Exercise construct the SLR parsing table for grammar

– Exercise: construct the SLR parsing table for grammar: S->L=R, S->R L->*R L->id R->L – The grammar can have shift/reduce conflict or reduce/reduce conflict. • What about shift/shift conflict?

– SLR does not always remember the right context -- using the follow set to determine when to reduce is sometimes too general. – LR(1) parsing table has more context information in its items. • An LR(1) item is of the form – terminal a has not effect when is not. – When is , an item of the form reduce only when the next input symbol is a. means to

• Computing sets of LR(1) items. – Closure(I) is now slightly different – Start the construction of the set of LR(1) items by computing the closure of

• Example 1: first page • Example 2: S’ -> S S -> CC C ->c. C C ->d

• Constructing the LR(1) parsing table

– The number of states in LR(1) parsing table is much more than that in SLR parsing table. – LALR reduces the number of states in LR(1) parsing table. • LALR (Look. Ahead LR) is less powerful than LR(1) • reducing states may introduce reduce-reduce conflict, but not shift-reduce conflict. • LALR has the same number of states as SLR, but more powerful. – Constructing LALR parsing table. • Combine LR(1) sets with the same sets of first parts (ignore lookahead). • Algorithms exist that skip constructing the LR(1) sets.

– Using ambiguous grammars • ambiguous grammars will results in conflicts • Can use precedence and assocativity to resolve the conflicts • May result in a smaller parsing table in comparison to using un-ambiguous grammars. • Example: E->E+E E->E*E E->(E) E->id