C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Position of test values in conditional expressions Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Ney André de Mello Zunino Guest Posted: Sun Jun 20, 2004 3:40 am    Post subject: Position of test values in conditional expressions Hello. I have noticed, in a lot of C and C++ code, that many programmers seem to prefer putting the test values first in conditional expressions. I.e., they would rather write this: if (-1 == foobar()) than this: if (foobar() == -1) The second form looks more natural and easier to read IMHO and is the one I have always used. However, given the high ocurrence of the first one, I would like to know the implications of using one way or the other. Thank you, -- Ney André de Mello Zunino Back to top Jack Klein Guest Posted: Sun Jun 20, 2004 4:11 am    Post subject: Re: Position of test values in conditional expressions On Sun, 20 Jun 2004 00:40:25 -0300, Ney André de Mello Zunino <zunino (AT) inf (DOT) ufsc.br> wrote in comp.lang.c++: Quote: Hello. I have noticed, in a lot of C and C++ code, that many programmers seem to prefer putting the test values first in conditional expressions. I.e., they would rather write this: if (-1 == foobar()) than this: if (foobar() == -1) The second form looks more natural and easier to read IMHO and is the one I have always used. However, given the high ocurrence of the first one, I would like to know the implications of using one way or the other. Thank you, The compiler does not care, one way or the other. And except for the case of a C++ function returning a reference to a non-const object, there is no gain. But when doing equality tests against constant expressions, consider this: int x; // assign some value to x if (x == 3) // do something Now consider what happens if the programmer commits the not uncommon error of typing only '=' when '==' was intended. The resulting code: if (x = 3) // do something ....is perfectly legal, and has the effect of evaluating the constant value assigned to x. If the constant value is 0, the test is always false and the conditional code is always skipped. If the constant is not 0, the test is always true and the conditional code is always executed. On the other hand, if you always write: if (3 == x) ....then if you accidentally leave off the second '=' the result is a constraint violation requiring the compiler to issue a diagnostic. -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html Back to top E. Robert Tisdale Guest Posted: Sun Jun 20, 2004 4:22 am    Post subject: Re: Position of test values in conditional expressions Ney André de Mello Zunino wrote: Quote: I have noticed, in a lot of C and C++ code, that many programmers seem to prefer putting the test values first in conditional expressions. I.e., they would rather write this: if (-1 == foobar()) than this: if (foobar() == -1) The second form looks more natural and easier to read IMHO and is the one I have always used. However, given the high ocurrence of the first one, I would like to know the implications of using one way or the other. What you have noticed is a *habit* that some programmers adopt to help them avoid a common mistake: int x = 0; // . . . if (x = -1) // Error! Should have been (x == -1) One problem with C and C++ is that operator== is too *close* to operator= and the programmer might mistype the assignment operator when the comparison operator was intended. This typographical error is difficult to spot when debugging your code so some programmers write: if (-1 == x) // . . . so that if they mistype Quote: cat main.cc int main(int argc, char* argv[]) { int x = 0; // . . . if (-1 = x); return 0; } Quote: g++ -Wall -ansi -pedantic -o main main.cc main.cc: In function `int main(int, char**)': main.cc:4: error: non-lvalue in assignment The get a diagnostic message from the compiler like the one above. In your example: Quote: cat main.cc int foobar(void) { return -1; } int main(int argc, char* argv[]) { if (foobar() = -1); return 0; } Quote: g++ -Wall -ansi -pedantic -o main main.cc main.cc: In function `int main(int, char**)': main.cc:6: error: non-lvalue in assignment You get a diagnostic if you type the assignment operator instead of the comparison operator because foobar(void) does *not* return an lvalue. This is probably just an example of *force-of-habit*. Programmers who use this trick get used to seeing the constant on the left-hand side of a comparison and feel more comfortable writing *all* comparison expressions that way even if they don't contain an lvalue. Back to top JKop Guest Posted: Sun Jun 20, 2004 9:42 am    Post subject: Re: Position of test values in conditional expressions Jack Klein posted: Quote: On the other hand, if you always write: if (3 == x) ...then if you accidentally leave off the second '=' the result is a constraint violation requiring the compiler to issue a diagnostic. Very clever indeed! -JKop Back to top Ney André de Mello Zunino Guest Posted: Sun Jun 20, 2004 1:46 pm    Post subject: Re: Position of test values in conditional expressions Thank you Jack and Robert for you clarifications. -- Ney André de Mello Zunino Back to top Walter Tross Guest Posted: Sun Jun 20, 2004 2:04 pm    Post subject: Re: Position of test values in conditional expressions Ney André de Mello Zunino 2004-06-20 : Quote: I have noticed, in a lot of C and C++ code, that many programmers seem to prefer putting the test values first in conditional expressions. I.e., they would rather write this: if (-1 == foobar()) than this: if (foobar() == -1) The second form looks more natural and easier to read IMHO and is the one I have always used. However, given the high ocurrence of the first one, I would like to know