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Webmaster Forum / HTML, CSS, Scripts / JavaScript / August 2005even random number, javascript
How would I generate a even random number each time? I can generate a random number each time but not an even one? Here is the code I use for the random number from 1-100. <script type="text/javascript"> no=math.random()*100 document.write (math.round(no)) </script> Thanks, Deb Debbie Lucier > How would I generate a even random number each time? I can generate a > random number each time but not an even one? Here is the code I use for > the random number from 1-100. Er, generate a random one from 1-50 and multiply it by 2. Cheers Richard. > How would I generate a even random number each time? I can generate a > random number each time but not an even one? Here is the code I use for [quoted text clipped - 4 lines] > document.write (math.round(no)) > </script> rf's answer should do the trick, but what about zero? Should it be included, or is your range 2-100?  SignatureRob > How would I generate a even random number each time? I can generate a > random number each time but not an even one? Here is the code I use for [quoted text clipped - 9 lines] > > *** Sent via Developersdex http://www.developersdex.com *** Math.round() has a bias toward the higher numbers (4.5 goes to 5 always and never to 4), so the starting number has a lower chance of occurring that all the others. A better distribution is from truncating the decimal part of numbers in a range between min and max plus one : <script type="text/javascript"> function genRandom( min, max ) { return min + (max-min+1)*Math.random() | 0; } document.write( genRandom( 0, 50 )*2 ); </script> SignatureZif Zif wrote on 30 aug 2005 in comp.lang.javascript: > Math.round() has a bias toward the higher numbers (4.5 goes to 5 > always and never to 4), Unimportant bias, because the chance of a pseudo random number being exactly 4.5 should be very very small. Other deviances between real random and pseudo random will have a much higher effect, though also unimportant in most script applications. > so the starting number has a lower chance of > occurring that all the others. what do you mean by "starting number"? SignatureEvertjan. The Netherlands. (Replace all crosses with dots in my emailaddress) >> no=math.random()*100 >> document.write (math.round(no)) > Math.round() has a bias toward the higher numbers (4.5 goes to 5 > always and never to 4), so the starting number has a lower chance of > occurring that all the others. That's not the problem with using Math.round and Math.random together. Since the range [3.5-4.5[ has the same length as [2.5-3.5[, the chance of hitting either is close to the same. A more serious problem is that Math.round(Math.random()*100) has 101 different possible outcomes, with 0 and 100 having only half the chance of the others. /L SignatureLasse Reichstein Nielsen - lrn@hotpop.com DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html> 'Faith without judgement merely degrades the spirit divine.' Lasse Reichstein Nielsen wrote on 30 aug 2005 in comp.lang.javascript: > A more serious problem is that Math.round(Math.random()*100) has 101 > different possible outcomes, with 0 and 100 having only half the > chance of the others. You are so right, Lasse. Let's try: Math.floor(Math.random()*100) SignatureEvertjan. The Netherlands. (Replace all crosses with dots in my emailaddress) >> A more serious problem is that Math.round(Math.random()*100) >> has 101 different possible outcomes, with 0 and 100 having [quoted text clipped - 5 lines] > > Math.floor(Math.random()*100) While we are at it, this seems like a good opportunity to use the truncating side-effect of bitwise operations as the OP wants only even numbers. Multiplying by two produces even numbers, but so does shifting left by one bit:- ((Math.random() * 51) << 1) // even integers 0 to 100 (assuming // a correct random implementation // with results 0 to <1) Richard. Richard Cornford wrote on 31 aug 2005 in comp.lang.javascript: > While we are at it, this seems like a good opportunity to use the > truncating side-effect of bitwise operations as