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how to handle  float numbers as keys to MAP containr?

Posted on 2011-02-17
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Last Modified: 2012-05-11
Hi,
I read Data to my program, numbers like 1.3434 100.34  etc
I put them inside floats variables.
1.3434 can be stored in the memory as 1.3433999 or 1.3433998 etc.

1) I want to use these numbers as keys in a map container.
How can I do it?
For example I want to put 1.3434 as a key in a MAP container,When I do calculations I get this number as 1.34339998 so converting it to 1.3434 would not be easy because I have many data types and I need to check how many digits after the decimal point each one of them has.

2) Are there special libraries for Visual C++ 2010 to represent these numbers in a different form ? (not as floats)
Thanks
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Question by:star90
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Author Comment

by:star90
ID: 34915667
I want to add that the data is from the Forex market.
Maybe there are special libraries to handle these numbers?
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Accepted Solution

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Infinity08 earned 1400 total points
ID: 34915703
Two options come to mind :

(a) Don't use floating point values. A fixed point data type doesn't have these comparison issues (and avoids other rounding/precision related issues as well).

(b) Specify a custom comparator for the map that compares two floats with an epsilon (ie. maximal difference between the two values that would still consider both equal).
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Author Comment

by:star90
ID: 34915778
Hi Infinity08,
1) How do I use fixed point data type in VC++? How do I declare them?
2) Can you please give me an example for f the custom comparator for the map?
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LVL 53

Expert Comment

by:Infinity08
ID: 34915839
>> 1) How do I use fixed point data type in VC++? How do I declare them?

You can either implement your own, or find an existing implementation that suits your needs.


>> 2) Can you please give me an example for f the custom comparator for the map?

Sure, something like :
struct CompareFloat {
  bool operator()(const float& left, const float& right) const {
    return (left < (right - 0.0001f));
  }
};

std::map<float, Data, CompareFloat> myMap;

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Author Comment

by:star90
ID: 34915898
If I write this fixed point data type then I need also to write the basic operations like - * + == etc?
Why is it not part of the C++ language or the Visual C++ libraries?

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Expert Comment

by:Infinity08
ID: 34915934
>> If I write this fixed point data type then I need also to write the basic operations like - * + == etc?

Yes.


>> Why is it not part of the C++ language or the Visual C++ libraries?

There are plenty of things that could have been in the C++ language heh. In the end, a decision was made as to what was included in it, and what was left to extensions or third party libraries. Fixed point datatypes are not commonly included by default in programming languages.

I don't know if an implementation is availabel in Visual C++ - I don't use it enough for that heh.
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Author Comment

by:star90
ID: 34915946
Thanks.
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Author Comment

by:star90
ID: 34916091
Assuming I write a fixed point data type using Lists of one digit numbers.
How do I tell MAP to recognize it?
Do I need to write a comparator for it? or how is it done?
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Author Comment

by:star90
ID: 34916197
OK I thought about it I understand that I need to write a comparator.
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Expert Comment

by:Infinity08
ID: 34916443
>> Do I need to write a comparator for it? or how is it done?

You can implement an operator== for your new data type, and that would be sufficient.
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Expert Comment

by:Infinity08
ID: 34916450
Sorry, I meant operator<
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