{"id":25119,"date":"2022-12-08T00:37:21","date_gmt":"2022-12-07T19:07:21","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-random-in-java-often-generating-same-value\/"},"modified":"2022-12-08T00:37:21","modified_gmt":"2022-12-07T19:07:21","slug":"solved-random-in-java-often-generating-same-value","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-random-in-java-often-generating-same-value\/","title":{"rendered":"[Solved] Random in Java often generating same value"},"content":{"rendered":"

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The Birthday Paradox<\/a> predicts that the probability of duplicates in random numbers is much higher than you’d think. For example, it predicts that, with a mere 23 random people, there’s a greater than 50% chance of two of them have the same birthday. By the Pigeonhole Principle<\/a>, it takes 367 people for there to be a 100% chance of a duplicate, but the probability of a duplicate is extremely high even before that.<\/p>\n

Here’s the probability distribution (from Wikipedia):
\n\"enter<\/a><\/p>\n

The rule of thumb<\/a> to approximate the number of numbers you have to generate before the probability of duplicates reaches sqrt(2m * p(n))<\/code>, where m<\/code> is the number of possible random numbers and p(n)<\/code> is the probability that you’re looking for. So, for example, if you’re generating random numbers in a 50-number range (e.g. if you pick a random number from 100 – 150), you’d only have to generate approximately sqrt((2 * 50) * 0.5) = 7.07<\/code> random numbers before the odds are just as good as not that you have a duplicate. If you generate 8 random numbers within a 50-number range, odds are better than not that’ll have a duplicate. (Note that this only works for p(n)<\/code> values of up to 1\/2).<\/p>\n

In your case, there are 8 possible values for any particular random value (5, 6, 7, 8, 9, 10, 11, 12), so you only need to generate sqrt(8) = 2.83<\/code> numbers before there’s a probability of 50% that you’ll have a duplicate. In other words, the Birthday Paradox predicts you only need to generate approximately 3 numbers for the chances to be better than not that you’ll have a duplicate.<\/p>\n

See also this Q&A<\/a>.<\/p>\n

One more point: beware the Gambler’s Fallacy<\/a>, in which people assume that if you randomly generate, for example, a 10, odds are the next one won’t be a 10. Actually, given that you’re generating random numbers, the odds of any particular number is 1\/8 regardless of what numbers came before. In other words, if you generate a 12, the probability of the next number being a 10 is 1\/8. If you generate a 7, the odds of the next number being a 10 is 1\/8. If you generate a 10, the probability of the next number being a 10 is still 1\/8. Each number is an independent event<\/em> (i.e. the numbers you’ve generated so far don’t influence the probability distribution of future numbers in the least).<\/p>\n

TL;DR<\/strong> You need to generate much fewer numbers than you think you do before you’re likely to start getting duplicates – if you’re generating random numbers within a small range of numbers in particular (like you are) the number is particularly low.<\/p>\n<\/p><\/div>\n

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solved Random in Java often generating same value <\/p>\n","protected":false},"excerpt":{"rendered":"

[ad_1] The Birthday Paradox predicts that the probability of duplicates in random numbers is much higher than you’d think. For example, it predicts that, with a mere 23 random people, there’s a greater than 50% chance of two of them have the same birthday. By the Pigeonhole Principle, it takes 367 people for there to … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[452,323,1014],"class_list":["post-25119","post","type-post","status-publish","format-standard","hentry","category-solved","tag-android","tag-java","tag-random"],"yoast_head":"\n[Solved] Random in Java often generating same value - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-random-in-java-often-generating-same-value\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] Random in Java often generating same value - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] The Birthday Paradox predicts that the probability of duplicates in random numbers is much higher than you’d think. 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