C++ Error when compiling c++ code using OpenMP

[jthorntonn]

I am trying to show the difference between the sequential and parallel versions of the selection sort. My code was working before but as soon as I added the clocks in to record the times I get errors referring to "cannot convert main():: lambda5 to 'int'." I am compiling my code with g++ -std=c++11 -openmp 'filename'. Here is my code:

#include <bits/stdc++.h>
#include <omp.h>
#include <algorithm>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
 
using namespace std;

void SeqSelectionSort(int* arr, int size)
{
    for (int i = size - 1; i > 0; --i)
    {
        int max = i;
        for (int j = i - 1; j >= 0; --j)
        {
            if (arr[j] > arr[max])
            {
                max = j;
            }
        }
        swap(arr[i], arr[max]);
    }
}

struct Compare { int val; int index; };
#pragma omp declare reduction(maximum : struct Compare : omp_out = omp_in.val > omp_out.val ? omp_in : omp_out)

void selectionsortomp(int* arr, int size)
{
    for (int i = size - 1; i > 0; --i)
    {
        struct Compare max;
        max.val = arr[i];
        max.index = i;
        #pragma omp parallel for reduction(maximum:max)
        for (int j = i - 1; j >= 0; --j)
        {
            if (arr[j] > max.val)
            {
                max.val = arr[j];
                max.index = j;
            }
        }
        int tmp = arr[i];
        arr[i] = max.val;
        arr[max.index] = tmp;
    }
}

int main(){
int vars = 100000;

int testarr1 = [vars];
int testarr2 = [vars];

for(int i = 0; i < vars; i++){
    testarr1[i] = rand();
    testarr2[i] = rand();
}

clock_t time_req1;
time_req1 = clock();
SeqSelectionSort(testarr1, 100000);
time_req1 = clock() - time_req1;
cout << "Execution time for Sequential Selection Sort: " << (float)time_req1 / CLOCKS_PER_SEC << " seconds\n" << endl;

clock_t time_req2;
time_req2 = clock();
selectionsortomp(testarr2, 100000);
time_req2 = clock() - time_req2;
cout << "Execution time for Parallel Selection Sort: " << (float)time_req2 / CLOCKS_PER_SEC << " seconds\n" << endl;

return 0;
}

 

[LTomy]

Hi jthorntonn. First, the lines 54 and 55 contain errors. In C and C++, we define arrays like that:

int testarr1[vars];
int testarr2[vars];

 

[LTomy]

On my side, with that change, the code compiles just fine.

 

[LTomy]

When defining arrays, we use the = operator only when we want to define the value of its elements:

int array[4] = {1, 2, 3, 4};

 

[LTomy]

Also, considering the size of the arrays, I would consider to use dynamic memory allocation instead (The operator new or the container std::vector for example). There is a (relatively small) limit to the amount of static memory that can be used. If the limit is surpassed, the program will crash.

 

[jthorntonn]

Hi jthorntonn. First, the lines 54 and 55 contain errors. In C and C++, we define arrays like that:

int testarr1[vars];
int testarr2[vars];

Wow that's embarrassing lol. Thank you for your help!

 

[LTomy]

Ahah 😉, you are welcome.