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I have a program that initializes a vector of 1,000,000 size and then finds all the prime numbers in it. The primes are stored in a new vector, I then iterate through that vector and try to find all of the happy numbers in it. I am trying to use task parallelism to split the work between 2 threads however I never return any output at all. here is the definition of a happy number A happy number is a number defined by the following process: Starting with any positive integer, replace the number by the sum of the squares of its digits, and repeat the process until the number either equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1. Those numbers for which this process ends in 1 are happy numbers, while those that do not end in 1 are unhappy numbers (or sad numbers).

#include <omp.h>
#include <stdio.h>
#include <iostream>
#include <stdlib.h>
#include<bits/stdc++.h>
#include<vector>
#include<numeric>

using namespace std;

void funcA();
void funcB();

int squaresum(int n){
    int sum=0;
    while(n){
        int d=n%10;
        sum+=d*d;
        n=n/10;
}
   return sum;
}

bool ishappy(int n){
set<int> s;
s.insert(n);

while(1){
    if(n==1) return true;
    n=squaresum(n);

    if(s.find(n)!=s.end()) return false;
    s.insert(n);
}
return false;
}
bool isprime(int n)
{

if (n <= 1)
    return false;
if (n <= 3)
    return true;


if (n % 2 == 0 || n % 3 == 0)
    return false;

for (int i = 5; i * i <= n; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0)
        return false;

return true;
}


int getHappyCount(int start, int size, int& happy)
{
int holder = size;
vector<int> num(size);
for(int i=start;i<start+size;i++) num[i]=i+1;

vector<int> prime;

for(int i=0;i<holder;i++)
{
    if(isprime(num[i])) prime.push_back(num[i]);
}

int count=0;
 for(int i=0;i<prime.size();i++)
{
     if(ishappy(prime[i])) count++;
}

#pragma omp atomic
happy += count;
}

int main()
{
int happy = 0;

#pragma omp parallel num_threads(2)
{
   #pragma omp sections
   {
      #pragma omp section

      {
        (void) getHappyCount(0, 500000, happy);
        printf("there are %d happy prime numbers \n", happy);
      }

      #pragma omp section

      {
        (void) getHappyCount(500000,  500000, happy);
        printf("there are %d happy prime numbers \n", happy);
      }
   }

}

    printf("there are %d happy prime numbers \n", happy);
    return 0;
 }
learmingslowly
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  • does you code produce correct output in a sequential version? – 463035818_is_not_an_ai Nov 23 '20 at 19:57
  • I have a sequential version that runs correctly, however, I added the getHappyCount function into this version and have been trying to parallelize it from here. – learmingslowly Nov 23 '20 at 19:59
  • _"I never return any output at all."_ — Could you please explain what does it mean? – Daniel Langr Nov 23 '20 at 20:44
  • when I say that I get no output, I mean that nothing returns when the code is run. So I am unsure if that is simply because the fout statements are wrong / implemented in the wrong way or if my attempt at parallelization is completely off base. – learmingslowly Nov 23 '20 at 21:00

1 Answers1

3
vector<int> num(size);
for (int i = start; i < start + size; i++)
  num[i] = i + 1;

This is obviously wrong unless start is zero. In all other cases, num[i] causes out-of-bound access of the num vector. You can switch to, for example:

for (int i = 0; i < size; i++)
  num[i] = start + i + 1;

Also, missing return statement in getHappyCount body implies undefined behavior. You should learn to understand warnings before you explicitly suppress them. Simply change the return type to void, or return something.

Daniel Langr
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