FAQ for Compiler Final Project 2005

1. What are the rules for constant def?
A:   legal_id_name = constant_string or a previously defined constant
     constant strings are either an integer or a float
     float follows the syntax of C except that we need a decimal point

     constant variable is not a normal variable.
     It has r-value, but no l-value. That is, a constant variable be
     on the left hand side of the assignement

2. How to return a function value?
A:   In each function, the name of the function is a variable that
     has l-value, but no r-value. The value of the variable at the end
     of the function execution returns back to the called procedure/function.
     Read page 3 of slide0.pdf for an example.

3. for loop裡的statement我猜是只能放1個statement,
   因為沒有END FOR之類的結束符
   號, 但if裡的也是statement, 所以老師的意思是if, for, while裡面只能有一行嗎??
A: No!
   A statement can be a sequence of statements enclosed by begin end.
   example:
	FOR i := 1 TO 10 DO
	BEGIN
		IF i / 2 * 2 == i
		THEN
			BEGIN
			 x := x + i;
			 x := x * x;
			END
		ENDIF;
		y := x + x;
	END

4. FOR var := int-expression-1 TO int-expression-2 DO
   其中的var要是之前有宣告過的嗎? 還是算是宣告, 可以放新變數?
A: var must be declared before.
   var must be integer.

   Same for FOR ... DOWNTO ... DO

5. 同4, int-expression-1只在進入FOR之前做一次, 但int-expression-2有兩種可能, 
一種是先算好再進入FOR, 一種是每次算, 不知道是哪一種?
A: int-expression-2每次算.

6. In arithmetic expression :    - no auto-type conversion
what does this mean?
A: It means the following.
       1.  int_expr <operator> float_expr    -->  error
       2.  int_var := float_expr    --> error 
       3.  float_var := int_expr    --> error 
   only the followings are correct.
       1. int_expr <operator> int_expr
       2. float_expr <operator> float_expr
       3. int_var := int_expr
       4. float_var := flaot_expr

7. "Can have "(" and ")" in expression"
   what is this ?
A: It means ( and ) and allowed in expressions.
That is,
     expression -> (expression)

8. 投影片中有要求
   check for divide by 0 in compile and run time
   need to check array out of bound in compile and run time
   如果只是檢查如 y := x / 0 這種還好
   萬一 y := x / z 這種也要檢查  是不是就要像interpreter算出 z 的值
   可是如果之前某行在算 z 的時候有syntax error
   這樣 z 算出來的值就可能沒有必定的準則了
   到時候判定 y := x / z 也可能會連帶出現問題
   check array out of bound 也有類似的問題  (如 a[i] 不知道 i 為何值)
A: You will know the value of variables in run time.
So you can generate a code like
        load the value of z into register4
        if value of register4 is zero then goto error_divid_by_zero
Write a C-- code for handling run time error
        error_divide_by_zero:
              error handling code in run time

9. What are the priority of operators?
A:
	The same as C.

10. How to use "not"?
A:
	not logical_expressions

11. 教授有說過浮點數必須要有小數點，那是否小數點前後都必須有數字呢?
因為如果不是的話 ARRAY[1..10] 裡的1應該會parse 成1. (float)
                                 10 應該會parse 成.10 (float)
A: 
小數點前後不須有數字, but at least one side must have digits.
In array declaration, there is no space between the two dots.
That is, ARRAY[1. .10] is illegal.

12.    function的回傳值是以 function name表示
那有沒有可能其 code中出現多次的 function name?
ex: FUNCTION  Foo( x : INTEGER ) : INTEGER
   BEGIN
         Foo := x + 3;
         Foo := x + 5;
         Other Statement;
   END
如果可能且是合法的話，是以最後出現的為準嗎?
又出現再其後的statement是否需要繼續做? 
還是說一看到 Foo 就馬上retuen?
A: A function name is a variable.
So your above example is legal.
You don't immediately perform a return when you see the function name
because it is simply a variable.

13. Type define中是不是也跟constant一樣也是剛宣告過的就可以用?
  Ex: TYPE
      Int1  = INTEGER;
      Int_Array  = ARRAY[1..10]  of Int1;
      ENDTYPE
A: Yes.

14. 請問logical operators(or, and, not, xor)的用法為何? 
例如or是用c的文法(a | b)還是(a or b)?

A: example:
       IF ((a > b) and (a >= c)) or ((not (c <= d)) xor (c <> a))
       THEN
          ...

15. 請問(or, and, not, xor), mod, return, of這些要列為reserved word嗎? 是否還
有其他特殊字串(大寫字串除外)要列為reserved word?
A: These are all resevered words.
   Our language uses resevered words.

16. 請問array的宣告在low跟..之間以及..跟upper之間有沒有空格?
ARRAY[low .. upper]
如果沒有的話在lex裡
就會被當作兩個合法的float number處理耶
A: Please also read FAQ#11.
We allow spaces in between low and "..", and in between ".." and upper.
We do not allow space in between "..", that is ". ." is illegal.
I am sure you can find some LEX tricks to handle this.

