C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  finding the minimum allowed value for a type Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Adam H. Peterson Guest Posted: Fri Sep 26, 2003 5:25 pm    Post subject: finding the minimum allowed value for a type Is there a standard way to find the minimum value for a data type? I'm thinking along the lines of std::numeric_limits<T>::min(). But that doesn't work for floating point types. I can't use (-std::numeric_limits<T>::max()) either, because that won't work for unsigned types (and technically won't work for signed integer types and others either, although it would probably be "good enough" for them). Do I need to implement a new template that uses std::numeric_limits<>::min() and just specialize it for floats/doubles? Or is this already solved somewhere in the library? Thanks, Adam H. Peterson Back to top David Rubin Guest Posted: Fri Sep 26, 2003 7:05 pm    Post subject: Re: finding the minimum allowed value for a type "Adam H. Peterson" wrote: Quote: Is there a standard way to find the minimum value for a data type? I'm thinking along the lines of std::numeric_limits<T>::min(). But that doesn't work for floating point types. Is there nothing analogous to DBL_MIN and FLT_MIN in C (limits.h)? Quote: I can't use (-std::numeric_limits<T>::max()) either, because that won't work for unsigned types Isn't the minimum value for unsigned types zero? Quote: (and technically won't work for signed integer types and others either, although it would probably be "good enough" for them). Signed integer types can use std::numeric_limits<T>::min() as you pointed out. Quote: Do I need to implement a new template that uses std::numeric_limits<>::min() and just specialize it for floats/doubles? Or is this already solved somewhere in the library? /david -- Andre, a simple peasant, had only one thing on his mind as he crept along the East wall: 'Andre, creep... Andre, creep... Andre, creep.' -- unknown Back to top Mike Wahler Guest Posted: Fri Sep 26, 2003 7:10 pm    Post subject: Re: finding the minimum allowed value for a type "Adam H. Peterson" <ahp6 (AT) email (DOT) byu.edu> wrote Quote: Is there a standard way to find the minimum value for a data type? I'm thinking along the lines of std::numeric_limits<T>::min(). That's the one. :-) Quote: But that doesn't work for floating point types. Sure it does. What have you tried? Quote: I can't use (-std::numeric_limits<T>::max()) either, Why would you want to do that? Quote: because that won't work for unsigned types Well, it doesn't make much sense to try to negate an unsigned type. Quote: (and technically won't work for signed integer types and others either, What have you tried that "wont' work"? Quote: although it would probably be "good enough" for them). Do I need to implement a new template that uses std::numeric_limits<>::min() and just specialize it for floats/doubles? The standard library already specializes the template for all scalar types except pointers. Quote: Or is this already solved somewhere in the library? It's already done. Why not post a small, compilable program that demonstrates the trouble you're having? -Mike Back to top Ron Natalie Guest Posted: Fri Sep 26, 2003 7:17 pm    Post subject: Re: finding the minimum allowed value for a type "Mike Wahler" <mkwahler (AT) mkwahler (DOT) net> wrote Quote: "Adam H. Peterson" <ahp6 (AT) email (DOT) byu.edu> wrote in message news:bl1sq5$2tah$1 (AT) acs2 (DOT) byu.edu... Is there a standard way to find the minimum value for a data type? I'm thinking along the lines of std::numeric_limits<T>::min(). That's the one. :-) No it's not. numeric_limits<double>::min() reports the smallest positilve representable number. What he wants is the mininum representable (numerically). What he wants is: template <typename T> T real_min() { if(numeric_limits<T>::is_integer) return numeric_limits<T>::min(); else return -numeric_limits<T>::max(); } Back to top Adam H. Peterson Guest Posted: Fri Sep 26, 2003 7:25 pm    Post subject: Re: finding the minimum allowed value for a type Mike Wahler wrote: Quote: "Adam H. Peterson" <ahp6 (AT) email (DOT) byu.edu> wrote in message news:bl1sq5$2tah$1 (AT) acs2 (DOT) byu.edu... Is there a standard way to find the minimum value for a data type? I'm thinking along the lines of std::numeric_limits<T>::min(). That's the one. :-) But that doesn't work for floating point types. Sure it does. What have you tried? For floating point types, it returns the minimum number greater than zero, which is not what I want. Back to top Adam H. Peterson Guest Posted: Fri Sep 26, 2003 7:28 pm    Post subject: Re: finding the minimum allowed value for a type David Rubin wrote: Quote: "Adam H. Peterson" wrote: snip Is there nothing analogous to DBL_MIN and FLT_MIN in C (limits.h)? snip Isn't the minimum value for unsigned types