cgw@mruxe.UUCP (C Waldman) (06/09/86)
Quick question- In a loop, an array element is incremented, then the array index is advanced to the next element. I recently coded this with .... total[i++] += f; .... After a couple of "Memory fault- core dumped" messages, I realized that i is getting incremented twice. I broke it into two lines and everything was ok, but I'm wondering why this construction doesn't work.
ark@alice.UucP (Andrew Koenig) (06/10/86)
> Quick question- > In a loop, an array element is incremented, then the array index > is advanced to the next element. I recently coded this with > .... > total[i++] += f; > .... > After a couple of "Memory fault- core dumped" messages, I realized > that i is getting incremented twice. I broke it into two lines and > everything was ok, but I'm wondering why this construction doesn't > work. Easy -- your compiler is broken. total[i++] += f; should be equivalent to total[i] += f; i++; If it isn't, your compiler is busted. Simple as that.
cgw@mruxe.UUCP (C Waldman) (06/10/86)
Thanks for your quick reply. The compiler in question is cc running on our V7 Release 2 Unix machine, and I'm amazed that I found a bug in it! I reported to root, awaiting response.
dant@tekla.UUCP (Dan Tilque) (06/11/86)
>From: cgw@mruxe.UUCP (C Waldman) > Quick question- > > In a loop, an array element is incremented, then the array index > is advanced to the next element. I recently coded this with > > .... > total[i++] += f; > .... > > After a couple of "Memory fault- core dumped" messages, I realized > that i is getting incremented twice. I broke it into two lines and > everything was ok, but I'm wondering why this construction doesn't > work. Your compiler seems to have a bug in the way it evaluates total[i++]. The gospel according to K&R (p. 191): The behavior of an expression of the form E1 op= E2 may be inferred by taking it as equivalent to E1 = E1 op (E2); however, E1 is evaluated only once. Your compiler seems to be evaluating total[i++] twice, once for the left side of the =, and once for the right. I ran a simple test on our compiler (cc) and it seems to work right. ========================================================================== Dan Tilque Googol in, Googolplex out. (tektronix!dadla!dant) ==========================================================================
gwyn@brl-smoke.ARPA (Doug Gwyn ) (06/15/86)
In article <108@mruxe.UUCP> cgw@mruxe.UUCP (C Waldman) writes:
- In a loop, an array element is incremented, then the array index
- is advanced to the next element. I recently coded this with
- ....
- total[i++] += f;
- ....
- After a couple of "Memory fault- core dumped" messages, I realized
- that i is getting incremented twice. I broke it into two lines and
- everything was ok, but I'm wondering why this construction doesn't
- work.
Either you were incrementing i somewhere else too (e.g. in the for() head),
or else your compiler has a bug. The whole point of the op= operators is
that the left side is supposed to be evaluated only once.davidsen@steinmetz.UUCP (Davidsen) (06/17/86)
In article <108@mruxe.UUCP> cgw@mruxe.UUCP (C Waldman) writes: > Quick question- > .... > total[i++] += f; > .... > > After a couple of "Memory fault- core dumped" messages, I realized > that i is getting incremented twice. I broke it into two lines and > everything was ok, but I'm wondering why this construction doesn't > work. I tried it on a VAX, a Sun, and XENIX, looking at the code with the -S option. They all work as desired. Therefore I believe that either your program has another problem or (most likely) your compiler is broken. Please let me know (or post) what you find, and which compiler/machine you are using. -- -bill davidsen ihnp4!seismo!rochester!steinmetz!--\ \ unirot ------------->---> crdos1!davidsen chinet ------/ sixhub ---------------------/ (davidsen@ge-crd.ARPA) "Stupidity, like virtue, is its own reward"