CODEWITHSUNDEEP
c++vector
ezdazuzena
ezdazuzena

Reputation: 6770

Empty a std::vector without change its size

I want to reuse a std::vector within a for loop. However, I need the vector to be empty for each iteration step of the for loop.

Question: How can I empty a vector rapidly without changing its capacity in the most efficient way?

What I used so far is

std::vector<int> myVec;
for(int i=0; i<A_BIG_NUMBER; ++i) {
    std::vector<T>().swap(myVec);
    myVec.reserve(STANDARD_MAXIMUM);

    /// .. doing business
}

Cheers!

Solution:

Thanks for the answers, here is how I implemented (checked) it:

#include <vector>
#include <iostream>

 int main() {

    int n = 10;
    std::vector< int > myVec;
    myVec.reserve(n);
    for(int j=0; j<3; ++j) {
            myVec.clear();
            for(int i=0; i<n; ++i) {
                    myVec.push_back(i);
            }
            for(int i=0; i<myVec.size(); ++i) {
                    std::cout << i << ": " << myVec[i] << std::endl;
            }
    }

    return 0;
}

EDIT: changed from operator[] to push_back.

Upvotes: 10

Views: 8258

Answers (5)

user420442
user420442

Reputation:

To retain the current size of a vector with default values for its content, you can assign default values to the vector. In the case of a vector of ints, you can do the following:

myVec.assign( myVec.size(), 0 );

Upvotes: 3

stefaanv
stefaanv

Reputation: 14392

Answer based on OP's solution:
The normal approach for containers is to start with an empty container and fill it up as needed with an exception for std::vector where you can reserve space eventhough there are still no objects in the container.
If you want a different approach where an "empty container" would be a container of default objects that you can access like an array (only works with std::vector and std::deque), then you need to start with resize() and you can "clean up" with fill:

int n = 10;
std::vector<int> myVec;
myVec.resize(n);
myVec[4] = 5;
std::cout << myVec[4] << "\n";
std::fill(myVec.begin(), myVec.end(), int()); // ofcourse for int, you can use 0 instead of int()
std::cout << myVec[4] << "\n";

Upvotes: 0

shofee
shofee

Reputation: 2129

use clear method as below:

std::vector<int> myVec;
    for(int i=0; i<A_BIG_NUMBER; ++i) 
    {
        std::vector<T>().swap(myVec);
        myVec.reserve(STANDARD_MAXIMUM);

        /// .. doing business
    myVec.clear();
    }

Upvotes: 2

Moo-Juice
Moo-Juice

Reputation: 38825

myVec.clear();

This is equivalent to myVec.erase(myVec.begin(), myVec.end()).

Upvotes: 5

Asha
Asha

Reputation: 11232

Use vector::clear method. It will clear the content without reducing its capacity.

Upvotes: 13

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