Creating Multiple Threads in For Loop to Process 2d Array - c++

Question

Ok so my code is probably a little messy - I'm new to this, so sorry in advance! Basically the code reads a file in to a 2d array and then creates multiple threads to perform calculations on each value in a number of rows of the array. The problem I'm having with this is the for loop at the bottom. It runs but does not loop. I have no idea why and have tried a number of things but I'm new to this so honestly have no idea. All the code is below and any help would be greatly, amazingly appreciated!!
#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <stdlib.h>
#include <time.h>
#include <pthread.h>
int threadArray[10][10];
int arrayVar[2];
using namespace std;
void *calc(void *arg){
int startPoint = arrayVar[0];
int endPoint = arrayVar[1];
int newArray[10][10];
int calculated;
for (int i = startPoint ; i < endPoint; i++){
for (int j = 0; j < 10; j++){
calculated = (threadArray[i][j] * 2 + 4) * 2 + 4;
newArray[i][j] = calculated;
}
}
for (int i = startPoint; i < endPoint; i++){
for (int j = 0; j < 10; j++){
cout << newArray[i][j] << " ";
}
cout << endl;
}
return NULL;
}
int main(){
int rc;
int start = 0;
int end;
ifstream numFile;
numFile.open("numbers.txt");
if (numFile.is_open()){
for (int row = 0; row < 10; row++){
std::string line;
std::getline(numFile, line);
std::stringstream iss(line);
for (int col = 0; col < 10; col++){
std::string num;
std::getline(iss, num, ' ');
std::stringstream converter(num);
converter >> threadArray[row][col];
}
}
cout << "Original 2D Array" << endl << endl;
for (int i = 0; i < 10; i++){
for (int j = 0; j < 10; j++){
cout << threadArray[i][j] << " ";
}
cout << endl;
}
cout << endl;
}
srand (time(NULL) );
const int rowArray[3] = {1, 2, 5};
int arrayIndex = rand() % 3;
int noOfRows = (rowArray[arrayIndex]);
end = noOfRows;
int noOfThreads = 10 / noOfRows;
pthread_t threads[noOfThreads];
arrayVar[2];
cout << "2D Array Altered" << endl << endl;
for (int t = 0; t < noOfThreads; t++){
arrayVar[0] = start;
arrayVar[1] = end;
rc = pthread_create(&threads[t], NULL, calc, NULL);
start = start + noOfRows;
end = end + noOfRows;
pthread_exit(NULL);
}
}

Answer 1

for (int t = 0; t < noOfThreads; t++)
{
arrayVar[0] = start;
arrayVar[1] = end;
rc = pthread_create(&threads[t], NULL, calc, NULL);
start = start + noOfRows;
end = end + noOfRows;
// pthread_exit(NULL); <-- this EXITS the loop and main
}
Comment out the above pthread_exit and have a second loop
for (int t = 0; t < noOfThreads; t++)
{
rc = pthread_join(threads[t], NULL);
}
pthread_exit(NULL);