stack overflow Math bit 1

[hemasaad]

when I test Math class I get this error "stack overflow Math bit 1". all I did is that
- declare static class named twoTothe and create it in the init function by size 15 according to A-instruction data represented by 15 bit and the most significant bit represents the sign of number
-then initialize these entries of the array by
i=0, n=1
while i<15 : twoTothe[i] = n; i = i+1; n= n+n;

in bit function(int x, int i):
i say if twoTothe[i] & x == 0 : return false ; else return true;

that is all I did. may be there is something I don't understand causes the stack overflow exception.

[cadet1620]

Stack overflows are usually caused by recursion gone wild.  For example (yours doesn't appear to) if bit() uses n=n*2 where '*' calls multiply() which calls bit() ... BOOM!

Another common error in initializing the twoToThe Array (again, I don't see this in your code) is for the while loop to run wild after the 14th doubling because 16384 + 16384 = -32768 in 2's-complement numbers.

If you want to, email me your Math.jack and I'll see if I can spot the problem.

--Mark

[hemasaad]

Thank you mark, you are right. a condition doesn't work correctly and that leads to overflow the stack I fixed it and pass the Math test but I notice this strange error. when I say
var boolean b;
let b = false;
and then write
if(b = false) I get this error a boolean value is illegal here.

but i think according to the grammar of expression it is legal.
ifstat : if '('expresssion')'......
expression : term (op term)*
term : ....|.....|constantkey|.......
constantkey : true| false| ......
op : ..|...|=|.....

I ask why I get this error?

[cadet1620]

hemasaad wrote

var boolean b;
let b = false;
if(b = false)

I get this error "A boolean value is illegal here".

but i think according to the grammar of expression it is legal.
ifstat : if '('expresssion')'......
expression : term (op term)*
term : ....|.....|constantkey|.......
constantkey : true| false| ......
op : ..|...|=|.....

I ask why I get this error?

You are correct that this is strictly legal by the Jack grammar.

This is a case where the compiler is trying to be helpful and save you from a common problem.
You should never compare to true and false.

The safe way to write your test is:

if (~b) 

--Mark

[hemasaad]

I have another simple question in string class which is when the main.jack says
let s = String.new(0);
i note in the new method in string class when i print the maxLength it is 0 and that is right
do s.dispose();
let s = String.new(6);
code to append 5 chars
but here the printed value isn't 6 but 3 and that is results in error because main will append 5 chars to string of length 3. i get 3 whatever the maxlength value passed by second string.new()

I don't know why that occurs. may there is problem in dispose () ?
all i do in dispose method is
do Memory.dealloc(this);
return;
 

       

[cadet1620]

The normal implementation of String is a class with 3 fields:

class String
    field int max_length;
    field int current_length;
    field Array data;       // character data

constructor String new(int maxLength) allocates the String object based on the number of fields, so all instances of String have fixed size = 3.
The data Array is a separate allocation of size maxLength.

String.dispose() needs to deallocate the data Array and then deallocate itself.

--Mark

[Ichiro]

I received a similar error "Stack overflow in Math.divide.12" when I executed the Math test. This is where Math.divide function recurses Math.divide. I showed a part of my code for Math.divide below.

//a part of Math.divide
function int divide(int x, int y){
   var int q;
   if (y>x){
      return 0;
   }
   let q = Math.divide(x, y+y);
   ...
}

I cannot see any issue but could you tell me the cause of the error?

Ichiro

[ashort]

Hard to say without seeing all your code, but one issue I can see is you are not checking for integer overflow.  If y is large, then y + y might be so large that it overflows (ask yourself what happens in a Hack computer with integer overflow).  So in that case the recursion doesn't return 0 when it should, resulting in a stack overflow because of too much recursion.

Where you return 0, you need to add a check for integer overflow of y.  If you understand what happens with integer overflow, you will realize it's an easy check.

[ashort]

of course you also have to handle either x or y or both being positive or negative... maybe you already do