Fastest way to reset every value of std::vector<int> to 0

215

What's the fastest way to reset every value of a std::vector<int> to 0 and keeping the vectors initial size ?

A for loop with the [] operator ?

This question is tagged with c++ reset stdvector

~ Asked on 2012-01-13 09:46:31

The Best Answer is


369

std::fill(v.begin(), v.end(), 0);

~ Answered on 2012-01-13 09:49:33


163

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 <vector>

#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<int> v(TEST_ARRAY_SIZE, 0);

   for(size_t i = 0; i < TEST_ITERATIONS; ++i) {
   #if TEST_METHOD == 1 
      memset(&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<int>::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)!

~ Answered on 2012-01-13 11:22:31


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