BlueBlade's blog

Be careful with log2() !!!!
By BlueBlade, history, 7 years ago, In English

What is the ans of floor of log2(288230376151711743) ??? Run a code with builtin log2() function, ans is 58 !!! Even google gives the ans 58 !!! Now write a code yourself to find the log2(x), and put x = 288230376151711743 . The actual answer is 57 !!!

couldn't get it !!!

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7 years ago, # |
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It uses double precision and it is 57.99999999999999999499464608778919110696411496790292147213... Check: [WolframAlpha](https://www.wolframalpha.com/input/?i=log2(288230376151711743))

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    7 years ago, # ^ |
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    thank you

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      7 years ago, # ^ |
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      but why my code is giving me ans 58?

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        7 years ago, # ^ |
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        double can only handle about 15-17 digits of precision. So C++ rounds the number to 58.00000000000000.

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7 years ago, # |
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try to output log2(x) + 1e-9

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