Shift-Reduce Parser Applet: Output Windows

Last Update Time-stamp: "97/06/27 19:15:18 umrigar"

The shift-reduce parser displays its output in three windows:

Parse Forest Window

The top window displays the parse forest as it is being constructed. Nodes which are still on the parse stack are shown in green (these nodes are always root nodes and appear across the top of the window --- tracing this line of green nodes gives the contents of the parse stack). When a node is removed from the parse stack, it's color is changed to red. As the parser is stepped, the window will always display the current node of interest which is highlighted in yellow.

Shift-Reduce Parse Table Window

This shows the action and goto tables. The rows correspond to the states; the initial columns indexed by terminals correspond to the action table; the subsequent columns indexed by nonterminal correspond to the goto table. The table entries are represented as follows:

Shift n

A shift on a terminal symbol to state number n is represented as sn.

Reduce n

A reduction by rule number n is represented as rn.

Goto n

A goto on a nonterminal symbol to state number n is represented as gn.

The currently active entry in the parse table is highlighted.

Trace Window

This window shows the trace history in the format used in classical compiler texts. Each trace line consists of the stack (in green) followed by the input (in black) which is yet to be processed. The current lookahead token is always the first token in the unexpended input. Each stack entry is of the form State/Sym where State is the state number and Sym is the grammar symbol which accesses that state. The stack contents correspond to the green nodes in the parse tree.

Each window displays a brief description of itself for a short period after the mouse pointer enters it.

Feedback: Please email any feedback to zdu@acm.org.

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