C++: Fastest method to check if all array elements are equal

Question

What is the fastest method to check if all elements of an array(preferable integer array) are equal. Till now I have been using the following code:

bool chec         


        

Answer 1

Here is a solid solution which is valid C++11. The advantages is that you do not need to manually play with the indexes or iterators. It is a best practice to

prefer algorithm calls to handwritten loops [Herb Sutter - C++ Coding Standards]

I think this will equally efficient as Paul R's solution.

bool check(const int a[], int n)
{
      return !std::all_of(a, a+n, [a](int x){ return x==a[0]; });
}

Answer 2

Find a library that's available on your platform that supports threading or parallel-for loops, and split the computation out such that different cores test different ranges of the array.

Some available libraries are listed here:

http://parallel-for.sourceforge.net/parallelfor.html

Or possibly, you can make use of the parallism that many GPU's offer.

Answer 3

bool check_identity (int a[], int b[], const int size)
{
    int i;
    i = 0;
    while ((i < size-1) && (a[i] == b[i]))   i++;

    return (a[i] == b[i]);

}

Answer 4

Once you have found a mismatching element you can break out of the loop:

bool check(const int array[], int n)
{   
    for (int i = 0; i < n - 1; i++)      
    {         
        if (array[i] != array[i + 1])
            return true;
    }
    return false;
}

If this is performance-critical then it can be further optimised slightly as:

bool check(const int array[], int n)
{   
    const int a0 = array[0];

    for (int i = 1; i < n; i++)      
    {         
        if (array[i] != a0)
            return true;
    }
    return false;
}

Answer 5

int check(const int a[], int n)
{   
    while(--n>0 && a[n]==a[0]);
    return n!=0;
}

Answer 6

bool check(int array[],int n)
{       
  // here 1st element is checked with others. This decreases the number of iteration by 1.
  // also it returns immediately. 
  // The requirement is to check if all the elements are equal. 
  // So if 1st element is equal to others then all elements are equal. 
  // Otherwise the  elements are not equal.
  for(int i=1;i<n;i++)      
  {         
    if(array[0]!=array[i])
      return false;
  }        
  return true;
}