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Guest
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 Posted: Thu Dec 14, 2006 10:11 am Post subject: how to return index of sorted vector |
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Hi, I am wondering if there is a simple and quick way to return the
indices of sorted vector.
for example, I have a vector<int> x containing {5, 2, 3, 0, 2}.
I can use
sort(x.begin(), x.end(), less<int>());
to sort the vector x. Then the sorted x becomes {0, 2, 2, 3, 5}. But
how can I obtain the indices of sorted x, which is {3, 1, 4, 2, 0}.
I know that I can insert each element of x paired with the index into a
multimap and read out the reordered indices. But that is not what I am
looking for since I do not want to spend memory to keep another copy of
my vector x in multimap.
Surapong L. |
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Guest
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| Posted: Thu Dec 14, 2006 10:11 am Post subject: Re: how to return index of sorted vector |
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boheman (AT) gmail (DOT) com wrote:
| Quote: | Hi, I am wondering if there is a simple and quick way to return the
indices of sorted vector.
for example, I have a vector<int> x containing {5, 2, 3, 0, 2}.
....
to sort the vector x. Then the sorted x becomes {0, 2, 2, 3, 5}. But
how can I obtain the indices of sorted x, which is {3, 1, 4, 2, 0}.
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Create a vector i = {0,1,2,3,4,5} of sort indexes (std::generate should
com in handy). Use the three argument sort, where the third argument
references your vector x, to compare the elements of i based not on
their own values but on the values they index in x:
class lt {
vector<int> &_x;
public:
lt( vector<int> & x ) : _x(x) {}
bool comp( int j, int k ) const { return _x[j] < _x[k]; }
};
....
sort( i.begin(), i.end(), lt(x) );
Incidentally, APL has a "grade" primitive which does exactly this. |
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