C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Simple Decimal class?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message tajmorton Guest Posted: Wed Nov 30, 2005 5:40 am    Post subject: Simple Decimal class? Hello, First, some back ground: I am learning C++ while writing a custom inventory program for our (seed) business. I am using Qt as my toolkit, as the app needs to be compatible with both Windows and Linux/X11. I have run into a problem with storing prices and weights (weights are in pounds). Obviously, float and double won't work. I've been googleing around, but haven't really found a easy to work with, portable, and open source class. Does anyone have any suggestions, or should I set about writing my own? Thanks! -- Taj Back to top Gianni Mariani Guest Posted: Wed Nov 30, 2005 5:47 am    Post subject: Re: Simple Decimal class? tajmorton wrote: Quote: Hello, First, some back ground: I am learning C++ while writing a custom inventory program for our (seed) business. I am using Qt as my toolkit, as the app needs to be compatible with both Windows and Linux/X11. I have run into a problem with storing prices and weights (weights are in pounds). Obviously, float and double won't work. I've been googleing around, but haven't really found a easy to work with, portable, and open source class. Does anyone have any suggestions, or should I set about writing my own? Why does double not work ? Is it a display issue ? BTW - you really need to drop these lb's and go metric :-) Back to top tajmorton Guest Posted: Wed Nov 30, 2005 6:07 am    Post subject: Re: Simple Decimal class? Gianni Mariani <gi2nos... (AT) mariani (DOT) ws> wrote: Quote: Why does double not work ? Is it a display issue ? Actually, I haven't tried double--only real. I was under the impression that double was just a bigger real. As for why it's not working; I can't do something like this: if (myreal == 0.5) Also, I've read that using real in currency is just a bad idea because of the "floating" nature of it. What I really need is something that when I assign it a value of 0.5--that's its exact value--no more, no less. Or am I completly missing something? Quote: BTW - you really need to drop these lb's and go metric Yeah, I know--I've been pushing for that. Thanks for the reply, -- Taj Back to top roberts.noah@gmail.com Guest Posted: Wed Nov 30, 2005 6:28 am    Post subject: Re: Simple Decimal class? tajmorton wrote: Quote: Gianni Mariani <gi2nos... (AT) mariani (DOT) ws> wrote: Why does double not work ? Is it a display issue ? Actually, I haven't tried double--only real. I was under the impression that double was just a bigger real. As for why it's not working; I can't do something like this: if (myreal == 0.5) Also, I've read that using real in currency is just a bad idea because of the "floating" nature of it. What I really need is something that when I assign it a value of 0.5--that's its exact value--no more, no less. Often money is implemented with an integral. Instead of counting $ you count cents. One dollar then is 100 instead of 1.00. If you can find a boundary for your weights you could do something similar. Now, when you do your calculations you may want to do so as reals instead and then do some rounding: int compute_tax(int amt) { return static_cast<int>(amt * .08 + .5); } and so on...this will happen so often you will want a round() function or macro. This is where the whole Superman/Office Space thing comes into play... Back to top Greg Guest Posted: Wed Nov 30, 2005 7:14 am    Post subject: Re: Simple Decimal class? tajmorton wrote: Quote: Gianni Mariani <gi2nos... (AT) mariani (DOT) ws> wrote: Why does double not work ? Is it a display issue ? Actually, I haven't tried double--only real. I was under the impression that double was just a bigger real. As for why it's not working; I can't do something like this: if (myreal == 0.5) Also, I've read that using real in currency is just a bad idea because of the "floating" nature of it. What I really need is something that when I assign it a value of 0.5--that's its exact value--no more, no less. Or am I completly missing something? Note that a double probably could be used here, since its precision is probably far greater than the precision needed to represent the weights. Otherwise, a fixed width number format can be used if the values must be exact. First, decide on the smallest unit that needs to be represented and treat one of those units as "1" and all other values scale from there. So a program would not compare a particular weight against 0.5, but against 50, assuming the scaling factor is 100. Fixed width arithmatic is integer arithmatic, so all comparisons are exact. And the only time when it is necessary to apply the scaling factor to the stored values is when they are displayed. To display a fixed width value as a floating point value, just convert the fixed point value to a doulbe and then divide by the scaling factor (in this example, 100): const int kScalingFactor = 100; int weight = 50; // half a pound printf("weight is %f", double(weight)/kScalingFactor); Greg Back to top tom.schultz Guest Posted: Wed Nov 30, 2005 7:36 am    Post subject: Re: Simple Decimal class? Greg wrote: Quote: tajmorton wrote: Gianni Mariani <gi2nos... (AT) mariani (DOT) ws> wrote: Why does double not work ? Is it a display issue ? Actually, I haven't tried double--only real. I was under the impression that double was just a bigger real. As for why it's not working; I can't do something like this: if (myreal == 0.5) Also, I've read that using real in currency is just a bad idea because of the "floating" nature of it. What I really need is something that when I assign it a value of 0.5--that's its exact value--no more, no less. Or am I completly missing something? Note that a double probably could be used here, since its precision is probably far greater than the precision needed to represent the weights. Otherwise, a fixed width number format can be used if the values must be exact. First, decide on the smallest unit that needs to be represented and treat one of those units as "1" and all other values scale from there. So a program would not compare a particular weight against 0.5, but against 50, assuming the scaling factor is 100. Fixed width arithmatic is integer arithmatic, so all comparisons are exact. And the only time when it is necessary to apply the scaling factor to the stored values is when they are displayed. To display a fixed width value as a floating point value, just convert the fixed point value to a doulbe and then divide by the scaling factor (in this example, 100): const int kScalingFactor = 100; int weight = 50; // half a pound printf("weight is %f", double(weight)/kScalingFactor); Greg 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 Page 1 of 1   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.