Question

这是我们教授写的一篇教程，我无法理解它。我可以想出推导，但我只能通过分析推导来提出语法。

在这种情况下，“匹配”是指什么？

你能解释一下match_if，matched_stmt，unmatched_if如何用简单的单词:)？

The following is an unambiguous grammar for the problem:

stmt → if_stmt | nonif_stmt
if_stmt → matched_if | unmatched_if
matched_if → 'if' logical_expr 'then' matched_stmt 'else' matched_stmt
matched_stmt → mathced_if | nonif_stmt
unmatched_if → 'if' logical_expr 'then' stmt
 | 'if' logical_expr 'then' matched_stmt 'else' unmatched_if
logical_expr → id '==' lit
nonif_stmt → assgn_stmt
assgn_stmt → id '=' expr
expr → expr '+' term | term
term → '(' expr ')' | id
id → 'A' | 'B' | 'C'
lit → '0' | '1' | '2'


Consider the following input:
if A == 0 then
  if B == 1 then
    C = A + B
  else
    B = C

Let us do a leftmost derivation for the input:

stmt
=> if_stmt
=> unmatched_if
=> 'if' logical_expr 'then' stmt
=> 'if' id '==' lit 'then' stmt
=> 'if' 'A' '==' lit 'then' stmt
=> 'if' 'A' '==' '0' 'then' stmt
=> 'if' 'A' '==' '0' 'then' if_stmt
=> 'if' 'A' '==' '0' 'then' matched_if
=> 'if' 'A' '==' '0' 'then' 'if' logical_expr 'then' matched_stmt 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' id '==' lit 'then' matched_stmt 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' lit 'then' matched_stmt 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' matched_stmt 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' nonif_stmt 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' assgn_stmt 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' id '=' expr 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' expr '+' term 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' term '+' term 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' id '+' term 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' 'A' + term 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' 'A' + 'B' 'else' matched_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' 'A' + 'B' 'else' nonif_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' 'A' + 'B' 'else' assgn_stmt
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' 'A' + 'B' 'else' id '=' expr
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' 'A' + 'B' 'else' 'B' '=' expr
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' 'A' + 'B' 'else' 'B' '=' term
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' 'A' + 'B' 'else' 'B' '=' id
=> 'if' 'A' '==' '0' 'then' 'if' 'B' '==' '1' 'then' 'C' '=' 'A' + 'B' 'else' 'B' '=' 'C'

Answer 1

“匹配”表示每个then都与else匹配。

if语句没有必要匹配else，但如果没有，则不能在匹配的if语句中，因为这意味着一些else匹配外部then而非内部then）。

所有语法都是上述形式化。

类似的问题是为普通的算术表达式编写一个语法，附加规则是你可以省去尾随的括号。（所以你可以写(1+2*(1+2*(1+2，例如。）这种语言显然是明确的，但是当你为它编写语法时，你需要处理不匹配的括号，因此需要处理包含无法匹配的括号的表达式。这与“匹配”这个词的使用相同·（并且解决方案有点类似）。

如果不这样做的话，无法为我创造明确的语法

1 个答案: