I wanted to remove the elements of the vector based on the index, say all the even indexed elements.
I have read about the erase remove idiom but can't see how to apply it.
This is what I tried:
vector<int> line;
line.reserve(10);
for(int i=0;i<10;++i)
{
line.push_back(i+1);
}
for(unsigned int i=0;i<line.size();++i)
{
//remove the even indexed elements
if(i%2 == 0)
{
remove(line.begin(),line.end(),line[i]);
}
}
line.erase( line.begin(),line.end() );
This erases the entire vector. I was hoping to only remove the elements that had been marked by the remove algorithm.
Then I tried this
for(unsigned int i=0;i<line.size();++i)
{
//remove the even indexed elements
if(i%2 == 0)
{
line.erase( remove(line.begin(),line.end(),line[i]),line.end() );
}
}
This again doesn't work as there is a problem while removing, the indices seem to shift whilst iterating over the vector.
What should be the correct approach to accomplish this.
Online Demo:
#include <vector>
#include <algorithm>
#include <iostream>
/*Check if Index is Even or Odd*/
bool is_IndexEven(int i)
{
static int k = 1;
/*Handle Index 0 as special case as per choice*/
if(k == 1)
{
k++;
return false;
}
if(k++ % 2)
return true;
else
return false;
}
int main()
{
using namespace std;
int elements[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
// create a vector that holds the numbers from 0-9.
vector<int> v(elements, elements + 10);
/*Display elements before removal*/
vector<int>::const_iterator iter = v.begin();
cout<<"Before\n";
for(iter;iter!= v.end();++iter)
{
cout<<*iter;
}
/*Remove_if + Erase Algorithm for one step removal*/
v.erase( remove_if(v.begin(), v.end(), is_IndexEven), v.end() );
/*Display result after elements removed*/
cout<<"\nAfter\n";
iter = v.begin();
for(iter;iter!= v.end();++iter)
{
cout<<*iter;
}
return 0;
}
By going from 0
to size
, you'll end up skipping half of the elements because the indices change as you erase the elements. Make your for
loop go from size()
to 0
:
for(unsigned int i = line.size(); i > 0; i--)
{
}
Why don't you use remove_if? Use a static variable inside the function to signal the index for the current element.
Here is how to use erase-remove method to remove odd numbers from a vector. I am not sure whether you can remove elements based on the index, because remove_if() applies the predicate on the values pointed by the iterators rather than the iterator itself.
See the following :
http://cplusplus.com/reference/algorithm/remove_if/
#include <iostream>
#include <algorithm>
#include <vector>
#include <iterator>
using namespace std;
int main()
{
vector<int> v;
v.push_back(11);
v.push_back(22);
v.push_back(33);
v.push_back(44);
v.push_back(55);
v.push_back(66);
v.push_back(77);
ostream_iterator<int> printit(cout, " ");
cout << "Before removing odd numbers" << endl;
copy(v.begin(), v.end(), printit);
v.erase(remove_if(v.begin(), v.end(),
[] (int e) { return e%2 == 1; }), v.end());
cout << endl;
cout << "After removing odd numbers" << endl;
copy(v.begin(), v.end(), printit);
cout << endl;
}
An answer that generalizes not only upon the type of container to be handled but also upon the type of container that holds the indices to be removed, is given in :
Erasing elements in stl::vector by using indexes