C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  How to tell if a number has decimal places different than 0 Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Gaijinco Guest Posted: Mon Sep 26, 2005 8:07 pm    Post subject: How to tell if a number has decimal places different than 0 Sooner or later everytime I found recreational programming challenges I stumble with how I test if a number is has decimal places differnt than 0? For example if I want to know if a number is a square number (i.e. a number which square root is a positive number as 4, 9, 16 have) I do something like: int square = sqrt(number); if((int)square==square) // number is a perfect square else // number is not a perfect square Is there a function or a language-specific-way to do this? Back to top Emmanuel Delahaye Guest Posted: Mon Sep 26, 2005 8:10 pm    Post subject: Re: How to tell if a number has decimal places different tha Gaijinco wrote on 26/09/05 : Quote: Sooner or later everytime I found recreational programming challenges I stumble with how I test if a number is has decimal places differnt than 0? For example if I want to know if a number is a square number (i.e. a number which square root is a positive number as 4, 9, 16 have) I do something like: int square = sqrt(number); if((int)square==square) // number is a perfect square else // number is not a perfect square Is there a function or a language-specific-way to do this? fmod() -- Emmanuel The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html The C-library: http://www.dinkumware.com/refxc.html "It's specified. But anyone who writes code like that should be transmogrified into earthworms and fed to ducks." -- Chris Dollin CLC Back to top John Harrison Guest Posted: Mon Sep 26, 2005 9:05 pm    Post subject: Re: How to tell if a number has decimal places different tha Gaijinco wrote: Quote: Sooner or later everytime I found recreational programming challenges I stumble with how I test if a number is has decimal places differnt than 0? For example if I want to know if a number is a square number (i.e. a number which square root is a positive number as 4, 9, 16 have) I do something like: int square = sqrt(number); if((int)square==square) // number is a perfect square else // number is not a perfect square Is there a function or a language-specific-way to do this? Language-specific? What do you mean by that? Your test is not perfect because it will fail when square is bigger than the biggest int. This test always works (within the limitations of floating point accuracy). #include <math> double square = sqrt(number); if (floor(square) == square) // number is a perfect square else // number is not a perfect square john Back to top Eric Sosman Guest Posted: Mon Sep 26, 2005 9:12 pm    Post subject: Re: How to tell if a number has decimal places different tha Gaijinco wrote On 09/26/05 16:07,: Quote: Sooner or later everytime I found recreational programming challenges I stumble with how I test if a number is has decimal places differnt than 0? For example if I want to know if a number is a square number (i.e. a number which square root is a positive number as 4, 9, 16 have) I do something like: int square = sqrt(number); if((int)square==square) // number is a perfect square .... and the test is a tautology. Quote: else // number is not a perfect square Is there a function or a language-specific-way to do this? The code you've shown will (if it doesn't invoke undefined behavior) declare that every number is a perfect square: 1, 2, 3.14, and even -42. To test whether a floating-point number is an integer with no fractional part, you could try if ((int)fpn == fpn) ... This runs into trouble when the magnitude of fpn is large, so large that its value is outside the range of numbers representable as `int'. As an improvement you might try if (fmod(fpn, 1.0) == 0.0) ... This is still vulnerable to the "graininess" of floating-point numbers, which are not mathematical real numbers with infinite precision. You cannot usually expect sqrt(fpn) to be the exact square root of fpn. sqrt(fpn) will be very close to the exact root, but will (in general) be just a little bit different from the true value. There could be several different fpn values for which sqrt(fpn) would deliver exactly the same slightly wrong answer: both sqrt(4.0) and sqrt(4.0 + tiny_number) might produce 2.0 as an answer. If you decide that a number is a perfect square if its computed square root turns out to be an integer, you will erroneously conclude that 4.0+tiny_number is a perfect square. A possibly more thorough test might compute the square root, test whether it's an integer, and then test whether its square equals the original number: double root = sqrt(number); if (fmod(root, 1.0) == 0.0 && root * root == number) .... but even this may have