Moderatorship in Computer Science and Language - IT Assignment Help

Assignment Task

1. Question concerns CFGs and their parsing

• (a) Assume the following outline of a CHART class:
• class CHART {
• public:
• /* a member ’chart’ such that chart[i][k] is the chart’s content concerning
• start = i, length = k */
• Grammar g;
• vector<string> input; // sequence to parse
• void initialize(); // set chart dimensions then put in length 1 entries
• void do_cell(int i, int k); // fill in all entries at chart[i][k]
• bool is_there(int i, int k, Category c); // check for c at chart[i][k]
• void add_edge(int i, int k, Category c); // add c to chart[i][k] if not there
• };
• The CHART::chart member is left underspecified: it must encode a 2-dimensional
• start-vs-length table, each slot of which represents a collection of Category objects.
• After CHART::initialize() has added all the length 1 entries,
• CHART::do_cell(i,k) is the central method of the CKY algorithm, repeatedly called
• in a nested loop structure. For start position i, length k, it should put every category
• C into chart[i][k] which is implied by rules applied to existing entries for shorter
• lengths k
• 0
• at appropriate positions. Provide an implementation of this method
• (assuming is there(..) and add edge(..) are already implemented). [15 marks]
• (b) Assuming the underspecified CHART::chart member is actually declared
• vector<vector<vector<Category>>> chart;
• give implementations of CHART::is there(..) and CHART::add(..). [7 marks]
• (c) let G1 be the grammar:
• initial(vp)
• vp --> va,np,[to],vp
• vp --> vb,np
• va --> [order]
• vb --> [find]
• np --> [it]
• np --> [him]
• i. The backtracking version of the top-down parser will accept ‘order him to find it’.
• Give its sequence of words+stack configurations (including some commentary on
• backtracking). [5 marks]
• ii. Give an example of a left-recursive grammar and a trace of an execution of the
• top-down algorithm failing to terminate when using it.

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