SE250:lab-9:hpan027

SE250:lab-9

Starting up

• Chose first task
• 10 minutes in, considering switching tasks.
• Chose second task

Reading the code

Parse by default calls the function for if-then-else statements - which has not been written. Hence code does not run.

Need to recode Parse into something like:

Given an input
Work out what type of statement it is <- hard part
Call appropriate method

Looking at the various inputs and attempting to translate them into code

"a=1",
"a=1;b=2;k;k;k",

Self explanatory?

"ia>btwtda=a*2ek",

if (a > b)
  then   while (t)
         do (a = a * 2)
  else k

"ict{x=1;wtda=a*2}ek",

if ( c )
then {
   x = 1;
   while (t)
     do (a = a*2);
}
else k;

"junk that won't parse",

Lols.

• Getting "a=2" to parse is pretty easy.
• Call Exp(tokens,2)
• Output

Parse("a=1"):
	=(a 1)
	a 1 =2

Onto the actual task

  S ::= "if" E "then" S [ "else" S ]
      | "while" E "do" S
      | "{" Ss "}"
      | Ide "=" E
      | "skip"

Code will need a series of if statements to check for the keywords

//if ("if")
//parse for if statements
//else if ("while")
//parse for while statements

etc

Very basic if-then-else

Tree* Stmt( TokenStream* tokens ) { //if then else statements

    if( current( tokens ) == TOK_IF )
	printf("True\n");

    advance( tokens );
    Tree* t1 = Exp( tokens, 1 );

    expect( tokens, TOK_THEN );

    Tree* t2 = Exp( tokens, 1 );

    expect( tokens, TOK_ELSE );

    Tree* t3 = Exp( tokens, 1 );
    
    Tree* t = mkNode3('?',t1,t2,t3);
   
    return t;
}

• Doesn't work for anything except the most basic form of if-then-else.
• Need to change the Exp calls into Stmt calls - but can't do that until the rest of the function is finished

Revised if-then-else code

Tree* Stmt( TokenStream* tokens ) { //if then else statements

    if( current( tokens ) == TOK_IF ) {
	advance( tokens );
	Tree* t1 = Exp( tokens, 1 );
	
	expect( tokens, TOK_THEN );
	
	Tree* t2 = Stmt( tokens );
	
	if (!end_of_tokens(tokens)) {
	    expect( tokens, TOK_ELSE );
 
	    Tree* t3 = Stmt( tokens );
	    
	    return mkNode3('?',t1,t2,t3);
	}

	return  mkNode2('?',t1,t2);
    }

    if( isVariable( current( tokens ) ) )
	return Exp( tokens, 0 );
 
    return mkNode('0',0);
}

• Seems to work for the input "ia=1ta=2ea=3"
• Need to write the if blocks for other things defined under S

Other S grammar added

Tree* Stmt( TokenStream* tokens ) { //if then else statements

    /* IF */
    if( current( tokens ) == TOK_IF ) {
	advance( tokens );
	Tree* t1 = Exp( tokens, 0 );
	expect( tokens, TOK_THEN );
	Tree* t2 = Stmt( tokens );
	if (!end_of_tokens(tokens)) {
	    expect( tokens, TOK_ELSE );
	    Tree* t3 = Stmt( tokens );
	    return mkNode3('?',t1,t2,t3);
	}
	return  mkNode2('?',t1,t2);
    }

    /* Variable assignment (e.g. a = 1) */
    if( isVariable( current( tokens ) ) )
	return Exp( tokens, 0 );

    /* While loops */
    if( current( tokens ) == TOK_WHILE ) {
	advance( tokens );
	Tree* t1 = Exp( tokens, 0 );
	expect( tokens, TOK_DO );
	Tree* t2 = Stmt( tokens );
	return mkNode2(TOK_WHILE,t1,t2);
    }

    /* { Curly brackets } */
    if( current( tokens ) == TOK_OPENBRACE ) {
	advance( tokens );
	Tree* t = StmtSeq(tokens);
	expect( tokens, TOK_CLOSEBRACE);
	return t;
    }

    /* SKIP */
    if( current( tokens ) == TOK_SKIP) {
	return mkNode(0,0);
    }

    /* Print */
    if( current( tokens ) == TOK_PRINT) {
	return mkNode1(TOK_PRINT, Exp(tokens,0));
    }
 
    /* Base fail case */
    return mkNode(0,0);
}

• Trying to get this code to run with "ia=btwa=1da=a*2ek"
• Having some problems at the moment
• The default test cases do not work at all
• Variable assignment needed to be rewritten

    if( isVariable( current( tokens ) ) ) {
	Tree* t1 = P(tokens);
	expect(tokens, TOK_EQ);
	return mkNode2(TOK_EQ, t1, Exp(tokens,0));
    }

• Still problems

• Order matters...variable assignment should be right at bottom as i, p, e, etc are all considered variables
• /facepalm
• However getting there
• Output currently ?(=(a b) w(=(a 1) =(a *(a 2)))
  • If statement seems to have been broken by the while
• Actually caused by the skip
• No idea how to produce an external node

• Changing test case to "ia=btwa=1da=a*2ea=2"

Parse("ia=btwa=1da=a*2ea=2"):
	?(=(a b) w(=(a 1) =(a *(a 2))) =(a 2))

• Seems to work

Conclusion

Very long lab for 2 hours. Might finish later.