Thomas W. Christopher

Consulting in High Performance Computing. Training in Concurrency, Networking, and Parallelism, especially in Python and Java.

Look-ahead demo applet

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This applet demonstrates look-ahead. The BACKUP symbols cause the parser to back up the input before continuing.

Try typing into the "input line," followed by ENTER:

A:B=1;

L:K:if e then if f then A:b else C;

VisualLLkApplet

Input grammar

fiducials: ";" then else.
start = labeled_statement .
labeled_statement = label statement STMT! .
label = NULL_LABEL! { id ":" LABEL!}.
statement = v "=" e ";" ASGN! .
statement = e ";" EXPRSTMT! .
statement = if e then labeled_statement IFTHEN! .
statement = if e then labeled_statement else labeled_statement IFTHENELSE! .
e = e "+" t ADD! .
e = e "-" t SUB!.
e = t .
t = f "*" t MPY!.
t = f "/" t DIV!.
t = f "%" t MOD!.
t = f .
f = v FETCH! .
f = num CVTNUM!.
f = "(" e ")" PARENS!.
f = id "(" e ")" CALL!.
f = "-" f NEG!.
v = id LOOKUPID!.
v = v "[" e "]" SUBSCR!.
v = v "." id FIELD!.

Transformed grammar

e = t "e:101".
"e:101" = "+" t ADD "e:101".
"e:101" = "-" t SUB "e:101".
"e:101" =.
f = id "f:103".
f = "f:105".
"f:103" = "(" e ")" CALL.
"f:103" = LOOKUPID "v:102" FETCH.
"f:105" = num CVTNUM.
"f:105" = "(" e ")" PARENS.
"f:105" = "-" f NEG.
label = NULL_LABEL "label_6_21:109".
"label_6_21:109" = id "label_6_21:110".
"label_6_21:109" = "(" BACKUP.
"label_6_21:109" = "-" BACKUP.
"label_6_21:109" = num BACKUP.
"label_6_21:109" = if BACKUP.
"label_6_21:110" = "%" "label_6_21:111".
"label_6_21:110" = "(" "label_6_21:111".
"label_6_21:110" = ";" "label_6_21:111".
"label_6_21:110" = "=" "label_6_21:111".
"label_6_21:110" = "*" "label_6_21:111".
"label_6_21:110" = ":" "label_6_21:112".
"label_6_21:110" = "-" "label_6_21:111".
"label_6_21:110" = "." "label_6_21:111".
"label_6_21:110" = "+" "label_6_21:111".
"label_6_21:110" = "/" "label_6_21:111".
"label_6_21:110" = "[" "label_6_21:111".
"label_6_21:111" = BACKUP BACKUP.
"label_6_21:112" = LABEL "label_6_21:109".
labeled_statement = label statement STMT.
start = labeled_statement.
statement = "f:105" "t:104" "e:101" ";" EXPRSTMT.
statement = id "statement:106".
statement = if e then labeled_statement "statement:107".
"statement:106" = "(" e ")" CALL "t:104" "e:101" ";" EXPRSTMT.
"statement:106" = LOOKUPID "v:102" "statement:108".
"statement:107" = IFTHEN.
"statement:107" = else labeled_statement IFTHENELSE.
"statement:108" = "=" e ";" ASGN.
"statement:108" = FETCH "t:104" "e:101" ";" EXPRSTMT.
t = f "t:104".
"t:104" = "*" t MPY.
"t:104" = "/" t DIV.
"t:104" = "%" t MOD.
"t:104" =.
"v:102" = "[" e "]" SUBSCR "v:102".
"v:102" = "." id FIELD "v:102".
"v:102" =.