Fastest way to zero out a vector >

Question

I've tried something like this:

vector<bool> x(10, 0);
vector<vector<bool> > hello(10, x);

/*some modifications on hello*/

memset(&hello, 0, sizeof(hello));

And my program compiles, but it breaks. Any idea how I can do this operation as quickly as possible? I know that the memset probably isn't working because of the nested vector, but I'm not sure how to accomplish this task.

is the second dimension always 10 in size? I mean always always? — Yakk - Adam Nevraumont, Jan 04 '14 at 04:31
You should be careful using `memset` in c++, see http://stackoverflow.com/questions/1975916/should-c-programmer-avoid-memset Even if it didn't result in a runtime error, you would have best case cleared the outer vector completely and leaked some memory on the way. — , Jan 04 '14 at 04:34

score 1 · Accepted Answer · answered Jan 04 '14 at 04:12

1

I would use this, which reuses the x variable you declared in the question.

std::fill(hello.begin(), hello.end(), x);

answered Jan 04 '14 at 04:12

Sam Harwell

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score 1 · Answer 2 · answered Jan 04 '14 at 04:12

1

for(auto& bv : hello) 
    std::fill(begin(bv), end(bv), false);

Or if you want to use x as the prototype

std::fill(begin(hello), end(hello), x);

answered Jan 04 '14 at 04:12

111111

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score 0 · Answer 3 · answered Jan 04 '14 at 05:13

With the code you have, I'd write …

for( auto& v : hello ) { v = x; }

assuming x has remained as all-zeros. It's clear enough and avoids dragging in <algorithm>.

Hewever, it will probably be faster to change the representation of your bit matrix, from a vector of vectors to a single vector, or if it's fixed size, to a single bitset.

Fastest way to zero out a vector >

3 Answers3