|
When we were CompSci students around 1980, ready-made packages were not readily available, so a group of us created an arbitrary precision arithmetic fortram package for a friend in theoretical physics:
This fellow was working on an analythical model to describe, at the micro level, what happens when two wave fronts collide head on. When applying this model in a simulation, even the Univac 1100 double precision 72 bit floating point causd discontinuities - more or less perfect square wave forms, which none of us really believed existed in the physical world, certainly not in liquids... So we gave him a library where we set up buffers for 200 decimal digits. (For character I/O, BCD was much better suited than a 600+ bits pure binary value - speed of calculation was not of any importance.)
Even with 600+ bits of precision, he experienced too large discontinuities. Then we realized that he was adding up elements of a series expansions from the head of the series, from the big towards the smaller elements. This lead us into a huge argument with him: As I said, he was in theoretical physics, and simply refused to accept that the order of adding together series elements would make a difference. We were several guys, over a period of many days, trying to explain, each in our own way, why "in theory, therory and practice are identical, but in practice they are not". Finally, he gave in and turned the addition around the other way. The waves came out so smooth that we silently suspected that the Univac 72 bits format would have been sufficient, if had just, from the very beginning, added the smallest elements first.
There was an intense discussion in academia around that time whether Computer Science is a science requiring its own scientists to do computer related jobs, or if every engineer, mathmatician or physicist should learn to handle a computer himself. I several times used this story to argue that scientists getting close to a computer must learn enough about them to avoid silly problems caused by poor understanding of how computers operate.
|
|
|
|
|
It says this project has no files.
|
|
|
|
|
Well, it depends...
First, rational numbers would slow things down.
But if you want accuracy in math, I think the answer is to use integers with IMPLIED Decimals.
Some currency types do this with 4 digits of implied decimal accuracy.
Personally, I have never seen the need for more than this, unless I was using matlab or something where maintaing 1/3 of important to backing out of the problem properly.
Finally, you could always implement your own classes to behave properly. Good luck with that, and the resulting speed.
Kirk Out!
|
|
|
|
|
Kirk 10389821 wrote: First, rational numbers would slow things down.
First, in quite a few applications, that doesn't matter. Whether the calculations are done in 10 or 20 milliseconds doesn't matter if you prepare a value that is displayed in an interactive dialog.
Second, if arithmetic calculations is only a small part of the handling, other formats than the traditional floating point may be faster in the other operations. Take an archetypical Cobol application: It reads in, moves around and displays values in counts and dollars and cents; every now and then doing a multiplication, but mostly just addition and subtraction. The time saved in I/O from a BCD format compared to a float format greatly offsets the slower BCD addition/subtraction.
Third: If you use rationals to avoid rounding errors, your float option might be an arbitrary precision float package. Adding 1/3 + 1/6 as rationals is likely to be much faster than calling a library function maintaining a list of mantiass elemnets to add 0.333333.... and 0.16666... for, say, 512 bits.
|
|
|
|
|
Smalltalk had a Fraction type that (I believe) worked like you would do it in elementary math.
1/3 -> new Fraction(1,3) (not the smalltalk notation!)
3/2 -> new Fraction(3,2)
(1/3) * (3/2) = new Fraction(1,3) * new Fraction(3,2) = new Fraction(1,2) -> 1/2
|
|
|
|
|
They had this in Assembler and FORTRAN a long time ago. In fact, it is a basic number type in System/370 machine language.
|
|
|
|
|
With modern language I think you can implement it yourself. All C# variable types derive from System.Object anyway. And then you can override arithmatic operator for this new type. I can sort of picture it, storing each value as two integer : numerator and denominator.
|
|
|
|
|
I wouldn't hesitate for a second to give that as the exercise of the week to second year programming students, implmenting the four basic arithmetic operations for rationals.
There is only a small problem: If the denominators are not identical in addition/subtraction, you can trivially find a common denominator by multiplying the two, but in the genral case, it is not the least common denominator. If you use 64 bit integers for the denominator you might delay handling this until the results are to be used (at least for small student level applications), but at some stage you will have to factorize the numerator and denominator, and remove common factors. In principle this is not difficult as long as the problem is of "student size", but Eratosthenes' sieve is not very efficient (certainly not in space!) - you wouldn't want to employ that for every addition or subtraction!
Factors of 2 are easy to get rid of: While both (numerator AND 1) and (denominator ANd 1) are nonzero, shift both right by one bit. This you can do for every basic arithmetic operation (or complex expression). Other factors are worse. If you find it acceptable to do a whole series of division/remainder operations (trying with all prime factors up to the square root of the smaller of numerator and denominator), then the space required is roughly limited to a single read-only table of known primes, but you'll certainly be keeping the division circuitry of the CPU hot  .
