# Documentation

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# poiindex

(Not recommended) Indices of points in canonical ordering for rectangular grid

`poiindex` is not recommended. To solve Poisson's equations, use `solvepde`. For details, see Solve Problems Using PDEModel Objects.

## Syntax

```[n1,n2,h1,h2,i,c,ii,cc] = poiindex(p,e,t,sd) ```

## Description

`[n1,n2,h1,h2,i,c,ii,cc] = poiindex(p,e,t,sd)` identifies a given grid `p`, `e`, `t` in the subdomain `sd` as an evenly spaced rectangular grid. If the grid is not rectangular, `n1` is 0 on return. Otherwise `n1` and `n2` are the number of points in the first and second directions, `h1` and `h2` are the spacings. `i` and `ii` are of length `(n1-2)*(n2-2)` and contain indices of interior points. `i` contains indices of the original mesh, whereas `ii` contains indices of the canonical ordering. `c` and `cc` are of length `n1*n2-(n1-2)*(n2-2)` and contain indices of border points. `ii` and `cc` are increasing.

In the canonical ordering, points are numbered from left to right and then from bottom to top. Thus if `n1 = 3` and ```n2 = 5```, then `ii = [5 8 11]` and ```cc = [1 2 3 4 6 7 9 10 12 13 14 15]```.