This program generates a table filled with random numbers in a thread, then counts the sum and avarage of each row in a separate thread and then counts the avarage of sums of all rows in a thread.

pthreadLinuxThreads.cpp

//compile with: g++ name.cpp -o name -lpthread

    #include <pthread.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <time.h>
    #include <iostream>
    #include <assert.h>

    using namespace std;
    int rows;
    int columns;
    int ** table;
    int * sum;

    void *generateRandom(void * n)
    {
        for(int i =0; i <rows;i++ ){
            for(int j =0; j<columns;j++)
                table[i][j]=rand() % 20 ;
        }
        pthread_exit(NULL);
    }

    void *countSum(void * n)
    {   int sum=0;
        int row = (int )n;
        for(int i =0; i <columns;i++ ){
            sum +=table[row][i];
        }
        cout << "Thread id: "<<row <<"sum of row "<< row << " is "<< sum<<endl;
        pthread_exit((void*)sum);
    }

    void *getAvarage(void *n){
        int sumOfSums=0;
        for(int i =0 ; i< rows; i ++){
            sumOfSums +=sum[i];
        }
        pthread_exit((void*)sumOfSums);     
    }


    int main (int argc, char *argv[])
    {
        srand (time(NULL));
        cout << "\nThis program generates a table filled with random numbers in a thread, then counts the sum and avarage of each row in a separate thread and then counts the avarage of sums of all rows in a thread."<<endl;
        cout <<endl<<"Please input the number of rows of the column:";
        cin>>rows;
        cout <<"Please input the number of columns of the column:";
        cin >> columns;

        
        sum = new int[rows];
        table= new int*[rows];
        for(int i =0 ;i < rows; i ++){
            table[i]= new int[columns];
        }
        pthread_t thread[rows+2];
        pthread_attr_t attr;
        long t;
        void *result;

        /* Initialize and set thread detached attribute */
        pthread_attr_init(&attr);
        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

        //creating a thread that generates random numbers in the table
        pthread_create(&thread[0], &attr, generateRandom,(void*)t);
        cout<<endl << "Creating a thread with id 0. Generating random numbers.."<<endl;
        
        //waiting for the thread to end
        pthread_join(thread[0], &result);
        cout << "Random number generation DONE. Table contains:"<<endl;
        
        //printing the generated table
        for(int i =0 ; i <rows; i ++){
            cout << "| ";
            for(int j = 0 ; j < columns;j++){
                cout << table[i][j]<<" | ";
            }
            cout << endl;
        }


        //creating threads with id from 1 to "rows".
        for(t=0; t<rows; t++) {
            cout <<"Main: creating thread "<< t+1<<endl;
            pthread_create(&thread[t+1], &attr, countSum, (void*)t);
        }

        //Joining threads (waiting for them to finish)
        for(t=0; t<rows; t++) {
            pthread_join(thread[t+1], &result);
            sum[t]= (int)result;
            cout << "Main: completed join with thread "<<t+1<<"."<<endl;
            
        }
        

        pthread_create(&thread[rows+1], &attr, getAvarage,(void*)t);
        cout <<endl<<"Creating a thread with id "<<rows+1<<". \nComputing the sum of ALL threads.."<<endl;
        
        //waiting for the thread to end
        pthread_join(thread[rows+1], &result);
        printf("Main: completed join with thread %ld.\n\nThe sum of ALL threads is %ld\n",rows+1,(int)result);
        
        //computing the avarage in all rows 
        float avarageOfAllRows=0;
        for(int i = 0; i < rows; i ++){
            avarageOfAllRows +=sum[i];
        }
        avarageOfAllRows/=rows;
        cout << "The avarage of ALL threads is "<< avarageOfAllRows<<endl;


    pthread_attr_destroy(&attr);        
    printf("Main: program completed with no errors. All threads finished. Exiting.\n");
    pthread_exit(NULL);
    }