
November 24th, 2009, 11:52 AM
#1
[RESOLVED] Original indices before partial_sort()?
If I wanted to use partial_sort() to find the first k of n values (because of the O(n log k) complexity), is there any way to retrieve the original indices of the sorted vector? I need them to access other arrays that don't need to be sorted, but that share ordering.
For instance, I have a list of n x and y coordinates. If I sort them by x coordinate, that reorders the x vector. However, I need to do something like this (forgive the pseudocode):
partial_sort(x);
for i = 1 to x.size()
plot x(i) and y(originalIndex(i))
end
in order to maintain the correct correspondence. This seems like a really common problem, but I can't find anything about it. I suppose I could just make a heap of the first k and push and pop, still making O(n log k), but that seems less...elegant.
Thanks for the help.
Last edited by fadecomic; November 24th, 2009 at 12:10 PM.
Reason: question resolved

November 24th, 2009, 11:57 AM
#2
Re: Original indices before partial_sort()?
One way is to create a vector of indices, pointers or iterators, then sort that based on the original vector's elements.
By the way, if you only want to find the first k of n values, then you should use std::nth_element, not std::partial_sort.
Last edited by laserlight; November 24th, 2009 at 12:00 PM.

November 24th, 2009, 12:01 PM
#3
Re: Original indices before partial_sort()?
Headsmacker. Of course. You mean just write a comparison that really compares the x value, right? I hate not seeing the obvious ones.

November 24th, 2009, 12:04 PM
#4
Re: Original indices before partial_sort()?
Originally Posted by fadecomic
You mean just write a comparison that really compares the x value, right?
Yes, it will compare based only on the x value of the elements of the original vector.

November 24th, 2009, 12:08 PM
#5
Re: Original indices before partial_sort()?
Thanks for the tip on nth_element. That clearly involves less work. That will at best be O(n) I see.
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