Uniform sampling

This is a discussion on Uniform sampling within the C Programming forums, part of the General Programming Boards category; Hi My goal is to build a C function that perform a uniform sampling. I mean that I have a ...

  1. #1
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    Uniform sampling

    Hi

    My goal is to build a C function that perform a uniform sampling. I mean that I have a big file with a lot of data and I'd like to take just some data samples accordly the uniform distribution.

    Es.
    file1: a b c d e f g h i l m n o p q

    An example would be to flip a coin for each letter and select all letters
    for which the coin showed the head.

    I have used the srand() system call, but it is not uniform.
    Can anyone suggest a way to start?
    Thanks

    D

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    rand() (not srand()) is uniform, but not necessarily on the range you want it to be on.

    But, in terms of drawing from a file, you need to choose every line, not necessarily 50-50, but in the ratio (number needed to still be drawn)/(number still in the file).

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    hi

    I'm reading about rand, so with:

    a=rand();

    I get a number uniformly distribuited between 0 and RAND_MAX

    Thanks for reply

    D

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    Don't forget to seed rand() using srand() or something similar. Time is always a good bet for a seed. If you want to flip a coin, do this:

    Code:
    a = (int) (rand() / RAND_MAX + 0.5)
    This will either give a 0 or a 1, with equal probability.

  5. #5
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by Dedalus View Post
    An example would be to flip a coin for each letter and select all letters for which the coin showed the head.
    To be clear, that is not uniform sampling. That is Poisson sampling.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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    This will either give a 0 or a 1, with equal probability.
    No, that will give zero with 100% probability.

    You need to cast one of the division operands to get the result you are looking for:
    Code:
    a = (int) ((double)rand() / RAND_MAX + 0.5)

  7. #7
    Registered User linuxdude's Avatar
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    Have you checked the FAQ. Prelude wrote a well written article on it here.

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