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Doubly subscripted arrays (matrices) can be constructed as a list of
vectors (lists), where each vector represents a row of the matrix. So
to make a matrix `b`

with `n`

rows and
`m`

columns we could do this:
**
**

** b = [[0 for i in range(m)] for j in range(m)]
**

**
**
(You need to be careful about getting all the parentheses and
brackets right.) Then we can refer to an element of the matrix with
two subscripts, namely, `b[i][j]`

. As you can see, setting
up matrices in Python is a bit clunky. Later, we use the `numpy`

package, which will make it more elegant.

Carleton DeTar
2017-09-19