---
jupyter:
  description: 'Automatic ids in a planex2d sketch: Line.1, Arc.2, Point.3 and Chain.1 for everything drawn, or your
    own with id=. Ids stay in the sketch, while names reach the faces. An L bracket drilled by its ids.'
---

# Automatic ids

Every element drawn in a sketch gets an id made of its kind and a number, such as `Line.1`, `Arc.2`, `Circle.1` or
`Spline.1`, as in CAD sketchers. Every point gets an id like `Point.3`, and every chain drawn in one go an id like
`Chain.1`. `id=` gives an element an id of your own instead.

Ids are the references within the sketch, and they never leave it. Faces, wires and exports carry only the names given
with `name=`, which travel with the geometry through Boolean operations and into the mesh.

```python
import math
import planex as px
```

```python nbsphinx="hidden"
from docs_figures import Row, Vertices
```

## An L bracket drawn quickly

The example is an L bracket with legs 40 and 30 long and 10 thick, drawn without a single `id=` (Figure 1). Its six
sides are therefore `Line.1` to `Line.6` in drawing order, and its corners `Point.1` to `Point.6`, from the start at
the origin. The re-entrant corner is `Point.4`.

```python
s = px.QuickSketch().Line(40).Turn(90).Line(10).Turn(90).Line(30)
s.Turn(-90).Line(20).Turn(90).Line(10).Close()
corner = s.Location("Point.4")
print("Point.1", s.Location("Point.1"), "Point.4", corner)
```

```python nbsphinx="hide-input"
sides = [(f"Line.{k}", s.Location(f"Line.{k}.mid")) for k in range(1, 7)]
Row((s, "An L bracket labeled with its automatic ids", {"labels": sides + [("Point.4", corner)], "marks": [corner]}))
```

## Referring to them

Later calls refer to the automatic ids like to any other (Figure 2):

- a bolt hole of 5 lies opposite the middle of `Line.2`, the end of the long leg, 8 in from it, and gets the name
  `bolt` for the export;
- the washer around it is a construction circle, concentric with `Circle.1`, the automatic id of the hole;
- a construction line from the re-entrant corner `Point.4` to the outer corner `Point.1` marks where a gusset would
  go.

```python
end = s.Location("Line.2.mid")
s.Circle(2.5, center=(end[0] - 8, end[1]), name="bolt")
s.Circle(5, concentric="Circle.1", construction=True)
s.Move("Point.4").Line("Point.1", id="gusset line", construction=True)
bracket = s.Face()
```

```python nbsphinx="hide-input"
Row((s, "The hole Circle.1, its washer and the gusset line, by their ids"))
```

Only the name given with `name=` reaches the face, while the ids stay in the sketch:

```python
print("area", bracket.mass, "exact", 40 * 10 + 20 * 10 - math.pi * 2.5**2)
print({e.name for e in bracket.edges})
```

Ids are also how the editing calls find their elements. `Fillet("Point.4", 5)`, for example, rounds the re-entrant
corner by the id of its point. See [Fillets and chamfers](fillets-chamfers.md) for details.