the OP wants only even [quoted text clipped - 4 lines] > // a correct random implementation > // with results 0 to <1) x = 1.234 alert(x) // 1.234 x = x << 1 alert(x) // 2 Why is the truncation as it is? x = 0.234 alert(x) // 0.234 x = x << 1 alert(x) // 0 !!!! Wrong, in the sense of your application! btw: x = -1.234 alert(x) // -1.234 x = x << 1 alert(x) // -2 SignatureEvertjan. The Netherlands. (Replace all crosses with dots in my emailaddress) Evertjan. wrote on 31 aug 2005 in comp.lang.javascript: > x = 0.234 > alert(x) // 0.234 > x = x << 1 > alert(x) // 0 !!!! > > Wrong, in the sense of your application! I seem not to be awake yet. It is correct as it should be! SignatureEvertjan. The Netherlands. (Replace all crosses with dots in my emailaddress) >> While we are at it, this seems like a good opportunity to use >> the truncating side-effect of bitwise operations as the OP wants [quoted text clipped - 11 lines] > > Why is the truncation as it is? The floating point number is internally converted into a signed 32 bit integer prior to the actual shift (this happens with all bitwise operations except - >>> -, which uses an unsigned 32 bit integer). It is this conversion that has the truncating (towards zero) side-effect, so the operand for the actual shift is the number one. Richard. Richard Cornford wrote on 31 aug 2005 in comp.lang.javascript: >> x = 1.234 >> alert(x) // 1.234 [quoted text clipped - 8 lines] > this conversion that has the truncating (towards zero) side-effect, so > the operand for the actual shift is the number one. So this is a legal Math.floor equivalent for positive numbers? x = x << 0 ========= different when negative: x = -71.234 x = x << 0 alert(x) // -71 x = -71.234 x = Math.floor(x) alert(x) // -72 SignatureEvertjan. The Netherlands. (Replace all crosses with dots in my emailaddress) >>> x = 1.234 >>> alert(x) // 1.234 [quoted text clipped - 10 lines] > > So this is a legal Math.floor equivalent for positive numbers? If the positive number can be accommodated in a 32 bit signed integer. > x = x << 0 or:- x = x | 0; - or any other bitwise operation that will not alter its operand. Though OR zero and shift zero have proved the (approximately equal) fastest of the possibilities (at between 4 and 14 times faster than Math.floor). > ========= > [quoted text clipped - 6 lines] > x = Math.floor(x) > alert(x) // -72 Yes, the truncating is always towards zero with bitwise operators. But they are still a good option when you want to quickly acquire integer values that will be non-negative and restricted in range, such as for pixel positioning in animation. Richard. JRS: In article <df41ds$p1m$1$830fa7a5@news.demon.co.uk>, dated Wed, 31 Aug 2005 11:39:54, seen in news:comp.lang.javascript, Richard Cornford <Richard@litotes.demon.co.uk> posted : >The floating point number is internally converted into a signed 32 bit >integer prior to the actual shift (this happens with all bitwise >operations except - >>> -, which uses an unsigned 32 bit integer). It is >this conversion that has the truncating (towards zero) side-effect, so >the operand for the actual shift is the number one. The result of X << Y (etc.) where Y is not a small non-negative integer look interesting, but one should check that actual results and ECMA agree (for which it is too late tonight) before contemplating actual use. Signature© John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v4.00 IE 4 © <URL:http://www.jibbering.com/faq/> JL/RC: FAQ of news:comp.lang.javascript <URL:http://www.merlyn.demon.co.uk/js-index.htm> jscr maths, dates, sources. <URL:http://www.merlyn.demon.co.uk/> TP/BP/Delphi/jscr/&c, FAQ items, links. > How would I generate a even random number each time? I can generate a > random number each time but not an even one? Here is the code I use for [quoted text clipped - 4 lines] > document.write (math.round(no)) > </script> n = (Math.random()*98 +2) & 0xFE ; // 2 - 98 inclusive n = (Math.random()*100 +2) & 0xFE ; // 2 - 100 inclusive n = (Math.random()*100) & 0xFE ; // 0 - 98 inclusive n = (Math.random()*101) & 0xFE ; // 0 - 100 inclusive -- S.C. |
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