MohammadParsaElahimanesh's blog

Floor log2 n in O(1) for C++
By MohammadParsaElahimanesh, history, 4 years ago, In English

Hello, many times you need to calculate lg2(n) and usually you need just to calculate floor of that. may these functions help you. 

/// there are in O(1) and they work for int and long long numbers that is greater than 0
int lg2(const int &x){return 31 - __builtin_clz(x);}
long long int lg2(const long long int &x){return 63 - __builtin_clzll(x);}
  • Vote: I like it
  • +7
  • Vote: I do not like it

»
4 years ago, # |
  Vote: I like it +19 Vote: I do not like it

You do not need const& for primitives. I'm not sure if the reference gets ignored or optimized out, but either way it's a code smell.

→ Reply
»
4 years ago, # |
  Vote: I like it +28 Vote: I do not like it

C++ (at least C++17) already offers the function __lg() that does exactly this.

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Ctrl+F through C++20 standard not found any __lg() function. Maybe it's just gcc implementation?

    → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Yes, I think it's only for gcc. Anyways, why would you choose another compiler instead of GCC? Official competitions (like ICPC/IOI) use GCC, and also you have some things in GCC that are not available in other compilers.

      → Reply
      • »
        »
        »
        »
        4 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it
        1. My remark just to avoid misleading that __lg() is C++ function.
        2. I just don't like any platform/compiler-dependent code. I think it's too bad to be used to rely on gcc-specific code and be surprised when same code not/slow work in other compilers. For same reason I always use whole header list instead of bits/stdc++.
        3. Maybe it's main reason — I write in Visual Studio :)
        → Reply