Vector iterator comparison

Question

I have a problem while comparing the values in two vectors.

Following is the sample code of my program:

  template <typename T> bool CompareVectors(std::vector<T> vector1, std::vector<T> vector2)
  {
    std::sort(vector1.begin(),vector1.end());
    std::sort(vector2.begin(),vector2.end());
    if (vector1.size() != vector2.size())
      return false;
    else
    {
      bool found = false;
      std::vector<T>::iterator it;
      std::vector<T>::iterator it2;
      for (it = vector1.begin();it != vector1.end(); it++)
      {      
        for(it2 = vector2.begin(); it2 != vector2.end(); it2++)
        {
          if(it == it2) // here i have to check the values in the itearators are equal.
          {
            found = true;
            break;
          }
        }
        if(!found)
          return false;
        else
          found = false;
      }
      return true;  
    }
    };

In this sample code I have to compare the two vectors. For that I have sorted the two vectors using std::sort(). Since the data type in the vector is a template (I am using a class object in the vector), the std::sort() is not working properly. Ie, sometimes the two vectors give different order of elements after sorting.

So I am not able to use the std::equal() function also.

For an alternative solution, I have used two iterators for the twi vectors.

And iterating one vector and searches that element in the other vector. For this the iterator comparison is cannot be usable.

How did you implement operator< for the sorting ? It is probably your problem ... My bet is you have a vector of pointers and that your items get sorted by their address instead of their values. — J.N.
– J.N., Commented Mar 12, 2012 at 4:55
Have you defined the < and == operator for the class that you're using? — howard
– howard, Commented Mar 12, 2012 at 4:56
yaa i have defined== , < , != operators for the class that i am using Yea... i am using poinetr element in the vectors for comparing.So thats the problem with std::sorting. So it is sorting with address. — Aneesh Narayanan
– Aneesh Narayanan, Commented Mar 12, 2012 at 5:03

Sarfaraz Nawaz · Accepted Answer · 2012-03-12 05:00:30Z

3

First you've to use typename keyword here:

typename std::vector<T>::iterator it;
typename std::vector<T>::iterator it2;

without typename your code wouldn't even compile.

To compare the values pointed to by iterators, you've to do this:

if( *it == *it2)

You could write you compare function as:

//changed the name from CompareVectors() to equal()
template <typename T> 
bool equal(std::vector<T> v1, std::vector<T> v2)
{
  std::sort(v1.begin(),v1.end());
  std::sort(v2.begin(),v2.end());
  if ( v1.size() != v2.size() )
       return false;
  return std::equal(v1.begin(),v1.end(), v2.begin());
};

edited Mar 12, 2012 at 5:00

answered Mar 12, 2012 at 4:55

Sarfaraz Nawaz

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2 Comments

Blastfurnace Over a year ago

Instead of testing the size and then calling std::equal you can simply say return v1 == v2;

Sarfaraz Nawaz Over a year ago

@Blastfurnace: Thats good. I didn't know that there exists == non-member function to test equality of two vectors.

user1263270 · Accepted Answer · 2012-03-12 04:59:38Z

1

Should this line:

if(it == it2)

be

if (*it == *it2)

The first line is comparing pointers not the values.

answered Mar 12, 2012 at 4:59

user1263270

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Comments

chrisaycock · Accepted Answer · 2012-03-12 05:07:02Z

0

There are multiple issues here. First, you say that std::sort() doesn't work. Have you overloaded the operator< for your class?

Also, you need to compare what the iterators are pointing to:

*it == *it2

Further, you need to iterate through both arrays at the same time (just one loop):

for (it = vector1.begin(), it2 = vector2.begin();
     it != vector1.end(), it2 != vector2.end();
     it++, it2++) {
  ...
}

Though really, you should just use std::equal() by overloading the operator==.

And from an efficiency standpoint, you should compare the size() values before you bother sorting the arrays.

edited Mar 12, 2012 at 5:07

answered Mar 12, 2012 at 5:01

chrisaycock

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