C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Re: typo in a for loop has a strange affect Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Thomas Mang Guest Posted: Thu Aug 28, 2003 10:39 am    Post subject: Re: typo in a for loop has a strange affect whocares schrieb: Quote: Hi group I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. Can anyone explain the logic of this to me. cheers G What implementation are you using? Actually, the body of your loop should not be executed at all. The conditional expression terminating the loop is i++; As i is initialized to zero, the result of i++ is zero too, which will be implicitely converted to false - so the loop should terminate immediately. regards, Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tonci Tomic Guest Posted: Thu Aug 28, 2003 10:40 am    Post subject: Re: typo in a for loop has a strange affect "whocares" <gvingoe (AT) hotmail (DOT) com> wrote Quote: I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. Can anyone explain the logic of this to me. Actually, it doesn't loop at all. i is set to 0, then it is incremented, but i++ returns 0 and that's all. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alf P. Steinbach Guest Posted: Thu Aug 28, 2003 10:43 am    Post subject: Re: typo in a for loop has a strange affect On 27 Aug 2003 11:33:34 -0400, "whocares" <gvingoe (AT) hotmail (DOT) com> wrote: Quote: Hi group I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. In that case you have a broken compiler; it should loop zero times. Quote: Can anyone explain the logic of this to me. Disregarding effect of 'continue', if any, the loop is equivalent to { int i = 0; while( i++ ) { // do something i < x; } } which again is equivalent to { int i = 0; while( true ) { int temp = i; i = i + 1; if( temp == 0 ) break; // do something i < x; } } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Stefan Heinzmann Guest Posted: Thu Aug 28, 2003 10:44 am    Post subject: Re: typo in a for loop has a strange affect Quote: for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. Can anyone explain the logic of this to me. Strange, I would have expected the loop to terminate immediately. Does your example contain another typo or are you using a weird compiler? Cheers Stefan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Falk Tannhäuser Guest Posted: Thu Aug 28, 2003 10:46 am    Post subject: Re: typo in a for loop has a strange affect Quote: for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. Can anyone explain the logic of this to me. It should not be executed at all (and it isn't with gcc 3.2 which generates btw a 'statement with no effect' warning for the misplaced 'i the value of the 'i++' will be 0. Starting with another value for i, or writing ++i instead of i++ would cause the loop to be executed at least until i reaches the platform dependent INT_MAX. Then the result of incrementing an int holding the value INT_MAX is implementation defined, but on the 2-complement implementations I'm aware of it will wrap around to INT_MIN, so the loop will eventually terminate when i reaches 0, rather than run for ever. Falk Tannhäuser [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Thu Aug 28, 2003 10:47 am    Post subject: Re: typo in a for loop has a strange affect whocares wrote: Quote: I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. Can anyone explain the logic of this to me. Are you sure you didn't make some error copying this ? Let me explain: 1. int i = 0 Creates the loop-counter. No surprises here. 2. i++ This postfix-increments i, which is at that time zero. The expression converted to a boolean yields false. Since this is evaluated to decide if the loop should be executed another time, the loop is not executed at all. 3. Nothing. Since the condition for looping was false, the program continues after the loop. Note: if you changed the 'i++' to a '++i', things change. Then it loops until you get an overflow[1] of the loop-counter. Uli [1] not sure if such behaviour is required by the standard. -- Questions ? see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first ! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Tutone Guest Posted: Thu Aug 28, 2003 2:31 pm    Post subject: Re: typo in a for loop has a strange affect "whocares" wrote: Quote: I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. Really? Then your compiler is broken. On my system, it executes the loop zero times (rather than once) and then exits, which I believe is the correct behavior. Are you sure you didn't accidentally type "++i" instead of "i++" ? My compiler (GCC 3.3) also gave me a warning, since the i that context. What compiler are you using? Best regards, Tom __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ron Guest Posted: Thu Aug 28, 2003 2:50 pm    Post subject: Re: typo in a for loop has a strange affect Quote: I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. Can anyone explain the logic of this to me. Sure. The loop evaluates the condition ("i++") before each iteration, and exits when it becomes false. s.6.5.3(1). Since the condition never reaches 0 (== false) unless i is just about to overflow, the loop lasts a long time. -- Ron [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Aaron Bentley Guest Posted: Thu Aug 28, 2003 2:54 pm    Post subject: Re: typo in a for loop has a strange affect whocares wrote: Quote: Hi group I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. Can anyone explain the logic of this to me. I think I know for loops pretty well, and I can't see how this would be true. The loop can be rewritten like this: int x; { int i=0; while (i++) { cout< i } } Since you used i++ not ++i, the value 0 should be passed to the while(), and since 0 is false, the body of the while loop should not be entered. If you had used ++i, I could explain the behaviour. Then the value 1 (e.g. true) would have been passed to the while loop, and then the value 2, 3, 4, etc. But even that won't loop forever. Eventually, i will reach INT_MAX, and then ++i will produce INT_MIN, (a negative number). Finally, your values will reach -2, -1 and then 0. At that point, the loop will exit. You can see this effect more quickly if you make i a signed char. HTH, Aaron -- Aaron Bentley www.aaronbentley.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Greg P. Guest Posted: Thu Aug 28, 2003 3:05 pm    Post subject: Re: typo in a for loop has a strange affect "whocares" <gvingoe (AT) hotmail (DOT) com> wrote Quote: I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } As you know, it should be: for(int i = 0; i < x; ++i){..} Any *proper* compiler will not generate code to have the for statement loop forever, or even at all. The structure of a for loop is: for (initializations; boolean expressions; post operations) In your case, the for loop is checking "i++" for a boolean result. However, "i++" does return an integer (in your case). Your compiler is probably guessing that since the return is a positive number, that the expression results in "true" causing it to keep looping. I tried your code with Digital Mars and it won't even execute the loop. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Nils Petter Vaskinn Guest Posted: Thu Aug 28, 2003 8:05 pm    Post subject: Re: typo in a for loop has a strange affect On Wed, 27 Aug 2003 11:33:34 -0400, whocares wrote: Quote: Hi group I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } That's the same as: int i = 0; while (i++) { { /* do something */ } i } hth NPV [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Corno Guest Posted: Thu Aug 28, 2003 8:05 pm    Post subject: Re: typo in a for loop has a strange affect "whocares" <gvingoe (AT) hotmail (DOT) com> wrote Quote: Hi group I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. Can anyone explain the logic of this to me. it will continue while the 'i++' returns true (thus any value except 0) and that will be forever (at least a mighty long time) Corno [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Thu Aug 28, 2003 8:33 pm    Post subject: Re: typo in a for loop has a strange affect "whocares" <gvingoe (AT) hotmail (DOT) com> wrote Quote: I found the following typo in my code for(int i=0; i++; i { /* do somthing*/ } re-reading it I would of thought it would execute the loop once then i would equal 1 and it would exit. but it actually loops for ever. Can anyone explain the logic of this to me. Are you sure? I would expect it never to enter the loop, and this is the behavior I get with the compilers I have access to (g++ and Sun CC). After not entering the loop, of course, i is 1. -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France, +33 (0)1 30 23 45 16 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Nils Petter Vaskinn Guest Posted: Fri Aug 29, 2003 8:28 am    Post subject: Re: typo in a for loop has a strange affect On Thu, 28 Aug 2003 11:05:52 -0400, Greg P. wrote: Quote: "whocares" <gvingoe (AT) hotmail (DOT) com> wrote in message news:biho3n$4b5$1 (AT) rdel (DOT) co.uk... | I found the following typo in my code | | for(int i=0; i++; i | { | /* do somthing*/ | } As you know, it should be: for(int i = 0; i < x; ++i){..} Any *proper* compiler will not generate code to have the for statement loop forever, or even at all. The structure of a for loop is: Bzzzt. Wrong. But thanks for playing. for (; { // do something } Is completely legal. "do something" may or may not do something that cause the program to exit or the loop to break; Quote: I tried your code with Digital Mars and it won't even execute the loop. Because the OP's loop shouldn't, he obviously made a typo when writing his post or his program hung somewhere else. regards NPV [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Fri Aug 29, 2003 8:29 am    Post subject: Re: typo in a for loop has a strange affect In article <F1b3b.2786$_F1.424997 (AT) news20 (DOT) bellglobal.com>, Aaron Bentley <aaron.bentley (AT) utoronto (DOT) ca> writes Quote: But even that won't loop forever. Eventually, i will reach INT_MAX, and then ++i will produce INT_MIN, (a negative number). No, then you enter the domain of undefined behaviour. If we were using an unsigned int all would be fine as adding one to UINT_MAX is required to result in zero but there is no similar requirement for (signed) int. -- Francis Glassborow ACCU 64 Southfield Rd Oxford OX4 1PA +44(0)1865 246490 All opinions are mine and do not represent those of any organisation [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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