drawline problem

[karam]

hello
im trying to implement the algorithm
of drawline() in screen class
i did exactly as the slides say
but when i try to run the program nothing is printed on the screen
ther is no error


function void drawLine(int x1, int y1, int x2, int y2) {
        var int a,b,diff,dx,dy;
        let a=0;
        let b=0;
  let diff=0;
        let dx=x2-x1;
        let dy=y2-y1;

        while(~(a>dx) & ~(b>dy)){
           if(diff<0){
             let a=a+1;
             let diff=diff+dy;
           }
           else{
            let b=b+1;
            let diff=diff-dx;
           }
           do Screen.drawPixel(x1 + a, y1 + b);
        }
        return;
    }

[WBahn]

Are you sure that the values your are using result in dx and dy both being positive?

Have you confirmed that your drawPixel() function is working properly?

[karam]

in the drawpixel() i get the x%16 bit as the slide say
and i set that bit to 1 if the color desire is black else white
 here is my code :
function void drawPixel(int x, int y) {
        var int address,value,modulo;
        let address=16384 +(32 * y) + (x / 16);
        let value=Memory.peek(address);
        let modulo=x-((x/16)*16);
        if(setblack){
          let value=value | twoToThe[modulo];
        }
        else{
         let value=value & (~twoToThe[modulo]);
        }
        do Memory.poke(address,value);
        return;
    }