(→Semantic tree and dependency graph) |
(→Attribute grammar using only synthesized attributes) |
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Line 114: | Line 114: | ||
"val" is a synthesized attribute (propagation in red). | "val" is a synthesized attribute (propagation in red). | ||
− | + | [[Image:attrgrams-ex02-semantic-tree-2.png|500px|Temporary handwritten solution]] | |
− | |||
− | [[Image:attrgrams-ex02-semantic-tree-2.png]] | ||
[[category:Compilers]] | [[category:Compilers]] | ||
[[category:Teaching]] | [[category:Teaching]] |
Considere a seguinte gramática atributiva:
s | -> | symbol 1 # symbol 2 | { | |
s. val = symbol 1. val + symbol 2. val | ||||
symbol 1. x = 1 | ||||
symbol 2. x = 1 | ||||
} | ||||
symbol 1 | -> | symbol 2 dig | { | |
symbol 2. x = symbol 1. x * 5 | ||||
symbol 1.val = symbol 2.val + dig.val * symbol 1.x | ||||
} | ||||
symbol | -> | dig | { | |
symbol.val = dig.val * symbol.x | ||||
} |
O elemento lexical dig representa um dígito de 0 a 9, e o seu atributo val representa o valor numérico correspondente.
"x" is an inherited attribute (propagation in blue); "val" is a synthesized attribute (propagation in red).
We start by noting that the semantic computed by the previous grammar corresponds to base 5-like numbering (although the digits do not belong to base 5).
The new grammar is trivial to write:
<text> s -> symbol_1 # symbol_2 { s.val = symbol_1.val + symbol_2.val } symbol_1 -> symbol_2 dig { symbol_1.val = symbol_2.val * 5 + dig.val } symbol -> dig { symbol.val = dig.val } </text>
"val" is a synthesized attribute (propagation in red).