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I am trying to sort a vector of coordinates. The vector has pointers to these coordinates. I want to sort them by x and by y. What I am currently thinking of how to do this is as follows make two copies of the vector and then sort them. I am unsure of the following two things: 1) How to make a copy of a vector of pointers 2) How to sort both the points by x and y in the vectors and ensure also that they are properly sorted as follows (1,4),(1,5)

I have been reading and trying to figure out if there were any built in functions but I am not sure if for example sort function would correctly sort the x and y properly orderwise.

Here is what I have so far and any help would be appreciated.

typedef struct{double x; double y;) pt;
vector<pt*>v1;
vector<pt*>*v2 = v1;
// allocate memory for the points and push_back on the vector
the vector would have the following points {(1,7),(4,4),(1,3),(-2,4)}

When sorted it for x, it would be X={(-2,4),(1,3),(1,7),(4,4)} and Y={(1,3),(-2,4),(4,4),(1,7)}

UPDATE:

I am currently at this stage but it is still not working... :(

bool compare(pt* m1, pt* m2){return(m1->x <= m2->x) && (m1->y <= m2->y);}

vector<pt*>v1_x = v1;
sort(v1_x.begin(), v1_x.end(), comparer);
Masterminder
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1 Answers1

2

It's fairly easy using a custom comparator to do the dereferencing, as well as ready-made lexicographic tuple comparison:

#include <algorithm>
#include <tuple>
#include <vector>

struct pt { double x, double y };

std::vector<pt*> v = /* ... */ ;

auto x = v, y = v;   // copies

std::sort(x.begin(), x.end(),
          [](pt * a, pt * b) -> bool
          { return std::tie(a->x, a->y) < std::tie(b->x, b->y); });

std::sort(y.begin(), y.end(),
          [](pt * a, pt * b) -> bool
          { return std::tie(a->y, a->x) < std::tie(b->y, b->x); });

Of course the objects pointed to by the pointers must live at least as long as you're using the pointers in v, x and y.

Kerrek SB
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  • is it really as simple as that? – Masterminder Oct 21 '13 at 00:11
  • @Masterminder: Modulo getting all the ownership semantics right, sure, why not. – Kerrek SB Oct 21 '13 at 00:11
  • So once I make the copy of the vector, the sort will acutally do that for me if I specify the tie part? Like will it actually rearrange the pointers? – Masterminder Oct 21 '13 at 00:13
  • The `x` and `y` vectors will be sorted according to the specified lexicographic ordering. The original vector `v` will be untouched. – Kerrek SB Oct 21 '13 at 00:14
  • Is there anyway that this could be rewritten? I get a few error messages and am trying to fix them but my compiler is complaining about it not having library support and that it is currently experimental. Also it says I should have a primary-expression before the [] – Masterminder Oct 21 '13 at 00:35
  • @Masterminder: you need C++11 for the lambdas (add `-std=c++0x` to GCC and Clang, for example). You can always rewrite the lambda as a standalone function, though. As for `tie`, you could manually make a pair of references, since `pair` also implements lexicographic comparison, or just spell out the comparison by hand. – Kerrek SB Oct 21 '13 at 11:17