Date: Sat, 08 May 2004 11:38:20 +0200 From: Henrik W Lund <henrik.w.lund@broadpark.no> To: Malcolm Kay <malcolm.kay@internode.on.net>, freebsd-questions@freebsd.org Subject: Re: segmentation fault-- is my array too long? Message-ID: <409CAA8C.6090203@broadpark.no> In-Reply-To: <200405080948.15164.malcolm.kay@internode.on.net> References: <20040506190033.A45C616A5CF@hub.freebsd.org> <409AAA48.50409@broadpark.no> <200405080948.15164.malcolm.kay@internode.on.net>
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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
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