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While solving competitive programming questions, sometimes it is asked to compute the final answer as
" Since this number may be large, compute it modulo 1,000,000,007 (10^9+7) ".
Also, it is the fact that in python3 plain int type is unbounded.
So, it is necessary to compute modulo 10^9+7 if I am solving my programming question in python 3 ?
It doesn't matter what anyone here believes is fair or warranted. What matters is what the automated system or human in charge expects as an answer.
Of course, since doing modular operations can slow down a possible solution within a given time frame, it would be best to find an optimum alternative, if permitted.
Are there any software tools for performing arithmetic on very large numbers in parallel? What I mean by parallel is that I want to use all available cores on my computer for this.
The constraints are wide open for me. I don't mind trying any language or tech.
Please and thanks.
It seems like you are either dividing really huge numbers, or are using a suboptimal algorithm. Parallelizing things to a fixed number of cores will only tweak the constants, but have no effect on the asymptotic behavior of your operation. And if you're talking about hours for a single division, asymptotic behavior is what matters most. So I suggest you first make sure sure your asymptotic complexity is as good as can be, and then start looking for ways to improve the constants, perhaps by parallelizing.
Wikipedia suggests Barrett division, and GMP has a variant of that. I'm not sure whether what you've tried so far is on a similar level, but unless you are sure that it is, I'd give GMP a try.
See also Parallel Modular Multiplication on Multi-core Processors for recent research. Haven't read into that myself, though.
The only effort I am aware of is a CUDA library called CUMP. However, the library only provides support for addition, subtraction and multiplication. Anyway, you can use multiplication to perform the division on the GPU and check if the quality of the result is enough for your particular problem.
To be clear I don't mean, provided the last two numbers in the sequence provide the next one:
(2, 3, -> 5)
But rather given any index provide the Fibonacci number:
(0 -> 1) or (7 -> 21) or (11 -> 144)
Adding two numbers is a very simple task for any machine learning structure, and by extension counting by ones, twos or any fixed number is a simple addition rule. Recursive calculations however...
To my understanding, most learning networks rely on forwards only evaluation, whereas most programming languages have loops, jumps, or circular flow patterns (all of which are usually ASM jumps of some kind), thus allowing recursion.
Sure some networks aren't forwards only; But can processing weights using the hyperbolic tangent or sigmoid function enter any computationally complete state?
i.e. conditional statements, conditional jumps, forced jumps, simple loops, complex loops with multiple conditions, providing sort order, actual reordering of elements, assignments, allocating extra registers, etc?
It would seem that even a non-forwards only network would only find a polynomial of best fit, reducing errors across the expanse of the training set and no further.
Am I missing something obvious, or did most of Machine Learning just look at recursion and pretend like those problems don't exist?
Update
Technically any programming language can be considered the DNA of a genetic algorithm, where the compiler (and possibly console out measurement) would be the fitness function.
The issue is that programming (so far) cannot be expressed in a hill climbing way - literally, the fitness is 0, until the fitness is 1. Things don't half work in programming, and if they do, there is no way of measuring how 'working' a program is for unknown situations. Even an off by one error could appear to be a totally different and chaotic system with no output. This is exactly the reason learning to code in the first place is so difficult, the learning curve is almost vertical.
Some might argue that you just need to provide stronger foundation rules for the system to exploit - but that just leads to attempting to generalize all programming problems, which circles right back to designing a programming language and loses all notion of some learning machine at all. Following this road brings you to a close variant of LISP with mutate-able code and virtually meaningless fitness functions that brute force the 'nice' and 'simple' looking code-space in attempt to follow human coding best practices.
Others might argue that we simply aren't using enough population or momentum to gain footing on the error surface, or make a meaningful step towards a solution. But as your population approaches the number of DNA permutations, you are really just brute forcing (and very inefficiently at that). Brute forcing code permutations is nothing new, and definitely not machine learning - it's actually quite common in regex golf, I think there's even an xkcd about it...
The real problem isn't finding a solution that works for some specific recursive function, but finding a solution space that can encompass the recursive domain in some useful way.
