Picat Tabling for Memoization 5

Question 1

What is the effect of placing the `table` declaration after the predicate clauses in Picat?

Accepted Answer

It causes a compilation error or warning because `table` must precede the predicate clauses.

Answer

In Picat, `table` must appear before the predicate definition; placing it after leads to an error.

Question 2

For the longest common subsequence (LCS) problem, why is tabling particularly effective in Picat?

Accepted Answer

LCS has overlapping subproblems of the form lcs(i,j) that are recomputed many times without tabling.

Answer

LCS subproblems overlap heavily (each cell depends on three adjacent cells), making tabling essential for polynomial performance.

Question 3

In Picat, what is the purpose of the `nt` (no-table) mode specifier within a `table` declaration?

Accepted Answer

It marks an argument that should not be used as part of the tabling key, effectively ignoring it in the lookup.

Answer

`nt` excludes an argument from the call pattern key, useful when an argument is always the same or irrelevant to caching.

Question 4

What is a potential pitfall when using global variables inside a tabled Picat predicate?

Accepted Answer

Global variable side effects are not captured in the table, so replayed answers may reflect stale or incorrect state.

Answer

Tabling only caches logical outputs; if the predicate relies on or modifies global state, cached replays won't re-execute that state change.

Question 5

Which property of a problem makes it well-suited for Picat tabling as a DP strategy?

Accepted Answer

Optimal substructure and overlapping subproblems

Answer

DP—and by extension tabling—works best when subproblems overlap and solutions compose optimally, exactly the conditions tabling exploits.

Picat Practice Test