|
Hi,
I've a problem with the proper conversion of double to it's string representation.
For example.
If I've in the code some double value:
double a = 69000.015;
the debugger in debug window will show 69000.014999999999 but not 69000.015
Generally I need the precision and number of significant digits for the conversion.
For sprintf(...) I have to specify the precision and specifier.
How can I get the correct precision of the double value?
Is there any solution/clases for this type of conversion?
Thanks.
|
|
|
|
|
Hi,
there is simply no solution to your problem. Floating point numbers, by their very nature, cannot always represent the intended value.
The simplest example is the outcome of 1.0/3.0
humans write it down as 0.333333..., computers perform the division in binary and get a reasonably
accurate quotient (say off by no more than 1 of the lowest bit position), which is after all
an approximation.
The binary approximation of 1/3 is something like 1/4 + 1/16 + 1/64 + 1/256 + ...
and it has to stop somewhere since there are only so many bits reserved for the mantissa.
Similar things will happen to almost all real numbers; in order to avoid it, the number must
happen to be an integer value possibly divided by a power of two.
Hence 1.0/4.0, 7.0/4.0, 23.0/256.0 etc can be represented exactly,
whereas numbers with a prime factor (other than 2) in the denominator will not be exact,
nor will irrational numbers (such as pi, or the square root of 2).
If you know how many decimals (i.e. digits behind the decimal point) are required to get an exact
representation, then order that number of decimals or fewer. Rounding will occur, and everything
will look very natural.
If you don't know the number of decimals required and care very much about the correctness of them,
say you start your own financial program or business, then you'd better have a look at the decimal type. It offers a smaller range of numbers, but in some sense a better accuracy.
|
|
|
|
|
Thanks Luc for the answer,
but unfortunally I'm not a Fortran developer, I do Visual C++ which do not have a decimal type.
But the 69000.015 is non preiodical double(not like 1/3=0.333333...) and internal CPU's data register representation will be a particular correspondent binary. I'm not a pro in math, but for me seems like it needs some analysis of the binary representation to get a precision.
|
|
|
|
|
This page [^]may show the internal representation of a decimal number as double.
It comes out, that 69000.015 is represented with 40F0D8803D70A3D,
i.e. 1.0528566741943360 as significand and 16 as exponent.
If you put the above number in windows calculator then you'll get:
1.0528566741943360 * 2^16 = 1,0528566741943360 * 65536 = 69000.0150000000040960
probably not exactly what is expected by you.
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke
[My articles]
|
|
|
|
|
No real simple solution due to the way floating point numbers are represented in binary form. You'll always have some odd round off to deal with.
You could always try using the Decimal type...
"The clue train passed his station without stopping." - John Simmons / outlaw programmer
"Real programmers just throw a bunch of 1s and 0s at the computer to see what sticks" - Pete O'Hanlon
|
|
|
|
|
You won't always have round off to deal with. Binary fractions like 1/2, 1/4, 3/4, 1/8, etc. can be exactly represented as a double. But you're right that most numbers can't (assuming that the word "most" can be used of an infinite set).
The reason we put up with the inexactness is that exactness doesn't exist in the physical world. If a piece of wood is "2.4 meters long", it's really "2.4 meters give or take a millimeter" (which would be written as "2.400" to emphasize this). So, then, what difference does it make if your computer represents the length as 2.399999999999999911182158029987476766109466552734375 m, off by a mere 8.88e-17 m, when the physical accuracy is nowhere near that good?
The one exception is money. If something costs $3.99, it costs exactly $3.99, not $3.9900000000000002. If the sales tax is 8.25%, it's exactly 8.25%, not 8.2500000000000004%. And, because we use decimal currency, Decimal classes are often used for monetary amounts.
You could use it as a general-purpose number class, but:
(1) It's not a panacea for rounding error. You still have to deal with 1/3 + 2/3 != 1.
(2) Due to the lack of hardware support, it's much slower than binary floats.
Therefore, I wouldn't just blindly recommend "use Decimal". If your only complaint with float or double is the string representation, then just use a smaller precision in sprintf or string.Format or the equivalent in your favorite language. (15 digits in the 'g' format will usually get the job done.)
|
|
|
|
|
Read this[^]
Peter
"Until the invention of the computer, the machine gun was the device that enabled humans to make the most mistakes in the smallest amount of time."
|
|
|
|
|
Hi,
I have a string with value "111733394601234567094987654321" . Now i want to divide this with 636. I convert it to double , then im getting 1.1173339460123456E+29 . but these both values are differnt finally im
getting wrong output. can anyone help me.....
