What does this not work for getchar() method?

This was my own experiment to understand what goes under the hood, what does this program mean to the compiler?

main()
{
  int c;
  printf("%d\n",c);
  printf("%d ", getchar());


  while ((c == getchar()) != EOF){
    putchar(c);
  }
}

When I say c must equal getchar() (c == getchar()), does it not proceed through the while loop? Now I am really confused of my own code, of what c must mean!

Also, in this code:

main()
{
int c;
c = getchar()
while ((c = getchar()) != EOF)
putchar(c);
}

if we modify the int c to int c = getchar(), why cannot we somply write like this:

while (c != EOF)(
    putchar(c);
    c = getchar();
    }

The compiler should know from the previous statement that c = getchar(), why have to write the statement again? Sorry, if I am confused.

Solution

while ((c==getchar()) != EOF) {
  ...
}

is a while loop. It evaluates the condition for each iteration of the loop and terminates only if the condition is false.

In your case, the condition is:

(c==getchar()) != EOF)

which is a nonsensical expression, but let's examine it anyway:

First, the program will evaluate:

    getchar()

This grabs a keystroke from standard input. The value of the expression is the value of the key.

Then:

 c==getchar()

This takes the result of getchar() and compares it to whatever is currently in c. In your first program, c is uninitialized, so its value is indeterminate. If c had a defined value, then c==getchar() would evaluate to either true or false. Since c had no defined value, c==getchar() also has no defined value.

Now the program evaluates:

(c==getchar())

Which would still be true or false, except that in your case it is undefined.

The program next considers:

(c==getchar()) != EOF

That is, it compares the true-false value to EOF; this makes no particular sense, and in your case we still have the undefined behavior of an uninitialized c.

In sum, if c were initialized, the expression would fetch a key from standard input and then compare either true or false to EOF. As I said, it is a nonsensical expression.