Pointer to pointers of structs.

[johnsolver]

Hi I hope I got the topic right..

that's what I'm doing:
create P(variable) polygons each having a variable numbers of vertices,
something in my mallocs is wrong... since I get weird results.

help?

#include <stdio.h>
#include <stdlib.h>

typedef struct point
{
double x, y;
} point;

typedef struct polygon
{
int n;
point *v;
} polygon;

typedef struct line
{
point p1, p2;
double A, B;
} line;

#define P 2

int main()
{
int i = 0, j = 0;

polygon *obstacles = malloc(sizeof *obstacles * P);

for(i = 0; i < P; i++)
{
obstacles[i].n = i + P * 2;
obstacles[i].v = malloc(sizeof obstacles[i].v * obstacles[i].n);

for(j = 0; j < obstacles[i].n; i++)
{
obstacles[i].v[j].x = obstacles[i].n;
obstacles[i].v[j].y = obstacles[i].n + 5;
}
}
}

[Keith Thompson]

jo********@gmai l.com writes:

Please be more specific: what do you mean by 'weird results' ?

To make it clearer, the program doesn't finish runing. I'm quite
certain I've made a copmlete mess out of the mem-allocation...

I think your question has already been answered elsethread, but I'll
just mention that "the program doesn't finish running" isn't a very
helpful description. Anyone trying to help you is going to be far
less interested in what your program *doesn't* do than in what it
*does* do. In other words, you need to specify exactly *how* it
"doesn't finish running", what output it produces, whether it crashes
or hangs, etc.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Richard G. Riley]

On 2006-02-19, jo********@gmai l.com <jo********@gma il.com> wrote:

Chris Torek wrote:
so the above can be

re-rewritten as:

(((p + i))->member + j)->member2 = val;

The intermediate form is *much* worse than the original array syntax,
and the final form is *still* worse than the original array syntax --
so stick with the array syntax. :-)

Yeah I've stumbled upon it when trying to rewrite, the thing is that
I've seen in many places advices on the use of the -> operator as being
'easier' and clearer way, seems like this isn't the case.

Many thanks.

Try them both and maybe use intermediate variables. personally I much
prefer something like (off top of head and not compilable)

char * pChar = malloc(n*sizeof (char));
for(char *p=pChar,int i=0;i<n;i++)
*p++=getChar();

to the array syntax.

It can often be horses for courses : but familarity with pointer
notation is a definite plus.

As the previous poster said though, it can be messy if
you dont use intermediate pointers.

--
Remove evomer to reply

[August Karlstrom]

jo********@gmai l.com wrote:

Bugger, I deserve this, mixing up i's and j's ... and forgetting the *,
many thanks.

I usually use k as index in single loops and j and k in doubly nested
loops. Much more distinct than i and/or j.
August

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