My recommendation would be to delay the factorization until it is needed for presentation or conversion to other formats. Viewing/interpreting the values through a debugger would be a next to impossible, and you should implement an exception handler removing common factors if an operation would cause the numerator/denominator to overflow the integer format - maybe doing a 2-factor removal as a fast first try, and going on to a series of division operations only if the values are still above a certain overflow risk threshold. If you use 64 bit ints, you might in the worst case have to do divisions for every prime less than 4G; that is quite some operation...
|
|
|
|
|
Not all that hard.
In my MS Computer Science course, All students had to write a compiler for a language that had only integers and rational numbers.
This was before the days of C++, classes, etc.
Not too much of a sweat.
|
|
|
|
|
How did you handle the (least common) denominator problem? Or did you simply ignore it, ignoring the risk that it might overflow? How did you present the results - as if they were floats (through a "(float)numerator/(float)denominator" division) using float output format? Did you do anything to detect common denominators, or did 1/3 + 1/3 end up as 6/9?
|
|
|
|
|
I don't remember the details because it was such a long time ago but I distinctly remember that I did determine the least common denominator for fractions and ensured that all rational numbers were stored in their reduced form.
|
|
|
|
|
While a rational number type has both advantages and disadvantages and we can discuss that forever, it is an undeniable truth that JSON should have had a rational number type, simply because some languages or libraries already support rational numbers and there should exist a portable way for applications using rational numbers to serialize and exchange these with other applications. A rational number that is within the range of a floating point number type can always be "downgraded" to that floating point number type, but the converse is problematic. For example, is the JSON expression [0.67,0.33] an array with the numbers 2/3 and 1/3 ? If not, then what about [0.6666666666666667,0.3333333333333333]? It would have been clear what was meant if we were allowed to write [2/3,1/3].
|
|
|
|
|
Retired.
Again.
Third time.
Got to go out back and make sure the grass is growing.
Going to sit in a rocking chair. Maybe after a few months, I will rock.
Lou 
If you can keep your head while those about you are losing theirs, perhaps you don't understand the situation.
|
|
|
|
|
So you have a new full time job as Lawn Inspector? Good for you! 
Sent from my Amstrad PC 1640
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
AntiTwitter: @DalekDave is now a follower!
|
|
|
|
|
The problem with retirement is that you never get a day off. 
I'm pretty sure I would not like to live in a world in which I would never be offended.
I am absolutely certain I don't want to live in a world in which you would never be offended.
Freedom doesn't mean the absence of things you don't like.
Dave
|
|
|
|
|
That is why I went back to work the other 2 times.
If you can keep your head while those about you are losing theirs, perhaps you don't understand the situation.
|
|
|
|
|
theoldfool wrote: Retired. ...
Going to sit in a rocking chair.
Congratulations, rock star!
|
|
|
|
|
Watching the grass grow is tedious work!
Everyone has a photographic memory; some just don't have film. Steven Wright
|
|
|
|
|
Mike Hankey wrote: Watching the grass grow
No World Cup posts! 
I'm pretty sure I would not like to live in a world in which I would never be offended.
I am absolutely certain I don't want to live in a world in which you would never be offended.
Freedom doesn't mean the absence of things you don't like.
Dave
|
|
|
|
|
|
Oh sh*t, I'm looking at my first attempt, I have been threatening to retire for the last 3 years but this time I actually changed my contract to working 10 days a month - from home! By the great Ghu I hope it works. I do NOT want to come back to working full time.
Never underestimate the power of human stupidity
RAH
|
|
|
|
|
I would have done it sooner but I never got around to commenting my codez.
If you can keep your head while those about you are losing theirs, perhaps you don't understand the situation.
|
|
|
|
|
Hi All,
The useful part of the day has come and gone, I'm too tired to do any more today. I have done what I intended to do and a bit more today (and oh joy the air con has died!) so I am now trying to look busy.
Looking to find secondary sources for components. This allows me to have Altium open and a circuit diagram in front of me with a browser open on a suppliers web page. 
|
|
|
|
|
Sorry, it's late - was doing the computers.
Individualist or covert conformist? (10, 2 words)
Whenever you find yourself on the side of the majority, it is time to pause and reflect. - Mark Twain
modified 9-Jul-18 7:04am.
|
|
|
|
|
BLACK SHEEP.
Sent from my Amstrad PC 1640
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
AntiTwitter: @DalekDave is now a follower!
|
|
|
|
General 
News 
Suggestion 
Question 
Bug 
Answer 
Joke 
Praise 
Rant 
Admin 
Submit an article or tip
