Is x=; and if(d<) invalid to use in c language

Question

At the end of this question there is Mid-Point circle drawing algorithm in c language. I saw the following two statements in the code

x=;

if(d<);

in the program online and in our book as well, so basically i thought that the compiler might automatically be initializing them to 0(zero) so i wrote the same code in our homework. But the teacher said that it is wrong and indeed it gives Invalid Expression in Turbo C++ compiler. Thats why I want to know if it is wrong then why is it written like this everywhere.

Here is the code :

#include<graphics.h>
#include<conio.h>
#include<stdio.h>

void main() {
   int gd=DETECT,gm;
   int i,r,x,y,xc,yc;
   float d;

   clrscr();

   initgraph(&gd,&gm,"c:\\tc\\");

   printf("Enter Radius\n");
   scanf("%d",&r);

   printf("Enter Center of circle\n");
   scanf("%d",&xc);
   scanf("%d",&yc);

   d=1.25-r;
   x=;
   y=r;
   do {
        if(d<) {
            x=x+1;
            d=d+2*x+1;
        }  else {
               x=x+1;
               y=y-1;
              d=d+2*x-2*y+10;
       }
       putpixel(xc+x,yc+y,5);
       putpixel(xc-y,yc-x,5);
       putpixel(xc+y,yc-x,5);
       putpixel(xc-y,yc+x,5);
       putpixel(xc+y,yc+x,5);
       putpixel(xc-x,yc-y,5);
       putpixel(xc+x,yc-y,5);
       putpixel(xc-x,yc+y,5);
    } while(x<y);
    getch();
}

Also it is the link of above code:

Mid Point Circle Drawing Algorithm Code in c Language

Please help me out. Thanks.

Answer 1

Ques. What is storage class of variable x?

ans. local variables i.e. var x used in a block statement( as you used inside the main() ) is belong to "AUTO" storage class in C.

One More Ques :-
Now, question arises in you mind that, what is the default value with ,auto variable initialised with**??**

Ans. Initialized with "Garbage Value".

In declaration of x:-

int i,r,x,y,xc,yc;   

Note:- x is initialized with garbage value.

but you write,

x=;

Note:- which is niether the declaration nor the initilaization statement, but it is the Incomplete statement which'll not understand by the compilor so it gives the error :
Invalid Expression

Now, come to your next doubt:-

if(d<);

it's also a invalid expresion because just think in general maths if you compute:- if 2 is greater than ______??

can you imagine the value in blank space?? yes, you can but, compilor(or computer) can'not :)

Answer 2

No, those expressions are not syntactically valid.

Books have typos, and fishy-looking reproductions of books online also have typos. That image looks like a cell phone photo or something, really dubious.

Just because something is said online, it doesn't have to be true¹.

¹ Said an anonymous stranger online. :)

Answer 3

The code on the link you provided has obviously been mutilated.

The correct code is here:

#include <stdio.h>
#include <conio.h>
#include <graphics.h>
#include <math.h>

void main()
{
  int gd = DETECT, gm;
  int r, x, y, p, xc = 320, yc = 240;

  initgraph(&gd, &gm, "C:\\TC\\BGI");
  cleardevice();    

  printf("Enter the radius ");
  scanf("%d", &r);    

  x = 0;
  y = r;
  putpixel(xc + x, yc - y, 1);

  p = 3 - (2 * r);

  for (x = 0; x <= y; x++)
  {
    if (p<0)
    {
      y = y;
      p = (p + (4 * x) + 6);
    }
    else
    {
      y = y - 1;    
      p = p + ((4 * (x - y) + 10));
    }

    putpixel(xc + x, yc - y, 1);
    putpixel(xc - x, yc - y, 2);
    putpixel(xc + x, yc + y, 3);
    putpixel(xc - x, yc + y, 4);
    putpixel(xc + y, yc - x, 5);
    putpixel(xc - y, yc - x, 6);
    putpixel(xc + y, yc + x, 7);
    putpixel(xc - y, yc + x, 8);    
  }

  getch();
  closegraph();
}

Note: this code as it stands here works only on the antique Turbo C. For other platforms adaptions need to be made.