Fearless_cod's blog

Peculiar behaviour of log2() function in C++.
By Fearless_cod, history, 2 years ago, In English

Hi reader. Coming straight to the point. The value of (1ll<<59) or 2^59 is 576460752303423488(say x).So if you do log2(x),the answer should come out to be 59.That's indeed the case. But here is the catch... .If you do log2(x-1) in C++ 14(not sure for other versions),it also gives 59 as output..(but actual answer is 58), which is wrong and that caused me a fst in recent Div 3 problem F.

So, can anyone explain why is this happening and how to avoid these types of things. Thanks for spending your time in reading this blog.

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2 years ago, # |
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Why log2((2^59)-1)=59?

Be careful with log2() !!!!

log2 function limitations!!! be careful with this function

The comment section of these blogs will answer your question.

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    2 years ago, # ^ |
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    Not only log2(). You should also be careful with pow(), sqrt(), and any other inbuilt functions that operate on integers $$$\ge 2^{31}$$$.

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2 years ago, # |
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Thanks bro. Got it.

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2 years ago, # |
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I use log2((long double)x).It even works for integers >2^31.

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2 years ago, # |
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Use __lg(x) for floored log2.

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2 years ago, # |
← Rev. 11   Vote: I like it +6 Vote: I do not like it

I use

This

I got to know about this when I was solving this question Here is my submission without using the above code which gave me a WA and here is my submission with the above code give gave me a AC. Hope was helpful

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