Given the following matrix

```
\def\points{{ {-1, 3}, {2, 7}, {3, 8} }}

```

which corresponds to the 3x2 matrix

[![enter image description here](https://i.stack.imgur.com/JaJWH.png)](https://i.stack.imgur.com/JaJWH.png)

how do I 

1. iterate (in `tikz`) over the number of columns?
2. iterate (in `tikz`) over the number of rows?
3. address an element directly, e.g. in row 1, column 0 (i.e. 2)

A similar question regarding an array of coordinates is posed at [tikz array of coordinates: iteration and addressing elements](https://tex.stackexchange.com/questions/404235/tikz-array-of-coordinates-iteration-and-addressing-elements)