So other than Neural Networks trained using Backpropagation hypothetically finding the closed form of a recursive function (if a closed form even exists, and they don't in most real cases where recursion is useful), or a non-forwards only network acting like a pseudo-programming language with awful fitness prospects in the best case scenario, plus the virtually impossible task of tuning exit constraints to prevent infinite recursion... That's really it so far for machine learning and recursion?
According to Kolmogorov et al's On the representation of continuous functions of many variables by superposition of continuous functions of one variable and addition, a three layer neural network can model arbitrary function with the linear and logistic functions, including f(n) = ((1+sqrt(5))^n - (1-sqrt(5))^n) / (2^n * sqrt(5)), which is the close form solution of Fibonacci sequence.
If you would like to treat the problem as a recursive sequence without a closed-form solution, I would view it as a special sliding window approach (I called it special because your window size seems fixed as 2). There are more general studies on the proper window size for your interest. See these two posts:
Time Series Prediction via Neural Networks
Proper way of using recurrent neural network for time series analysis
Ok, where to start...
Firstly, you talk about 'machine learning' and 'perfectly emulate'. This is not generally the purpose of machine learning algorithms. They make informed guesses given some evidence and some general notions about structures that exist in the world. That typically means an approximate answer is better than an 'exact' one that is wrong. So, no, most existing machine learning approaches aren't the right tools to answer your question.
Second, you talk of 'recursive structures' as some sort of magic bullet. Yet they are merely convenient ways to represent functions, somewhat analogous to higher order differential equations. Because of the feedbacks they tend to introduce, the functions tend to be non-linear. Some machine learning approaches will have trouble with this, but many (neural networks for example) should be able to approximate you function quite well, given sufficient evidence.
As an aside, having or not having closed form solutions is somewhat irrelevant here. What matters is how well the function at hand fits with the assumptions embodied in the machine learning algorithm. That relationship may be complex (eg: try approximating fibbonacci with a support vector machine), but that's the essence.
Now, if you want a machine learning algorithm tailored to the search for exact representations of recursive structures, you could set up some assumptions and have your algorithm produce the most likely 'exact' recursive structure that fits your data. There are probably real world problems in which such a thing would be useful. Indeed the field of optimisation approaches similar problems.
The genetic algorithms mentioned in other answers could be an example of this, especially if you provided a 'genome' that matches the sort of recursive function you think you may be dealing with. Closed form primitives could form part of that space too, if you believe they are more likely to be 'exact' than more complex genetically generated algorithms.
Regarding your assertion that programming cannot be expressed in a hill climbing way, that doesn't prevent a learning algorithm from scoring possible solutions by how many much of your evidence it's able to reproduce and how complex they are. In many cases (most? though counting cases here isn't really possible) such an approach will find a correct answer. Sure, you can come up with pathological cases, but with those, there's little hope anyway.
Summing up, machine learning algorithms are not usually designed to tackle finding 'exact' solutions, so aren't the right tools as they stand. But, by embedding some prior assumptions that exact solutions are best, and perhaps the sort of exact solution you're after, you'll probably do pretty well with genetic algorithms, and likely also with algorithms like support vector machines.
I think you also sum things up nicely with this:
The real problem isn't finding a solution that works for some specific recursive function, but finding a solution space that can encompass the recursive domain in some useful way.
The other answers go a long way to telling you where the state of the art is. If you want more, a bright new research path lies ahead!
See this article:
Turing Machines are Recurrent Neural Networks
http://lipas.uwasa.fi/stes/step96/step96/hyotyniemi1/
The paper describes how a recurrent neural network can simulate a register machine, which is known to be a universal computational model equivalent to a Turing machine. The result is "academic" in the sense that the neurons have to be capable of computing with unbounded numbers. This works mathematically, but would have problems pragmatically.
Because the Fibonacci function is just one of many computable functions (in fact, it is primitive recursive), it could be computed by such a network.
Genetic algorithms should do be able to do the trick. The important this is (as always with GAs) the representation.
