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Searching vs. Computation

  1. Aug 21, 2013 #1
    This is likely going to be a stupid question given that I am not in com sci, have very little com sci knowledge with regards to information storage/algorithms.

    I was wondering, if this question has any meaning, what are the costs of scanning for something vs carrying out some integer operations such as adding, multiplying, etc. (time/algorithmic efficiency/machine cost/whatever related things apply)?

    I know this is very contingent on exactly what I'm trying to do, so I'm going to try to lay out the situation. I'm just doing some sparse matrix problems. But there are going to be lots of zeroes and the sparsity doesn't really have any general pattern to it. I know Gaussian Elimination is generally considered to be in the range of (2/3)n^3, don't know what this is in relation to computational time which I would expect to be dependent on the system specifications of the computer. So what would the cost be, in relation to computer taxation, of searching for rows with two non-zero elements as opposed to going through matrix calculations?

    I'm guessing that is very vague or possibly not answerable because there might be no equivalence between the two operations. Lemme try a simpler case, what are the machine costs of doing some integer operations as opposed to doing comparisons or searching for something?
     
  2. jcsd
  3. Aug 21, 2013 #2

    jim mcnamara

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    Searching is expensive. Jumping to a given memory location often invalidates the cpu cache, slowing the whole process down. And. At each memory location at least one compare is required. A binary search of an ordered array has this problem, for example. The cpu is orders of magnitude faster than memory access, so if it can work on data without having to do time-consuming memory operations, efficiency goes way up. This is called locality. So it depends.

    Plus.

    I'm not sure this is what you mean by sparse in your cont3ext, but in the CS world a sparse array or matrix is often represented by a small data structure which is accessed via a hash table. Not all possible slots for every possible value actually exist. Each "slot" is created on demand. In a problem with a set 1000 items, the potential domain of those items could be from zero to a large number of possible slots. Computers often do not have enough memory to allocate in order to create empty slots for all the "unused" slots for non-existent items. The hash directs the memory search to just the relevant slots. This can be time-conserving as well as resource conserving.

    Is this what you are asking, or are you after time complexity for an algorithm? I'm sure this is not answering you in a way you need.
     
  4. Aug 21, 2013 #3

    AlephZero

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    In the context of Gaussian Elimination, there is an important issue here: the elimination process may introduces more non-zero terms into the matrix.

    There are many different "standard" algorithms for solving sparse matrix equations. Which is "best" depends on the "pattern" of sparseness in the equations. They vary from straightforward (e.g. for a banded matrix where all the non-zero elements are close to the diagonal) to very general methods which first do a symbolic (non-numerical) decomposition of the matrix to find a good data structure with a small amount of "fill-in" of the original zero terms.

    There are also iterative methods ....

    I agree, the OP's question is too vague to give a good answer.
     
  5. Aug 21, 2013 #4

    jim mcnamara

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    Staff: Mentor

    Better idea. Instead of having us floundering around badly - please tell us what you want to do? Maybe the answer is to solve a lot of matrix problems very efficiently.

    The right algorithm usually solves 90% of the speed of computation problems.
     
  6. Sep 2, 2013 #5

    harborsparrow

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    I don't know if this will help, but sparse matrices can be stored as linked lists (basically, coordinates + value) so that any non-zero values in the row or column can be found pretty quickly. However, the matrix has to be preprocessed to convert it into the easily-usable form.
     
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