While loop problem

[SimpleMind]

Hi. This is my first post. I'm a complete beginner with C, but have some ability with Python, and know a little Java.

Anyway, my problem involves while loops and pointers. I'm just getting the hang of pointers and have been making a few mess-around programs. One such program is supposed to iterate a pointer.

int x;
int *y;

x = 500; //arbitrary value.
y =&x;

printf("%d", x); //print 500
printf("%d", y); // print memory space value.

*y = 200;
printf("%d",x); //print 200
//the above stuff is fine. Its the below while loop that I have a problem with. 
*y = 1;
while( *y<10){
printf("%d",x);
*y++
}

In the while loop, I expected the output to be 1,2,3,4...9. But it doesn't seem to loop. I don't know why. It compiles fine, but the output is simply 1. Then it stops.

I'm sure I'm making a completely stupid mistake, but as a "N00b" can't figure out what is wrong.

[quzah]

The * dereferences the pointer. You should change the last part to "y++" instead of "*y++".


Quzah.

[SimpleMind]

Thanks, but I'm not sure I understand.
Should I initialize y to 1?
But y refers to the memory space holding x (Im probably mutilating the jargon there).
If I iterate through y how will this change the value of x?

[User Name:]

> Should I initialize y to 1?
Assuming you mean *y, yes, unless you want it to equal 200.

> But y refers to the memory space holding x
Yes, and the * tells the compiler that the operator(+-/*) should affect the memory location referred to by y, instead of y itself.

> If I iterate through y how will this change the value of x?
If you mean *y, then yes.

[whiteflags]

I find what you're actually doing to be kind of odd but your actual problem is that the postfix increment operator is higher on the operator precedence table than the dereference operator (++ and * respectively).

*y++; will move the pointer first, then dereference. Well placed parentheses will do the right thing.

[SimpleMind]

Ah... I think I see what you mean. I should write (*y)++, perhaps. I'll give it a try later. (I'm using a different computer nat the moment, with no compiler.)

As to the purpos eof what I'm doing, well there isn't really one. Other than to mess around with pointers. I'm still waiting for Amazon to deliver my C and C++ books, so I'm messing around getting used to the syntax first.

Thanks for the help.

[quzah]

*y++; will move the pointer first, then dereference.

No it doesn't.

#include<stdio.h>
int main( void )
{
    char foo[] = "hello world";
    char *p = foo;
    char c = *p;
    
    printf( "%p : %c : %c : %s\n", p, c, *p, foo );
    c = *p++;
    printf( "%p : %c : %c : %s\n", p, c, *p, foo );
    c = ++*p;
    printf( "%p : %c : %c : %s\n", p, c, *p, foo );
    c = *++p;
    printf( "%p : %c : %c : %s\n", p, c, *p, foo );

    return 0;
}

Quzah.

[whiteflags]

Oh, right. Well hopefully that will make things clear. The parens then is just about incrementing the pointed to value the postfix way.

[Bayint Naung]

Operator precedence
If y is pointer, and you want to increment *what it points to*
use
(*y)++;
++*y;
*y += 1;
*y = *y + 1;

But not
*y++; //deference what pointer points & increment pointer p.


>*y++; will move the pointer first, then dereference. Well placed parentheses will do the right thing.
@whiteflag, hum?

[whiteflags]

Read the thread.

[SimpleMind]

I did as said and used (*y)++ instead of *y++. It works perfectly. Thanks everyone.

Incidentally, I'm using the Open WATCOM compiler. I have VSC++ installed, but I prefer the simplicity of WATCOM's compiler for these short programs.
Can anyone tell me whether it is worth using WATCOM. It doesn't seem as if anyone else uses it much.

[ssharish2005]

I personally haven't used WATCOM, but many people here use VSC++/Dev-C++/GCC etc. I suppose WATCOM is C99 compliance support so don’t think you would have much of a problem in terms of portability. But it would still suggest using compiler which many here use and you could expect much more effective help. But it wouldn't make too much of a difference. Its really a personal preference and need of your application. There are few employers out there still using Turbo C cos they need it for their application.

ssharish