some problems. I am always uneasy when comparing floating-point quantities for exact equality, mostly because fpn's are usually regarded as approximations to begin with. You usually need an "approximately equal" test of some kind, and such a test isn't well suited to the purely yet/no nature of perfect squaredom. For "number-theoretic" calculations you'll usually be much better off using integers of some flavor. If the numbers grow large you may need to resort to a "bignum" package; several are available. -- [email]Eric.Sosman (AT) sun (DOT) com[/email] Back to top Mark McIntyre Guest Posted: Mon Sep 26, 2005 9:18 pm    Post subject: Re: How to tell if a number has decimal places different tha On Mon, 26 Sep 2005 21:05:09 GMT, in comp.lang.c , John Harrison <john_andronicus (AT) hotmail (DOT) com> wrote: (of testing to see if a float is a perfect square) Quote: This test always works (within the limitations of floating point accuracy). I'd not be too sure of that. Remember floating point is not an exact representation. Quote: double square = sqrt(number); if (floor(square) == square) // number is a perfect square else // number is not a perfect square -- Mark McIntyre CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html> CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt> http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- Back to top John Harrison Guest Posted: Mon Sep 26, 2005 9:20 pm    Post subject: Re: How to tell if a number has decimal places different tha Mark McIntyre wrote: Quote: On Mon, 26 Sep 2005 21:05:09 GMT, in comp.lang.c , John Harrison [email]john_andronicus (AT) hotmail (DOT) com[/email]> wrote: (of testing to see if a float is a perfect square) This test always works (within the limitations of floating point accuracy). I'd not be too sure of that. Remember floating point is not an exact representation. That's why I said 'within the limitations of floating point accuracy'. My test always checks if a floating point number is integral. Obviously an integral return from sqrt does not necessarily mean the sqrt parameter was a perfect square. john Back to top Eric Sosman Guest Posted: Mon Sep 26, 2005 9:32 pm    Post subject: Re: How to tell if a number has decimal places different tha John Harrison wrote On 09/26/05 17:20,: Quote: Mark McIntyre wrote: On Mon, 26 Sep 2005 21:05:09 GMT, in comp.lang.c , John Harrison [email]john_andronicus (AT) hotmail (DOT) com[/email]> wrote: (of testing to see if a float is a perfect square) This test always works (within the limitations of floating point accuracy). I'd not be too sure of that. Remember floating point is not an exact representation. That's why I said 'within the limitations of floating point accuracy'. My test always checks if a floating point number is integral. Obviously an integral return from sqrt does not necessarily mean the sqrt parameter was a perfect square. On the machine in front of me right now, sqrt(1.0) and sqrt(1.0000000000000002) both give 1.0 as the root. -- [email]Eric.Sosman (AT) sun (DOT) com[/email] Back to top Martin Ambuhl Guest Posted: Mon Sep 26, 2005 9:37 pm    Post subject: Re: How to tell if a number has decimal places different tha Gaijinco wrote: Quote: Sooner or later everytime I found recreational programming challenges I stumble with how I test if a number is has decimal places differnt than 0? For example if I want to know if a number is a square number (i.e. a number which square root is a positive number as 4, 9, 16 have) I do something like: int square = sqrt(number); if((int)square==square) // number is a perfect square else // number is not a perfect square Is there a function or a language-specific-way to do this? using modf, you can test either the integral part or the fractional part. Look askance at any solution that involves ints. #include <stdlib.h> #include <stdio.h> #include <time.h> #include <math.h> int main(void) { double x, fp, ip; int loop, cnt; srand(time(0)); for (loop = cnt = 0; cnt < 10; loop++) { x = (int) (100. * rand() / (1. + RAND_MAX)) / 10.; fp = modf(x, &ip); if (ip != x || fp) continue; printf("%4d: x = %g, fractional part (fp) = %g," "integer part (ip) = %gn" " (ip %s x, fp %s 0)n", loop, x, fp, ip, (ip == x) ? "==" : "!=", (fp == 0) ? "==" : "!