zero? snip Signed integer types can use std::numeric_limits<T>::min() as you pointed out. But I'm interested in doing this in a template, where I don't know if I'll be passed a signed type, an unsigned type, or a floating point type (or possibly a UDT for which I would insist that the proper template be specialized). Inside the template, I don't know which of these cases to apply. So what is the best approach for an arbitrary type? Thanks for the response. Adam H. Peterson Back to top Adam H. Peterson Guest Posted: Fri Sep 26, 2003 7:44 pm    Post subject: Re: finding the minimum allowed value for a type Quote: It's already done. Why not post a small, compilable program that demonstrates the trouble you're having? Here you go, and thanks for taking the time to respond: #include <limits> #include <iostream> #include <ostream> template<typename T> void find_min_max(T const *array, int unsigned size, T &min, T &max) { min=std::numeric_limits<T>::max(); max=std::numeric_limits<T>::min(); while (size--) { if (min>array[size]) min=array[size]; if (max max=array[size]; } } int main() { double sampledata[]={ -1.0, -2.0, -4.5, -0.5, -3.5 }; double min,max; find_min_max(sampledata, 5, min, max); std::cout << "Values range from " << min << " to " << max << std::endl; } The problem comes when find_min_max gives a positive (very small) value for max, even though all values in the array are negative. On my machine, it prints: Values range from -4.5 to 2.22507e-308 Adam H. Peterson Back to top Kevin Goodsell Guest Posted: Fri Sep 26, 2003 8:09 pm    Post subject: Re: finding the minimum allowed value for a type Ron Natalie wrote: Quote: What he wants is: template <typename T> T real_min() { if(numeric_limits<T>::is_integer) return numeric_limits<T>::min(); else return -numeric_limits<T>::max(); Is there a requirement that the smallest (most negative) value for a floating point type is the negation of the largest value? -Kevin -- My email address is valid, but changes periodically. To contact me please use the address from a recent posting. Back to top Pete Becker Guest Posted: Fri Sep 26, 2003 8:09 pm    Post subject: Re: finding the minimum allowed value for a type "Adam H. Peterson" wrote: Quote: It's already done. Why not post a small, compilable program that demonstrates the trouble you're having? Here you go, and thanks for taking the time to respond: #include <limits #include #include template void find_min_max(T const *array, int unsigned size, T &min, T &max) { min=std::numeric_limits max=std::numeric_limits<T>::min(); while (size--) { if (min>array[size]) min=array[size]; if (max max=array[size]; } } You don't need the minimum and maximum possible values. Initialize min and max with array[size - 1], and run your loop down from there. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) Back to top Adam H. Peterson Guest Posted: Fri Sep 26, 2003 8:23 pm    Post subject: Re: finding the minimum allowed value for a type Quote: You don't need the minimum and maximum possible values. Initialize min and max with array[size - 1], and run your loop down from there. Thank you. That works in this case, but there are other cases where that's not necessarily an option. This program was by way of example. Adam H. Peterson Back to top Ron Natalie Guest Posted: Fri Sep 26, 2003 8:39 pm    Post subject: Re: finding the minimum allowed value for a type "Kevin Goodsell" <usenet1.spamfree.fusion (AT) neverbox (DOT) com> wrote Quote: Ron Natalie wrote: What he wants is: template <typename T> T real_min() { if(numeric_limits<T>::is_integer) return numeric_limits<T>::min(); else return -numeric_limits<T>::max(); Is there a requirement that the smallest (most negative) value for a floating point type is the negation of the largest value? Probably not, but I don't think I've ever encountered a floating point format where that wasn't the case. Back to top Mike Wahler Guest Posted: Sat Sep 27, 2003 4:56 am    Post subject: Re: finding the minimum allowed value for a type "Ron Natalie" <ron (AT) sensor (DOT) com> wrote Quote: "Mike Wahler" <mkwahler (AT) mkwahler (DOT) net> wrote "Adam H. Peterson" <ahp6 (AT) email (DOT) byu.edu> wrote in message news:bl1sq5$2tah$1 (AT) acs2 (DOT) byu.edu... Is there a standard way to find the minimum value for a data type? I'm thinking along the lines of std::numeric_limits<T>::min(). That's the one. :-) No it's not. numeric_limits<double>::min() reports the smallest positilve representable number. Oops, I overlooked that when I looked it up. Not surprising, since most of my applications are financial or business type, and I almost always use integral types for currency, inventory quantites, etc. Floating-point stuff is not as familiar or common to me as with many other folks. Quote: What he wants is the mininum representable (numerically). What he wants is: template <typename T> T real_min() { if(numeric_limits<T>::is_integer) return numeric_limits<T>::min(); else return -numeric_limits<T>::max(); } So I've learned two things: 1. numeric_limits<double>::min() returns smallest *positive* value 2. Pay closer attention to detail :-) -Mike Back to top Mike Wahler Guest Posted: Sat Sep 27, 2003 5:01 am    Post subject: Re: finding the minimum allowed value for a type "Adam H. Peterson" <ahp6 (AT) email (DOT) byu.edu> wrote Quote: David Rubin wrote: "Adam H. Peterson" wrote: snip Is there nothing analogous to DBL_MIN and FLT_MIN in C (limits.h)? snip Isn't the minimum value for unsigned types zero? snip Signed integer types can use std::numeric_limits<T>::min() as you pointed out. But I'm interested in doing this in a template, where I don't know if I'll be passed a signed type, an unsigned type, or a floating point type (or possibly a UDT for which I would insist that the proper template be specialized). Inside the template, I don't know which of these cases to apply. So what is the best approach for an arbitrary type? I think you could test numeric_limits<T>::is_integer and numeric_limits<T>::is_signed to do special processing for floating point types, e.g. what Ron showed: -numeric_limits<T>::max() -Mike Back to top Gianni Mariani Guest Posted: Sat Sep 27, 2003 7:23 am    Post subject: Re: finding the minimum allowed value for a type Adam H. Peterson wrote: Quote: Is there a standard way to find the minimum value for a data type? I'm thinking along the lines of std::numeric_limits<T>::min(). But that doesn't work for floating point types. I can't use (-std::numeric_limits<T>::max()) either, because that won't work for unsigned types (and technically won't work for signed integer types and others either, although it would probably be "good enough" for them). Do I need to implement a new template that uses std::numeric_limits<>::min() and just specialize it for floats/doubles? Or is this already solved somewhere in the library? This below I think gives you what you want - however it's impractical because of the excessive compilation time but ... if compilers were a little faster, it may actually be practical. However, it's just a toy I played with that shows computation of numeric constants using templates. You could easily extend this to compute compile time constants for things like e, pi, ln 10 nCr, factorial, and even a prime number in such a way that the constants would be compatible for different machine formats. Besides that, it's also an interesting compiler stress test. #include <iostream> template <typename T, int N> struct pow_n { static const T value = pow_n<T, N-1>::value/2; }; template <typename T> struct pow_n<T, 0> { static const T value = static_cast<T>( 1 ); }; template <typename T, int N, bool v> struct min_f; template <typename T, int N> struct min_f<T,N,true> { static const T min_value = pow_n<T,N-1>::value; static const int min_exponent = N-1; }; template <typename T, int N, bool v> struct min_f : min_f<T, N+1, pow_n { }; template<typename T> struct min_limit { static const int min_exponent = min_f<T,0,false>::min_exponent; static const T min_value = min_f<T,0,false>::min_value; }; int main() { std::cout << "Floatn"; std::cout << min_limit< float >::min_exponent << "n"; std::cout << min_limit< float >::min_value << "n"; std::cout << "Doublen"; std::cout << min_limit< double >::min_exponent << "n"; std::cout << min_limit< double >::min_value << "n"; }; The code above took 3min 14sec in compilation time on a 800Mhz x86-P3 machine with gcc 3.3.1 with the -ftemplate-depth-10000 flag. (Obviously miniscule execution time). g++ -ftemplate-depth-10000 -Wreturn-type -W -Wpointer-arith -pipe -ggdb3 -fcheck-new -fPIC -D_POSIX_THREADS -D_POSIX_THREAD_SAFE_FUNCTIONS -D_REENTRANT -DACE_HAS_AIO_CALLS -DBUILD_VERSION=0309262338-i686-uluru-gx86-gianni -DBUILD_ARCH=gx86 -I ./ -I work.gx86 -I /home/gianni/limbo/asmx/makexs/src/hello_library/code/ qqq.cpp -o qqq 171.260u 2.830s 3:13.92 89.7% 0+0k 0+0io 3932pf+0w [gianni@uluru makexs]$ time qqq Float 149 1.4013e-45 Double 1074 4.94066e-324 0.000u 0.000s 0:00.00 0.0% 0+0k 0+0io 219pf+0w Back to top Jerry Coffin Guest Posted: Sat Sep 27, 2003 12:06 pm    Post subject: Re: finding the minimum allowed value for a type In article <bl3du4$45a (AT) dispatch (DOT) concentric.net>, [email]gi2nospam (AT) mariani (DOT) ws[/email] says... [ ... ] Quote: The code above took 3min 14sec in compilation time on a 800Mhz x86-P3 machine with gcc 3.3.1 with the -ftemplate-depth-10000 flag. Hmmm...strange -- it gave identical results for me, but compiled in only 21 seconds with gcc 3.2 (the mingw port). This was on a 1.2 GHz P3, so I expected it to be a _little_ faster, but not nearly this much. -- Later, Jerry. The universe is a figment of its own imagination. 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