2.3 节的练习

2.3 节的练习

2.3.1

构建一个语法制导翻译方案，该方案把算数表达式从中缀表达式翻译成前缀表达式。

解答

产生式：

expr -> expr + term 
      | expr - term 
      | term
term -> term * factor 
      | term / factor 
      | factor
factor -> digit | (expr)

翻译方案：

expr -> {print("+")} expr + term 
      | {print("-")} expr - term 
      | term
term -> {print("*")} term * factor 
      | {print("/")} term / factor 
      | factor
factor -> digit {print(digit)} 
        | (expr)

2.3.2

构建一个语法制导翻译方案，该方案把算数表达式从后缀表达式翻译成中缀表达式。

解答

产生式：

expr -> expr expr + 
      | expr expr - 
      | expr expr * 
      | expr expr / 
      | digit

翻译方案：

expr -> expr {print("+")} expr + 
      | expr {print("-")} expr - 
      | {print("(")} expr {print(")*(")} expr {print(")")} * 
      | {print("(")} expr {print(")/(")} expr {print(")")} / 
      | digit {print(digit)}

疑问

参考答案是：

E -> {print("(")} E {print(op)} E {print(")"}} op | digit {print(digit)}

起初我的答案也是这样，但发现括号冗余比较严重，才改成现在的解答。虽然在多级优先级运算中，其实只能减少最低优先级运算的括号，不过看起来在这样的情况下还是能节约比较多括号的。这个解法正确否？

2.3.3

构建一个将整数翻译成罗马数字的语法制导翻译方案。

解答

辅助方法：

repeat(sign, times) // 将 sign 重复 times 次输出，如 repeat('a',2) = 'aa'

翻译方案：

num -> thousand hundred ten digit 
       { num.roman = thousand.roman || hundred.roman || ten.roman || digit.roman;
         print(num.roman)}
thousand -> low {thousand.roman = repeat('M', low.v)}
hundred -> low {hundred.roman = repeat('C', low.v)}
         | 4 {hundred.roman = 'CD'}
         | high {hundred.roman = 'D' || repeat('X', high.v - 5)}
         | 9 {hundred.roman = 'CM'}
ten -> low {ten.roman = repeat('X', low.v)}
     | 4 {ten.roman = 'XL'}
     | high {ten.roman = 'L' || repeat('X', high.v - 5)}
     | 9 {ten.roman = 'XC'}
digit -> low {digit.roman = repeat('I', low.v)}
       | 4 {digit.roman = 'IV'}
       | high {digit.roman = 'V' || repeat('I', high.v - 5)}
       | 9 {digit.roman = 'IX'}
low -> 0 {low.v = 0}
     | 1 {low.v = 1}
     | 2 {low.v = 2}
     | 3 {low.v = 3}
high -> 5 {high.v = 5}
      | 6 {high.v = 6}
      | 7 {high.v = 7}
      | 8 {high.v = 8}

2.3.4

构建一个将罗马数字翻译成整数的语法制导方案。

解答

产生式：

romanNum -> thousand hundred ten digit
thousand -> M | MM | MMM | ε 
hundred -> smallHundred | C D | D smallHundred | C M
smallHundred -> C | CC | CCC | ε
ten -> smallTen | X L | L smallTen | X C
smallTen -> X | XX | XXX  | ε
digit -> smallDigit | I V | V smallDigit | I X
smallDigit -> I | II | III | ε 

翻译方案：

romanNum -> thousand hundred ten digit {romanNum.v = thousand.v || hundred.v || ten.v || digit.v; print(romanNun.v)}
thousand -> M {thousand.v = 1} 
          | MM {thousand.v = 2} 
          | MMM {thousand.v = 3} 
          | ε {thousand.v = 0} 
hundred -> smallHundred {hundred.v = smallHundred.v}
         | C D {hundred.v = smallHundred.v}
         | D smallHundred {hundred.v = 5 + smallHundred.v}
         | C M {hundred.v = 9}
smallHundred -> C {smallHundred.v = 1}
              | CC {smallHundred.v = 2}
              | CCC {smallHundred.v = 3}
              | ε {hundred.v = 0}
ten -> smallTen {ten.v = smallTen.v}
     | X L  {ten.v = 4}
     | L smallTen  {ten.v = 5 + smallTen.v}
     | X C  {ten.v = 9}
smallTen -> X {smallTen.v = 1}
          | XX {smallTen.v = 2} 
          | XXX {smallTen.v = 3}
          | ε {smallTen.v = 0}
digit -> smallDigit {digit.v = smallDigit.v}
       | I V  {digit.v = 4}
       | V smallDigit  {digit.v = 5 + smallDigit.v}
       | I X  {digit.v = 9}
 smallDigit -> I {smallDigit.v = 1}
            | II {smallDigit.v = 2}
            | III {smallDigit.v = 3} 
            | ε {smallDigit.v = 0}

疑问

参考答案和我的解法有一个显著的区别，如下：

LowHundreds → ε { LowHundreds.val = 0; }
            | C { LowHundreds.val = 100; }
            | CC { LowHundreds.val = 200; }
            | CCC { LowHundreds.val = 300; }

所以他的最后一步获得翻译后的数字是用的加法：

RomanNumeral → Thousands Hundreds Tens Ones
               { RomanNumeral.val = Thousands.val + Hundreds.val + Tens.val + Ones.val;
               printf(“%d\n”, RomanNumeral.val;) }

看起来我的解法因为使用了更少的0而节约了一些空间，其他也没看出什么坏处，是这样？

2.3.5

构建一个将后缀表达式翻译成前缀表达式的语法制导翻译方案。

解答

产生式：

expr -> expr expr op | digit

翻译方案：

expr -> {print(op)} expr expr op | digit {print(digit)}