Thanks
Prathap
|
|
|
|
|
If you do this for the cryptograpy purposes use any kind of Crypto API, it works with very long integers.
|
|
|
|
|
I know there's lots - but does anyone have favorites, for ease of use and scope?
|
|
|
|
|
You might get better results asking for an algorithm rather than code.
If you don't have the data, you're just another a**hole with an opinion.
|
|
|
|
|
Hahaha! It didn't even occur to me that people would think - quite naturally - that my question was a homework problem! It's been so long since I've been outta school - HAHAHAHA!!
I'm familiar with the standard algorithms (unless a new one has become prominent in the last 10 years) - I was just looking to save myself the hassle with a lil homebrew tool for a game (eve).
Nevermind - thanks for being nice about it tho!
(giggles) Homework.
|
|
|
|
|
sherifffruitfly wrote: question was a homework problem
Typically this type of problem is a homework problem for an algorithms class. Sounds like it isn't in your case.
"The clue train passed his station without stopping." - John Simmons / outlaw programmer
"Real programmers just throw a bunch of 1s and 0s at the computer to see what sticks" - Pete O'Hanlon
|
|
|
|
|
Yupyup - It's all good - I should have realized how my question would appear to the reader.
|
|
|
|
|
To qualify as a homework problem, it need to have plz, urgent, hlp me, or any variation of a time constrained desperation.
I did find the Hamiltonian Cycle algorithm my Algorithms professor did a few years ago, a real fun one to read back through, but it works 
"The clue train passed his station without stopping." - John Simmons / outlaw programmer
"Real programmers just throw a bunch of 1s and 0s at the computer to see what sticks" - Pete O'Hanlon
|
|
|
|
|
hahaha - yeah, gamer-speak is definitely typical. Lots of exclamation marks too!!!elevens!!! 
|
|
|
|
|
Hi,
I'm just looking to rework part of an application I'm working on that catgorises files by comparing the file path and file name to a lookup table table with multiple string values. Are there any established patterns or algorithms that apply to this scenario?
Something along the lines of how Levenshtien Distance works for comparing two strings, the difference here being that I'm looking at whole words compared to an array of possible matches.
Cheers,
|
|
|
|
|
Why can't you use the Levenshtein distance with words, i.e. increment the cost for a different word instead of a letter? You could even use fractional costs according to how much the words differ.
|
|
|
|
|
Good Day,
Is there a mathematical formula/theory that can tell how much different (in numbers or percentage) two objects are?
Say, I have 2 files, File A and File B, and the formula can tell how much are they different?
Regards,
Ian
It is said that the most complex structures built by mankind are software systems. This is not generally appreciated because most people cannot see them. Maybe that's a good thing because if we saw them as buildings, we'd deem many of them unsafe.
|
|
|
|
|
You need some kind of metric by which to measure the difference. For example, length. Text files could differ by the number of different words if you are comparing an original file with a modified one.
So you need to specify what, exactly, you imply by "differ".
|
|
|
|
|
Well, I will be comparing binary data. A string of 1's and 0's.
It is said that the most complex structures built by mankind are software systems. This is not generally appreciated because most people cannot see them. Maybe that's a good thing because if we saw them as buildings, we'd deem many of them unsafe.
|
|
|
|
|
|
Thank you. But I would be comparing binary data. But this is somewhat along the line of what I need.
It is said that the most complex structures built by mankind are software systems. This is not generally appreciated because most people cannot see them. Maybe that's a good thing because if we saw them as buildings, we'd deem many of them unsafe.
|
|
|
|
|
You still have to define "different".
Say file REF holds 99 bytes of data.
File APPEND holds the same 99 bytes of data followed by one more byte.
File PREFIX holds that same one byte followed by the original 99 bytes.
Now these three files are different. How different are they? That is up to you.
One would say APPEND is very similar to REF.
One could argue PREFIX is completely different, or very similar. Take your pick.
And some parsers would object to "very similar"; if REF represents a valid image (say something
Image.FromFile understands), then APPEND is not likely to be a valid image (as per Image.FromFile).
|
|
|
|
|
I see.
I thought there was a general formula to test for "difference".
It is said that the most complex structures built by mankind are software systems. This is not generally appreciated because most people cannot see them. Maybe that's a good thing because if we saw them as buildings, we'd deem many of them unsafe.
|
|
|
|
General 
News 
Suggestion 
Question 
Bug 
Answer 
Joke 
Praise 
Rant 
Admin 
Submit an article or tip