If you define the search space to be syntax trees representing arithmetic formulas and provide enough training data (as you would with any machine learning algorithm), it probably will converge to the closed-form solution for the Fibonacci numbers, which is:
Fib(n) = ( (1+srqt(5))^n - (1-sqrt(5))^n ) / ( 2^n * sqrt(5) )
[Source]
If you were asking for a machine learning algorithm to come up with the recursive formula to the Fibonacci numbers, then this should also be possible using the same method, but with individuals being syntax trees of a small program representing a function.
Of course, you also have to define good cross-over and mutation operators as well as a good evaluation function. And I have no idea how well it would converge, but it should at some point.
Edit: I'd also like to point out that in certain cases there is always a closed-form solution to a recursive function:
Like every sequence defined by a linear recurrence with constant coefficients, the Fibonacci numbers have a closed-form solution.
The Fibonacci sequence, where a specific index of the sequence must be returned, is often used as a benchmark problem in Genetic Programming research. In most cases recursive structures are generated, although my own research focused on imperative programs so used an iterative approach.
There's a brief review of other GP research that uses the Fibonacci problem in Section 3.4.2 of my PhD thesis, available here: http://kar.kent.ac.uk/34799/. The rest of the thesis also describes my own approach, which is covered a bit more succinctly in this paper: http://www.cs.kent.ac.uk/pubs/2012/3202/
Other notable research which used the Fibonacci problem is Simon Harding's work with Self-Modifying Cartesian GP (http://www.cartesiangp.co.uk/papers/eurogp2009-harding.pdf).
(Is this the right site for this question?)
I've recently been looking into circuits, specifically the one's used to preform mathematical functions such as adding, subtracting, multiplication, and dividing. I got a book from the library about circuits in general and mainly read the part about math, but they didn't seem to have any part about division. I fully understood all the logic gates and their uses in addition, subtraction, and multiplication, but the book had nothing on division. Google proved to be not much of a help either. So my questions are
A) Do processors do division? Or is it done later on, like in the machine code or higher level programming language?
If the answer to the beginning of that is yes, then i would like to know
B) How do they preform division? What binary method of division do they use? And what arrangement of logic gates does it use (a diagram of the gates preferably)?
A) Yes, in many cases (x86 is one example). In other cases, there may be opcodes that perform parts of the division operation. In yet other cases, the whole thing may have to be emulated in software.
B) A variety of techniques. This book has a whole chapter on division techniques: Finite Precision Number Systems and Arithmetic.
Binary restoring division is perhaps the easiest to understand, it's equivalent to the long division that you would have done in school. Binary non-restoring division is the same thing but rearranged, which results in needing fewer operations. SRT division takes it even further. You can then get into non-binary division (i.e. based on higher radices).
On top of the basic division algorithm, you'll also need to handle negative numbers, special cases, and floating-point (if you're into that sort of thing). Again, lots of techniques exist.
Each method has trade-offs; I doubt it's common knowledge which particular variant e.g. Intel uses.
I was glancing through the contents of Concrete Maths online. I had at least heard most of the functions and tricks mentioned but there is a whole section on Special Numbers. These numbers include Stirling Numbers, Eulerian Numbers, Harmonic Numbers so on. Now I have never encountered any of these weird numbers. How do they aid in computational problems? Where are they generally used?
Harmonic Numbers appear almost everywhere! Musical Harmonies, analysis of Quicksort...
Stirling Numbers (first and second kind) arise in a variety of combinatorics and partitioning problems.
Eulerian Numbers also occur several places, most notably in permutations and coefficients of polylogarithm functions.
A lot of the numbers you mentioned are used in the analysis of algorithms. You may not have these numbers in your code, but you'll need them if you want to estimate how long it will take for your code to run. You might see them in your code too. Some of these numbers are related to combinatorics, counting how many ways something can happen.
Sometimes it's not enough to know how many possibilities there are because you need to enumerate over the possibilities. Volume 4 of Knuth's TAOCP, in progress, gives the algorithms you need.
Here's an example of using Fibonacci numbers as part of a numerical integration problem.
