| Re: Generating N random numbers that add to a TOTAL [message #89234 is a reply to message #89228] |
Thu, 07 August 2014 07:18   |
Russell Ryan
Messages: 122
Registered: May 2012
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Senior Member |
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You might have better luck with:
n_rand_var = 1000
requested_total=1000.
rand=randomu(seed,n_rand_var,/double)
rand*=(requested_total/total(rand))
This looks to be uniformly distributed, however it's not clear over what range it's uniform because the total(rand) in the denominator isn't necessarily the same. In the limit of n_rand_var -> infinity, then I think the total will converge to n_rand_var/2 and so the range will be
2*requested_total/n_rand_var
but for n_rand_var != infinity then it's a bit vague.
R
On Wednesday, August 6, 2014 11:52:47 PM UTC-4, Gianguido Cianci wrote:
> Hi all,
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> I am wondering if anybody has suggestions on how to improve the function below. It seems ok for floating precision numbers.
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> For integers it's a different story:
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> It works great if N<<TOTAL. When N approaches TOTAL I get a few numbers and then a bunch of zeros... Also, setting /DIFFERENT makes it run for ever if N is large. Also, the sum of res adds up TOTAL=/-1, not always to TOTAL exactly...
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> Suggestions?
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> Thanks,
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> Gianguido
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> FUNCTION nrndaddto, n, total, integers = integers, different = different
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> compile_opt idl2
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> res = dblarr(n)
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> res[0] = randomu(seed, 1, /double)*(total)
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> FOR i = 1, n-2 DO BEGIN
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> res[i] = randomu(seed, 1, /double)*(total-total(res[0:i-1], /double))
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> ENDFOR
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> res[n-1] = total-total(res[0:n-2], /double)
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> IF ~keyword_set(integers) THEN integers = 0
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> IF keyword_set(integers) THEN res = round(res)
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> IF keyword_set(different) THEN BEGIN
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> IF n_elements(res) NE n_elements(unique(res, /sort)) THEN res = $
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> nrndaddto(n, total, integers = integers, different = 1)
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> ENDIF
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> RETURN, res
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> END
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