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

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).

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

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:

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a = (int) (rand() / RAND_MAX + 0.5)

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This will either give a 0 or a 1, with equal probability.

Quote:

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Originally Posted by Dedalus

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

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To be clear, that is not uniform sampling. That is Poisson sampling.

Quote:

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This will either give a 0 or a 1, with equal probability.

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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:

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a = (int) ((double)rand() / RAND_MAX + 0.5)

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Have you checked the FAQ. Prelude wrote a well written article on it here.