Mark: 22 revision(s)

2008-11-03T05:20:35Z

22 revision(s)

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[[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.

SE250:lab-9:hpan027 - Revision history

Mark: 22 revision(s)