17. part10的投影片內的第三頁，有期末project的範例程式，
其中的CONST與TYPE，老師用的assign符號是'='，但是下面的statement部分
老師用的又是":="，請問是不是兩種都要做('=' for CONST, TYPE declaration)
A: Yes, 兩種都要做.
"=" for equivalent.
":=" for assignment.

18. 關於 type equivalent 的部分我觀念不太清楚
Example:
=====
TYPE
    type1 = INTEGER;
    type2 = type1;
ENDTYPE
=====
type1, type2, INTEGER三者算是equivalent？
A: We use name equivalence.
Hence type1 and INTEGER are equivanent.
type2 and type1 are also equivalent.
Since equivalence is a transitive relation, so
type1, typ1 and INTEGER are all equivalent.

19. More questions about type equivalent.
那
=====
TYPE
    type3 = ARRAY[1..5] of INTEGER;
    type4 = ARRAY[1..5] of INTEGER;
    type5 = type3;
    type6 = ARRAY[2..6] of INTEGER;
    type7 = ARRAY[2..7] of INTEGER;
ENDTYPE
=====
(type3, type4, type5)相同，(type6)一種，(type7)一種？
A: You are right.
(type3, type4, type5)相同，(type6)一種，(type7)一種.

20. operator的部分
logical: and, or, not, xor
這四個只會和boolean variable做運算
而boolean variable是指integer非零為true
還是logical operator只會和comparison的expression做運算
例如(a > b) and (c = d)
A: Boolean expression means a comparison of variable
with equal elementary types.
Logical operator only can be applied on boolean expressions.
There is no value for boolean expressions.
That is, we use flow-of-control, not numerical representation.
If a, b, c and d are all INTEGER,
then the following is illegal.

/* illegal usage of logical operator "and" */
BEGIN
     VAR
        a,b : INTEGER;
      ENDVAR
      IF (a and b)
      THEN ...
...

21. 看了FAQ 20之後，我對 and, or, xor, not 的使用還是不太懂，
這四種只適用於Boolean expression，意思是應該等到做到phase 4時，
才需要考慮這些operator嗎？
phase 1的expression，需要做的是不是就只有+. -, *, /, %, (, )呢？
也就是說這些不是像C語言中的bitwise operator囉，是嗎？
A: Yes for all three questions.

22. 因為規定的語言中boolean expressions沒有數值，如果寫
 if ( (a<b) xor (b<c) ) 這樣是合法的嗎？
又如果這個寫法可以，那代表的是什麼意思呢？
是若 a < b 為真、b < c 為真， 則這個 if 為假嗎？
A: For the first question, yes.
The grammar for using xor is
           logic-expression_1 xor logic-expression_2
It is true if either logic_expression_1 is true or logic-expression_2
is true, but not both of them are true at the same time.
A legal example of using logic operators can be found at FAQ#14.
Hence is (a<b) is true, and (b < c) is true, then
((a<b) xor (b < c)) is false.

23. 老師Q&A的第三題有說到
  " A statement can be a sequence of statements enclosed by begin end."
  那這樣的區塊裡是不是也可以有新的變數宣告 VAR  ENDVAR ?
  又如果 for / while loop只有一行是不是就不需要有 BEGIN END?
A: Yes for both questions.
You can declare VAR ENDVAR inside any begin..end block.
For one signle statement, yu can enclose it by BEGIN END, but
it is not required.
example:
      ...
      FOR i := 1 to y DO
      BEGIN
          VAR
            t : INTEGER;
          ENDVAR
          t := X[i];
          X[i] := X[2*y-i];
          X[2*y-i] := t;
      END
      FOR i := y DOWNTO 10 DO
           z := z * z;
      FOR i := z DOWNTO q DO
      BEGIN
           k := k * 2;
      END
      ...

24. ARRAY可以用int-expression當index嗎?
    如 x[y+z-2] 之類的?
A: Yes. The array index can be any integer-expression.

25. 關於For loop: "if the loop is not executed, then the value of the 
looping variable stays unchanged."意思是會保留之前的值嗎(不做assign的動作)?
另外, 會讓迴圈沒有被執行的情況是不是當(expr1 > expr2)的情形(在loop variable遞
增的情況下)? (expr1 = expr2)還是會執行? 若有執行loop, 則在離開loop之後loop 
variable的值應該等於expr2?
A: Yes for both questions.

26. 想請問 for loop
投影片說 if the loop is not executed then the value of the looping
variable stays unchanged
如果 FOR i:=1000 TO 10
這樣 i會是1000還是原本的值?
A: The value of i is 原本的值.

27. 另外，the value of the looping variable at the end of the loop is
the last looping value
如果 FOR i:=1TO 10
這樣 i會是11還是10?
A: The value of i is 10.