Harmonic numbers are a discrete analog of logarithms and so they come up in difference equations just like logs come up in differential equations. Here's an example of physical applications of harmonic means, related to harmonic numbers. See the book Gamma for many examples of harmonic numbers in action, especially the chapter "It's a harmonic world."
These special numbers can help out in computational problems in many ways. For example:
You want to find out when your program to compute the GCD of 2 numbers is going to take the longest amount of time: Try 2 consecutive Fibonacci Numbers.
You want to have a rough estimate of the factorial of a large number, but your factorial program is taking too long: Use Stirling's Approximation.
You're testing for prime numbers, but for some numbers you always get the wrong answer: It could be you're using Fermat's Prime test, in which case the Carmicheal numbers are your culprits.
The most common general case I can think of is in looping. Most of the time you specify a loop using a (start;stop;step) type of syntax, in which case it may be possible to reduce the execution time by using properties of the numbers involved.
For example, summing up all the numbers from 1 to n when n is large in a loop is definitely slower than using the identity sum = n*(n + 1)/2.
There are a large number of examples like these. Many of them are in cryptography, where the security of information systems sometimes depends on tricks like these. They can also help you with performance issues, memory issues, because when you know the formula, you may find a faster/more efficient way to compute other things -- things that you actually care about.
For more information, check out wikipedia, or simply try out Project Euler. You'll start finding patterns pretty fast.
Most of these numbers count certain kinds of discrete structures (for instance, Stirling Numbers count Subsets and Cycles). Such structures, and hence these sequences, implicitly arise in the analysis of algorithms.
There is an extensive list at OEIS that lists almost all sequences that appear in Concrete Math. A short summary from that list:
Golomb's Sequence
Binomial Coefficients
Rencontres Numbers
Stirling Numbers
Eulerian Numbers
Hyperfactorials
Genocchi Numbers
You can browse the OEIS pages for the respective sequences to get detailed information about the "properties" of these sequences (though not exactly applications, if that's what you're most interested in).
Also, if you want to see real-life uses of these sequences in analysis of algorithms, flip through the index of Knuth's Art of Computer Programming, and you'll find many references to "applications" of these sequences. John D. Cook already mentioned applications of Fibonacci & Harmonic numbers; here are some more examples:
Stirling Cycle Numbers arise in the analysis of the standard algorithm that finds the maximum element of an array (TAOCP Sec. 1.2.10): How many times must the current maximum value be updated when finding the maximum value? It turns out that the probability that the maximum will need to be updated k times when finding a maximum in an array of n elements is p[n][k] = StirlingCycle[n, k+1]/n!. From this, we can derive that on the average, approximately Log(n) updates will be necessary.
Genocchi Numbers arise in connection with counting the number of BDDs that are "thin" (TAOCP 7.1.4 Exercise 174).
Not necessarily a magic number from the reference you mentioned, but nonetheless --
0x5f3759df
-- the notorious magic number used to calculate inverse square root of a number by giving a good first estimate to Newton's Approximation of Roots, often attributed to the work of John Carmack - more info here.
Not programming related, huh? :)
Is this directly programming related? Surely related, but I don't know how closely.
Special numbers, such as e, pi, etc., come up all over the place. I don't think that anyone would argue about these two. The Golden_ratio also appears with amazing frequency, in everything from art to other special numbers themselves (look at the ratio between successive Fibonacci numbers.)
Various sequences and families of numbers also appear in many places in mathematics and therefore, in programming too. A beautiful place to look is the Encyclopedia of integer sequences.
I'll suggest this is an experience thing. For example, when I took linear algebra, many, many years ago, I learned about the eigenvalues and eigenvectors of a matrix. I'll admit that I did not at all appreciate the significance of eigenvalues/eigenvectors until I saw them in use in a variety of places. In statistics, in terms of what they tell you about uncertainty of an estimate from a covariance matrix, the size and shape of a confidence ellipse, in terms of principal component analysis, or the long term state of a Markov process. In numerical methods, where they tell you about convergence of a method, be it in optimization or an ODE solver. In mechanical engineering, where you see them as principal stresses and strains.
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