the implications of using one way or the other. As others have already explained, it helps in avoiding the error of writing an assignment where an equality comparison was intended. I personally dislike this trick, because it only helps in *initially* avoiding a mistake, while the more difficult to read code *remains*. If you rely on this kind of trick to write correct code, it means that you are not doing any testing, which is the worst mistake you can make. Walter Tross Back to top Ben Measures Guest Posted: Sun Jun 20, 2004 3:26 pm    Post subject: Re: Position of test values in conditional expressions Walter Tross wrote: Quote: Ney André de Mello Zunino 2004-06-20 : I have noticed, in a lot of C and C++ code, that many programmers seem to prefer putting the test values first in conditional expressions. I.e., they would rather write this: if (-1 == foobar()) than this: if (foobar() == -1) As others have already explained, it helps in avoiding the error of writing an assignment where an equality comparison was intended. I personally dislike this trick, because it only helps in *initially* avoiding a mistake, while the more difficult to read code *remains*. If you rely on this kind of trick to write correct code, it means that you are not doing any testing, which is the worst mistake you can make. Good point well put. -- Ben M. Back to top Dave Guest Posted: Mon Jun 21, 2004 1:56 pm    Post subject: Re: Position of test values in conditional expressions E. Robert Tisdale wrote: Quote: Ney André de Mello Zunino wrote: I have noticed, in a lot of C and C++ code, that many programmers seem to prefer putting the test values first in conditional expressions. I.e., they would rather write this: if (-1 == foobar()) than this: if (foobar() == -1) The second form looks more natural and easier to read IMHO and is the one I have always used. However, given the high ocurrence of the first one, I would like to know the implications of using one way or the other. What you have noticed is a *habit* that some programmers adopt to help them avoid a common mistake: int x = 0; // . . . if (x = -1) // Error! Should have been (x == -1) It also makes the code slightly easier to read, if you're only interested in this function and not the stuff that it calls: if (POSS_1 == some_complicated_expression) { // this code is executed if something we're not interested in // just yet evaluates to POSS_1 } So here we're only really interested in POSS_1 and what happens when we encounter that result. How POSS_1 is calculated isn't really important right now. If this reads the other way round then you have to scan to the end of the equality test before you find the stuff you're interested in. Semantically of course there's no difference; if A == B then B == A, but visually it can make code easier to understand. Dave. Back to top Andre Kostur Guest Posted: Mon Jun 21, 2004 3:21 pm    Post subject: Re: Position of test values in conditional expressions Ben Measures <saint_abroadremove (AT) removehotmail (DOT) com> wrote in news:11iBc.947$VA4.8163369 (AT) news-text (DOT) cableinet.net: Quote: Walter Tross wrote: Ney André de Mello Zunino 2004-06-20 : I have noticed, in a lot of C and C++ code, that many programmers seem to prefer putting the test values first in conditional expressions. I.e., they would rather write this: if (-1 == foobar()) than this: if (foobar() == -1) As others have already explained, it helps in avoiding the error of writing an assignment where an equality comparison was intended. I personally dislike this trick, because it only helps in *initially* avoiding a mistake, while the more difficult to read code *remains*. If you rely on this kind of trick to write correct code, it means that you are not doing any testing, which is the worst mistake you can make. Good point well put. IMHO: I prefer to use the (foobar() == -1) form because I know my compiler will issue a warning if I try to "if (foobar() = 1)". (Um, one of my two compilers.... so, yeah, I know it's not a diagnostic that the compiler is _required_ to issue, I just know that my tool _does_). Back to top Corey Murtagh Guest Posted: Tue Jun 22, 2004 1:22 pm    Post subject: Re: Position of test values in conditional expressions Andre Kostur wrote: <snip> Quote: IMHO: I prefer to use the (foobar() == -1) form because I know my compiler will issue a warning if I try to "if (foobar() = 1)". (Um, one of my two compilers.... so, yeah, I know it's not a diagnostic that the compiler is _required_ to issue, I just know that my tool _does_). If foobar() returns a reference to a non-const object then your compiler likely /won't/ return a diagnostic unless you tell it to be /really/ anal. For example: int some_int = 0; int& foobar() { return some_int; } int main() { if (foobar() = 1) return 1; return 0; } -- Corey Murtagh The Electric Monk "Quidquid latine dictum sit, altum viditur!" Back to top Andre Kostur Guest Posted: Tue Jun 22, 2004 3:37 pm    Post subject: Re: Position of test values in conditional expressions Corey Murtagh <emonk (AT) slingshot (DOT) no.uce> wrote in news:1087911000.283118 (AT) radsrv1 (DOT) tranzpeer.net: Quote: Andre Kostur wrote: snip IMHO: I prefer to use the (foobar() == -1) form because I know my compiler will issue a warning if I try to "if (foobar() = 1)". (Um, one of my two compilers.... so, yeah, I know it's not a diagnostic that the compiler is _required_ to issue, I just know that my tool _does_). If foobar() returns a reference to a non-const object then your compiler likely /won't/ return a diagnostic unless you tell it to be /really/ anal. For example: int some_int = 0; int& foobar() { return some_int; } int main() { if (foobar() = 1) return 1; return 0; } Yep... the