The left corner of a production rule in a context-free grammar is the left-most symbol on the right side of the rule.
For example, in the rule A→Xα, X is the left corner.
The left corner table associates a symbol with all possible left corners for that symbol, and the left corners of those symbols, etc.
Given the grammar
- S→VP
- S→NP VP
- VP→V NP
- NP→DET N
| Symbol | Left corner(s) |
|---|---|
| S | VP, NP, V, DET |
| NP | Det |
| VP | V |
Left corners are used to add bottom-up filtering of a top-down parser.
You can use the left corners to do top-down filtering of a bottom-up parser.
Famous quotes containing the words left and/or corner:
“I sometimes left a good fire when I went to take a walk in a winter afternoon; and when I returned, three or four hours afterward, it would be still alive and glowing. My house was not empty though I was gone. It was as if I had left a cheerful housekeeper behind. It was I and Fire that lived there; and commonly my housekeeper proved trustworthy.”
—Henry David Thoreau (1817–1862)
“In the Corner Store, near the village center, hangs a large sign reading: ‘After 40 years of credit business, we have closed our book of Sorrow.’”
—For the State of Maine, U.S. public relief program (1935-1943)