Note

Click here to download the full example code

# DGL Basics¶

**Author**: Minjie Wang, Quan Gan, Yu Gai,
Zheng Zhang

In this tutorial, you learn how to create a graph and how to read and write node and edge representations.

## Creating a graph¶

The design of `DGLGraph`

was influenced by other graph libraries. You
can create a graph from networkx and convert it into a `DGLGraph`

and
vice versa.

```
import networkx as nx
import dgl
g_nx = nx.petersen_graph()
g_dgl = dgl.DGLGraph(g_nx)
import matplotlib.pyplot as plt
plt.subplot(121)
nx.draw(g_nx, with_labels=True)
plt.subplot(122)
nx.draw(g_dgl.to_networkx(), with_labels=True)
plt.show()
```

The examples here show the same graph, except that `DGLGraph`

is always directional.

You can also create a graph by calling the DGL interface.

In the next example, you build a star graph. `DGLGraph`

nodes are a consecutive range of
integers between 0 and `number_of_nodes()`

and can grow by calling `add_nodes`

.
`DGLGraph`

edges are in order of their additions. Note that
edges are accessed in much the same way as nodes, with one extra feature: *edge broadcasting*.

```
import dgl
import torch as th
g = dgl.DGLGraph()
g.add_nodes(10)
# A couple edges one-by-one
for i in range(1, 4):
g.add_edge(i, 0)
# A few more with a paired list
src = list(range(5, 8)); dst = [0]*3
g.add_edges(src, dst)
# finish with a pair of tensors
src = th.tensor([8, 9]); dst = th.tensor([0, 0])
g.add_edges(src, dst)
# Edge broadcasting will do star graph in one go!
g.clear(); g.add_nodes(10)
src = th.tensor(list(range(1, 10)));
g.add_edges(src, 0)
import networkx as nx
import matplotlib.pyplot as plt
nx.draw(g.to_networkx(), with_labels=True)
plt.show()
```

## Assigning a feature¶

You can also assign features to nodes and edges of a `DGLGraph`

. The
features are represented as dictionary of names (strings) and tensors,
called **fields**.

The following code snippet assigns each node a vector (len=3).

Note

DGL aims to be framework-agnostic, and currently it supports PyTorch and MXNet tensors. The following examples use PyTorch only.

```
import dgl
import torch as th
x = th.randn(10, 3)
g.ndata['x'] = x
```

`ndata`

is a syntax sugar to access the state of all nodes.
States are stored
in a container `data`

that hosts a user-defined dictionary.

```
print(g.ndata['x'] == g.nodes[:].data['x'])
# Access node set with integer, list, or integer tensor
g.nodes[0].data['x'] = th.zeros(1, 3)
g.nodes[[0, 1, 2]].data['x'] = th.zeros(3, 3)
g.nodes[th.tensor([0, 1, 2])].data['x'] = th.zeros(3, 3)
```

Out:

```
tensor([[True, True, True],
[True, True, True],
[True, True, True],
[True, True, True],
[True, True, True],
[True, True, True],
[True, True, True],
[True, True, True],
[True, True, True],
[True, True, True]])
```

Assigning edge features is similar to that of node features, except that you can also do it by specifying endpoints of the edges.

```
g.edata['w'] = th.randn(9, 2)
# Access edge set with IDs in integer, list, or integer tensor
g.edges[1].data['w'] = th.randn(1, 2)
g.edges[[0, 1, 2]].data['w'] = th.zeros(3, 2)
g.edges[th.tensor([0, 1, 2])].data['w'] = th.zeros(3, 2)
# You can also access the edges by giving endpoints
g.edges[1, 0].data['w'] = th.ones(1, 2) # edge 1 -> 0
g.edges[[1, 2, 3], [0, 0, 0]].data['w'] = th.ones(3, 2) # edges [1, 2, 3] -> 0
```

After assignments, each node or edge field will be associated with a scheme containing the shape and data type (dtype) of its field value.

```
print(g.node_attr_schemes())
g.ndata['x'] = th.zeros((10, 4))
print(g.node_attr_schemes())
```

Out:

```
{'x': Scheme(shape=(3,), dtype=torch.float32)}
{'x': Scheme(shape=(4,), dtype=torch.float32)}
```

You can also remove node or edge states from the graph. This is particularly useful to save memory during inference.

```
g.ndata.pop('x')
g.edata.pop('w')
```

### Working with multigraphs¶

Many graph applications need parallel edges. To enable this, construct `DGLGraph`

with `multigraph=True`

.

```
g_multi = dgl.DGLGraph(multigraph=True)
g_multi.add_nodes(10)
g_multi.ndata['x'] = th.randn(10, 2)
g_multi.add_edges(list(range(1, 10)), 0)
g_multi.add_edge(1, 0) # two edges on 1->0
g_multi.edata['w'] = th.randn(10, 2)
g_multi.edges[1].data['w'] = th.zeros(1, 2)
print(g_multi.edges())
```

Out:

```
(tensor([1, 2, 3, 4, 5, 6, 7, 8, 9, 1]), tensor([0, 0, 0, 0, 0, 0, 0, 0, 0, 0]))
```

An edge in multigraph cannot be uniquely identified by using its incident nodes
\(u\) and \(v\); query their edge IDs use `edge_id`

interface.

```
eid_10 = g_multi.edge_id(1, 0)
g_multi.edges[eid_10].data['w'] = th.ones(len(eid_10), 2)
print(g_multi.edata['w'])
```

Out:

```
tensor([[ 1.0000, 1.0000],
[ 0.0000, 0.0000],
[ 0.1254, -0.5753],
[ 0.2376, 0.2539],
[-0.5524, 0.4663],
[ 0.1794, -1.8850],
[-0.5403, -0.2288],
[ 0.2820, 0.3863],
[ 0.1882, 0.4798],
[ 1.0000, 1.0000]])
```

Note

- Nodes and edges can be added but not removed.
- Updating a feature of different schemes raises the risk of error on individual nodes (or node subset).

## Next steps¶

In the next tutorial you learn the DGL message passing interface by implementing PageRank.

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