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combining two vectors

Posted on 2008-10-06
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Last Modified: 2012-06-27
Hi Guys,

What's the best way to combine the contents of  two vectors like so:

If value in v1 is found in v2, add together values and insert into v3.

if value in v1 is not found in v2 insert v1 value into v3.

if value in v2 is not found in v1 insert v2 value into v3.

Many thanks.

- Lee.
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Question by:schneiderit
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Expert Comment

by:Infinity08
Comment Utility
>> add together values

What do you mean by that ? If the value from v1 is found in v2, then that means that the values are the same. Do you mean to double that value and store it in v3 ? Or do you simply mean that you want v3 to be the union of v1 and v2 ?

I assume it's the latter, and in that case, take a look at the STL set_union algorithm :

        http://www.cplusplus.com/reference/algorithm/set_union.html
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Author Comment

by:schneiderit
Comment Utility
sorry, more detail.

struct Level
{
   int price_;
   int volume_;
}

The vectors contain level objects as above.  if the vectors contains level objects that have matching price, I need to add the volumes together and insert a new level object into  third vector.

if a level object exists in v1 with a price that the level objects in v2 don't have, then insert v1 object into v3.

if a level object exists in v2 with a price  that the level objects in v 1 don't have, then insert v2 object into v3.

Hope this is clearer.

Thanks.

- Lee.
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LVL 53

Expert Comment

by:Infinity08
Comment Utility
>> if the vectors contains level objects that have matching price, I need to add the volumes together

That's a bit different than what I understood from your first post ;)

First, instead of a vector of structs, consider using a map, so you can link the price and volume together more easily (see below).
Then you can simply insert all elements from v1 into v3. After that, start adding the elements from v2 into v3, but check for duplicates - if a duplicate is found, simply add the two volumes.

typedef int Price;

typedef int Volume;

std::map<Price, Volume> v1;

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Author Comment

by:schneiderit
Comment Utility
Tthe objects need to be stored based on index position, not key. We are unable to use a map for this unfortunately.
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Expert Comment

by:Infinity08
Comment Utility
>> We are unable to use a map for this unfortunately.

Then you'll just have to do a bit more work yourself :) Since apparently you cannot sort the data either ... (because that would change the order)

Which makes me wonder, how do you know what order the levels have to be in v3 ?
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Author Comment

by:schneiderit
Comment Utility
The order of levels in v3 doesn't matter, just wondered if there was a nice way of consolidating the two.  
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Expert Comment

by:Infinity08
Comment Utility
>> The order of levels in v3 doesn't matter

Ok. Can you sort the data in v1 or v2 too ? Because that would make things easier.
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Author Comment

by:schneiderit
Comment Utility
No not currently,  but I could add a sort function.
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Expert Comment

by:Infinity08
Comment Utility
>> No not currently,  but I could add a sort function.

STL already has a sort function. What I meant is : are you allowed to sort one of the two vectors ? Which will change its order ...
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Accepted Solution

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Infinity08 earned 125 total points
Comment Utility
What I had in mind was something like :

#include <vector>

#include <algorithm>
 

typedef struct Level {

  int price;

  int volume;
 

  bool operator<(const Level &l) { return (this->price < l.price); }         // <-- defines the order needed for sorting

} Level;
 

void merge(std::vector<Level> &result, std::vector<Level> &v1, std::vector<Level> &v2) {

  result.clear();

  sort(v1.begin(), v1.end());

  sort(v2.begin(), v2.end());
 

  std::vector<Level>::iterator it1, it2;

  for (it1 = v1.begin(), it2 = v2.begin(); (it1 != v1.end()) && (it2 != v2.end());) {

    if (*it1 < *it2) { result.push_back(*it1++); }

    else if (*it2 < *it1) { result.push_back(*it2++); }

    else { result.push_back(*it1++); result.back().volume += (*it2++).volume; }

  }

  while (it1 != v1.end()) { result.push_back(*it1++); }

  while (it2 != v2.end()) { result.push_back(*it2++); }

}

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Author Closing Comment

by:schneiderit
Comment Utility
that should do it,  thanks.
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