syntax -- grammar: putting together keywords etc the right way

semantics -- the meaning

S -> N V N .
N -> cat | ufo | apple | fox | dog | hotdog | ipod | itec
V -> bite | run | jump | hit | love | spin | hear

cat bite ufo.

S -> N V N .
   -> cat V N .
   -> cat V ufo .
   - > cat bite ufo .

A derivation: Take a string containing a nonterminal,
and replace that nonterminal with a right-hand-side from the grammar.

We say a string can be "dervied" from S if there is a series of 0 or more derivations that start from "S" and reach the target string.

For class, we'll use *left-most* derivations: always replace the left-most nonterminal when making one derivation.
S -> N V N .
   -> cat V N .
   -> cat bite N .
   - > cat bite ufo .

This is also equivalent to a tree:
                S
             /  |  \ \
            N  V  N  .
            |   |    |
        cat bite ufo

The "parse tree", or "syntax tree":


--------- new grammar --------
S -> NP CV NP .
NP -> ART N
ART -> a | an | the
CV -> V s | V ed
N -> cat | ufo | apple | fox | dog | hotdog | ipod | mongo | itec
V -> bite | run | jump | hit | love | spin | hear

t/f:
I can derive "the cat biteed the ufo." from S ?

How about "a apple spin hotdogs."?
S -> NP CV NP .
   -> ART N CV NP .
   -> a N CV NP .
   -> a apple CV NP .
   -> a apple V s NP .
   -> a apple spin s NP.  -- oops, doesn't seem possible.
   
   
----------------

 
  E -> E + E  |  E * E  |  N
  N -> any number

Can E derive "2+3*4" ?  Yes.

  E -> E + E 
     -> N + E   // n.b. you can't jump to "2+E"
     -> 2 + E
     -> 2 + E * E
     -> 2 + N * E
     -> 2 + 3 * E
     -> 2 + 3 * N
     -> 2 + 3 * 4
     
          E
     /    |   \
     E   +   E
     |        /|\
     N      E * E 
     |       |    |
     2      N   N
             |    |
             3   4
             
HOWEVER, there is a second way to derive "2+3*4" from E,
using a different rule on the very first derivation:
 E -> E * E
    -> ...
    -> 2 + 3 * 4.
    
          E
     /    |   \
     E    *   E
   / | \      |
  E  + E      N
  |      |      |
  N    N    4
  |      |     
  2     3

 A grammar that has some string with two different parse trees
 is called _____ambiguous_____.

 We don't like ambiguous.

(Java: 39 operators, with 13 levels of precedence...)


VD -> TYPE IDENT ;  |  ACCMOD TYPE IDENT ;
ACCMOD -> public | private | synchronized 
TYPE -> int | char | short | double | IDENT
IDENT -> any valid identifier