="); cnt++; } return 0; } 1: x = 8, fractional part (fp) = 0,integer part (ip) = 8 (ip == x, fp == 0) 36: x = 0, fractional part (fp) = 0,integer part (ip) = 0 (ip == x, fp == 0) 63: x = 6, fractional part (fp) = 0,integer part (ip) = 6 (ip == x, fp == 0) 70: x = 1, fractional part (fp) = 0,integer part (ip) = 1 (ip == x, fp == 0) 79: x = 5, fractional part (fp) = 0,integer part (ip) = 5 (ip == x, fp == 0) 87: x = 5, fractional part (fp) = 0,integer part (ip) = 5 (ip == x, fp == 0) 128: x = 4, fractional part (fp) = 0,integer part (ip) = 4 (ip == x, fp == 0) 147: x = 1, fractional part (fp) = 0,integer part (ip) = 1 (ip == x, fp == 0) 148: x = 8, fractional part (fp) = 0,integer part (ip) = 8 (ip == x, fp == 0) 162: x = 1, fractional part (fp) = 0,integer part (ip) = 1 (ip == x, fp == 0) Back to top John Harrison Guest Posted: Mon Sep 26, 2005 9:42 pm    Post subject: Re: How to tell if a number has decimal places different tha Quote: On the machine in front of me right now, sqrt(1.0) and sqrt(1.0000000000000002) both give 1.0 as the root. Am I missing something? What point are you making? john Back to top Keith Thompson Guest Posted: Mon Sep 26, 2005 10:15 pm    Post subject: Re: How to tell if a number has decimal places different tha John Harrison <john_andronicus (AT) hotmail (DOT) com> writes: [...] Quote: #include <math double square = sqrt(number); if (floor(square) == square) // number is a perfect square else // number is not a perfect square I suggest that "square" is a really bad name for a variable that hold the result of a call to sqrt(). -- Keith Thompson (The_Other_Keith) [email]kst-u (AT) mib (DOT) org[/email] San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst> We must do something. This is something. Therefore, we must do this. Back to top Pete Becker Guest Posted: Mon Sep 26, 2005 11:31 pm    Post subject: Re: How to tell if a number has decimal places different tha Mark McIntyre wrote: Quote: I'd not be too sure of that. Remember floating point is not an exact representation. Floating point is exact. Unlike real numbers, it is not continuous. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) Back to top E. Robert Tisdale Guest Posted: Mon Sep 26, 2005 11:43 pm    Post subject: Re: How to tell if a number has decimal places different tha Pete Becker wrote: Quote: Mark McIntyre wrote: I'd not be too sure of that. Remember floating point is not an exact representation. Floating point is exact. Unlike real numbers, it is not continuous. In general, *finite precision* floating-point arithmetic is *inexact*. The set real numbers includes irrational numbers which have *no* finite precision digital representation in this universe -- they exist only in the minds of mathematicians. Back to top Keith Thompson Guest Posted: Tue Sep 27, 2005 12:21 am    Post subject: Re: How to tell if a number has decimal places different tha Pete Becker <petebecker (AT) acm (DOT) org> writes: Quote: Mark McIntyre wrote: I'd not be too sure of that. Remember floating point is not an exact representation. Floating point is exact. Unlike real numbers, it is not continuous. It depends on how you look at it. A given floating-point representation can be viewed either as an exact value, or as an inexact approximation of any of the infinitely many real numbers close to the exact represented value. The latter, though arguably incorrect, is probably the more common interpretation, especially given things like: double one_third = 1.0/3.0; Integers are usually considered to be exact because they typically aren't thought of as being approximations of nearby numbers; 42 really is 42, not an approximation of 42.0625. -- Keith Thompson (The_Other_Keith) [email]kst-u (AT) mib (DOT) org[/email] <http://www.ghoti.net/~kst> San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst> We must do something. This is something. Therefore, we must do this. Back to top Dik T. Winter Guest Posted: Tue Sep 27, 2005 1:19 am    Post subject: Re: How to tell if a number has decimal places different tha In article <dha13e$ko8$1 (AT) nntp1 (DOT) jpl.nasa.gov> [email]E.Robert.Tisdale (AT) jpl (DOT) nasa.gov[/email] writes: Quote: Pete Becker wrote: Mark McIntyre wrote: I'd not be too sure of that. Remember floating point is not an exact representation. Floating point is exact. Unlike real numbers, it is not continuous. In general, *finite precision* floating-point arithmetic is *inexact*. I think you misunderstand Pete Beckers meaning. Assuming IEEE, given two operands and an operation, it is precisely predictable what the result is. But I understand you are pretty good at misunderstanding. -- dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131 home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/ Back to top Eric Sosman Guest Posted: Tue Sep 27, 2005 2:10 pm    Post subject: Re: How to tell if a number has decimal places different tha John Harrison wrote: Quote: On the machine in front of me right now, sqrt(1.0) and sqrt(1.0000000000000002) both give 1.0 as the root. Am I missing something? What point are you making? Just giving a concrete example of the point you made (and snipped): Quote: [...] Obviously an integral return from sqrt does not necessarily mean the sqrt parameter was a perfect square. -- Eric Sosman [email]esosman (AT) acm-dot-org (DOT) inva[/email]lid 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.