access matrix coefficients. These are given in one pair of brackets. This distinguishes the array syntax from a list of lists. There, two pairs of brackets are needed to access elements.

M = array([[1., 2.],[3., 4.]])
M[0, 0] # first row, first column: 1.
M[-1, 0] # last row, first column: 3.

Basic array slicing

Slices are similar to those of lists except that there might now be in more than one dimension:

• M[i,:] is a vector filled by the row i of M.
• M[:,j] is a vector filled by the column i of M.
• M[2:4,:] is a slice of 2:4 on the rows only.
• M[2:4,1:4] is a slice on rows and columns.

The result of matrix slicing is given in the following figure (Figure 4.1):

Figure 4.1: The result of matrix slicing

Note

Omitting a dimension

If you omit an index or a slice, NumPy assumes you are taking rows only. M[3] is a vector that is a view on the third row of M and M[1:3] is a...