compiler complained about it: test.cpp: In function `int main ()': test.cpp:10: warning: suggest parentheses around assignment used as truth value (Happens to be gcc v2.96, with -Wall as a compiler flag. When I get really picky, I turn on -Werror too, to make all warnings errors....) (In all of my makefiles, -Wall is always supplied.) Back to top William Guest Posted: Tue Jun 22, 2004 8:37 pm    Post subject: Re: Position of test values in conditional expressions "Walter Tross" <walter (AT) waltertross (DOT) com> wrote Quote: If you rely on this kind of trick to write correct code, it means that you are not doing any testing, which is the worst mistake you can make. I think any "trick" that helps ensure correct code is worthwhile. Since I've seen a number of problems where tests would not reveal that the intended == was replaced with =, I wouldn't rely solely on the tests either. How can the tests not reveal the problem? When the code that is excuted every time as a results of the ( x = 3) or whatever doesn't alter the final outcome. This can happen when the test is used to bypass an operation when it would be redundant and avoiding the overhead is worthwhile. (I saw a problem like this in a graphics routine - sped up quite a bit once the problem was fixed. Had the original programmer applied a trick or two it would have been flagged and fixed months earlier.) -Wm Back to top Darrell Grainger Guest Posted: Tue Jun 22, 2004 9:09 pm    Post subject: Re: Position of test values in conditional expressions On Tue, 22 Jun 2004, William wrote: Quote: "Walter Tross" <walter (AT) waltertross (DOT) com> wrote in message news:1kw2w7m6eyvbt.sjqua03ibh5t$.dlg (AT) 40tude (DOT) net... If you rely on this kind of trick to write correct code, it means that you are not doing any testing, which is the worst mistake you can make. I think any "trick" that helps ensure correct code is worthwhile. Since I've seen a number of problems where tests would not reveal that the intended == was replaced with =, I wouldn't rely solely on the tests either. How can the tests not reveal the problem? When the code that is excuted every time as a results of the ( x = 3) or whatever doesn't alter the final outcome. This can happen when the test is used to bypass an operation when it would be redundant and avoiding the overhead is worthwhile. (I saw a problem like this in a graphics routine - sped up quite a bit once the problem was fixed. Had the original programmer applied a trick or two it would have been flagged and fixed months earlier.) -Wm Just something to consider... If it costs you nothing to do this or if it is something you are willing to spend your time learning to do (rather than the company's time) I see nothing wrong with doing it. If your productivity is going to drop because you are trying to get into the habit of using this trick then I would have a problem with it. I'd consider whether buying a good lint application would be cheaper then the lost productivity. Additionally, I would not impose this policy on someone else. I'd suggest it. If they didn't feel it was worth while, do I want to spend the time (which costs the company money) arguing the point? Would it be more economical to just buy a lint application? By the way, my company uses a lint application that always catches when programmers use an assignment in a conditional expression. No need for the programmers to learn a trick to make the compiler catch it MOST of the time when I can get an application that catches it ALL of the time. -- Send e-mail to: darrell at cs dot toronto dot edu Don't send e-mail to [email]vice.president (AT) whitehouse (DOT) gov[/email] Back to top Programmer Dude Guest Posted: Tue Jun 22, 2004 9:38 pm    Post subject: Re: Position of test values in conditional expressions writes: Quote: How can the tests not reveal the problem? Inept compiler or no use of a lint tool? Quote: (I saw a problem like this in a graphics routine - sped up quite a bit once the problem was fixed. Had the original programmer applied a trick or two it would have been flagged and fixed months earlier.) -Wm Just as another data point, I've never experienced the error in over two decades of experience. Of course, my compilers always whined about it and I never use assignment in a test expression. (So any compiler warnings about this sort of thing ARE errors.) -- Somewhere in the Midwest... Chris Sonnack <Chris (AT) Sonnack (DOT) com> in Training... http://www.Sonnack.com/ Back to top CBFalconer Guest Posted: Wed Jun 23, 2004 3:21 am    Post subject: Re: Position of test values in conditional expressions Darrell Grainger wrote: Quote: .... snip ... By the way, my company uses a lint application that always catches when programmers use an assignment in a conditional expression. No need for the programmers to learn a trick to make the compiler catch it MOST of the time when I can get an application that catches it ALL of the time. But I often *want* to embed an assignement in a conditional, and your gang of hackers should also: void foo(int count, char *fn) { FILE *fp = NULL; bar *buf; if (!(buf = malloc(count * sizeof *buf))) err("No mem"); else if (!(fp = fopen(fn, "r")) err("can't open"); else { /* do my thing with fp and buf */ } if (fp) fclose(fp); free(buf); } and I can concentrate on doing my thing. The wrappers are solid, clean, and automatic. I haven't checked fclose. -- Chuck F (cbfalconer (AT) yahoo (DOT) com) (cbfalconer (AT) worldnet (DOT) att.net) Available for consulting/temporary embedded and systems. <http://cbfalconer.home.att.net> USE worldnet address! Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Goto page 1, 2  Next Page 1 of 2   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.