Fastest way to reset every value of std vector int to 0

Question

What s the fastest way to reset every value of a std  vector lt int gt  to 0 and keeping the vectors initial size     A for loop with the    operator

User · Accepted Answer

std  fill v begin    v end    0

User · Answer

How about the assign member function   some vector assign some vector size    0

User · Answer

As always when you ask about fastest  Measure  Using the Methods above  on a Mac using Clang    Method         executable size     Time Taken  in sec                   -O0       -O3       -O0         -O3         ------------ --------- --------- ----------- ----------  1  memset     17 kB     8 6 kB    0 125       0 124      2  fill       19 kB     8 6 kB    13 4        0 124      3  manual     19 kB     8 6 kB    14 5        0 124      4  assign     24 kB     9 0 kB    1 9         0 591        using 100000 iterations on an vector of 10000 ints   Edit  If changeing this numbers plausibly changes the resulting times you can have some confidence  not as good as inspecting the final assembly code  that the artificial benchmark has not been optimized away entirely  Of course it is best to messure the performance under real conditions  end Edit   for reference the used code     include  lt vector gt    define TEST METHOD 1 const size t TEST ITERATIONS   100000  const size t TEST ARRAY SIZE   10000   int main int argc  char   argv        std  vector lt int gt  v TEST ARRAY SIZE  0       for size t i   0  i  lt  TEST ITERATIONS    i        if TEST METHOD    1        memset  amp v 0   0  v size     sizeof v 0        elif TEST METHOD    2       std  fill v begin    v end    0       elif TEST METHOD    3       for  std  vector lt int gt   iterator it v begin    end v end    it  end    it              it   0              elif TEST METHOD    4       v assign v size   0       endif          return EXIT SUCCESS      Conclusion  use std  fill  because  as others have said its most idiomatic

User · Answer

try   std  fill   and also  std  size siz   vec size      no memory allocating vec resize 0   vec resize siz  0

User · Answer

I had the same question but about rather short vector lt bool gt   afaik the standard allows to implement it internally differently than just a continuous array of boolean elements   Hence I repeated the slightly modified tests by Fabio Fracassi  The results are as follows  times  in seconds                -O0       -O3          --------  -------- memset     0 666     1 045 fill      19 357     1 066 iterator  67 368     1 043 assign    17 975     0 530 for i     22 610     1 004   So apparently for these sizes  vector lt bool gt   assign   is faster  The code used for tests     include  lt vector gt   include  lt cstring gt   include  lt cstdlib gt    define TEST METHOD 5 const size t TEST ITERATIONS   34359738  const size t TEST ARRAY SIZE   200   using namespace std   int main int argc  char   argv         std  vector lt int gt  v TEST ARRAY SIZE  0        for size t i   0  i  lt  TEST ITERATIONS    i     if TEST METHOD    1         memset  amp v 0   false  v size     sizeof v 0     elif TEST METHOD    2         std  fill v begin    v end    false       elif TEST METHOD    3         for  std  vector lt int gt   iterator it v begin    end v end    it  end    it                 it   0                elif TEST METHOD    4       v assign v size   false       elif TEST METHOD    5       for  size t i   0  i  lt  TEST ARRAY SIZE  i                v i    false           endif            return EXIT SUCCESS      I used GCC 7 2 0 compiler on Ubuntu 17 10  The command line for compiling    g   -std c  11 -O0 main cpp g   -std c  11 -O3 main cpp

User · Answer

If it s just a vector of integers  I d first try   memset  amp my vector 0   0  my vector size     sizeof my vector 0      It s not very C    so I m sure someone will provide the proper way of doing this

[c++] Fastest way to reset every value of std::vector<int> to 0

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