segmentation fault-- is my array too long?

[Henrik W Lund]

Malcolm Kay wrote:
> On Friday 07 May 2004 06:42, Henrik W Lund wrote:
> ....
> 
>>malloc() is your friend! :-)
>>
>>--> double *ncost = malloc(sizeof (double) * persons * scens); <--
>>
>>This ought to do the trick. Just remember to make sure that malloc
>>returns a valid pointer, otherwise you'll have another seg fault.
>>
>>I'm pretty sure you can adress the pointer like you do with the array
>>there (ncost[persons][0], etc...); if not, you can always do
>>ncost(sizeof(double) * persons + 0), etc...
>>
>>/* AMENDMENT!!! */
>>In my haste, I totally forgot my pointer dereferencing. The correct way
>>to reference a pointer as a two dimensional array is, of course, thus:
>>
>>*(ncost + (sizeof(double) * persons) + 0)) = 0.00;
> 
> 
> You've still got it wrong!
> ncost increments in units of size equal to that which it points
> so it should be:
>   *(ncost + person*scens + scen)
> where person is the first index and scen the second.
> or in the particular instance
>   *(ncost + person*scens + 0) = 0.00;
> 
> For easier to read code it would be better to use:
>    double (*ncost)[scens] = malloc( persons * sizeof *ncost );
> 
> and dereference as:
>    ncost[person][scen]
> 
> or in particular
>    ncost[person][0] = 0.0;
> 
> And for the OP it is usual to write constants generated
> with #define in upper-case. It generally seems to help
> to make the code easier to follow. In this case:
>  PERSONS instead of persons
> and
>  SCENS instead of scens
> 
> This also make the distinction between PERSONS and person
> more evident while retaining their implied connection.
> 
> Malcolm
> 
> 

Thank you for the rectification. While the code did compile and run fine 
the way I wrote it, I suppose that's just lucky. I guess stuff like this 
is the reason why some of my C/C++ programs seg fault on rare occasions. ;-)

I generally handle pointers well, but evidently not perfectlu well. :-D

-Henrik W Lund