11.1 Active Edges

In this lecture we move from passive chart parsing to active chart parsing. Active chart parsing is based on a very simple idea. A passive chart, like the one we worked with last week, is used to keep a record of complete constituents that we have found. (For example, on a passive chart we can record the information that the string between nodes 2 and 4 is an NP.) In an active chart we additionally record hypotheses --- that is, we record what it is we are actually looking for, and how much of it we have found so far. Such information is recorded in active edges (or active arcs).

Here's an example of an active edge:

This is an arc between nodes 0 and 2. The important thing to note is the arc label: $S\ \rightarrow\ N\!P\ .\ V\!P$ This says: ``I am trying to build an s, consisting of an np followed by a vp. So far I have found an np arc, and I'm still looking for the vp.'' So the insignificant looking `.' in the middle of the rule is very important: it marks the boundary between what we have found so far, and what we are still looking for.

Here's another example:

This says: ``I am trying to build an s, consisting of an np followed by a vp, starting at node 0. So far I haven't found either the np or the vp that I need.''

Here's another example:

This says: ``I am trying to build a vp, consisting of a dv followed by an np followed by a pp, starting at node 4. So far I have found a dv arc and an np arc, and I'm still looking for the pp.'' The np-arc ends in node 8, so the pp-arc will have to start in node 8.

Here's another example:

This says: ``I am trying to build an vp consisting of a tv followed by an np starting at node 4 --- and I've done it!'' Note that the `.' is at the end of the arc label. This means that everything we need to find has been found. Such an arc is called a passive edge, or a passive arc. It records complete information. In effect, these were the arcs we used last week, though we didn't use the `.' notation.

One final example:

This says: ``I am trying to build an s consisting of an np and a vp starting at node 0 --- and I've done it! I've found both the np and the vp, and moreover, I've done it in a way that the arc goes all the way from the first to the last node. Hey! This means I've recognized the sentence!.''