How to make a program that finds the number of happy numbers between 1 and 1 million

Question

I am trying to write code that calculates the right number of happy numbers between 1 and 1 million. What ends up being the result, however, is that either my output window remains blank and keeps on running, or else I get an output of 0, which is not correct. Does anybody have any suggestions? Here's the code that I'm using:

From the main function:

for (x = 2; x < 10; x++)                 
{
    if(is_happy(x) == 1)
        happy_number++;
}

  cout << "There are " << happy_number << " happy prime numbers between 1 and 1 million" << endl;

Note: happy_number starts off with a value of 0;

Then there's the function that calculates whether or not a number is happy:

int is_happy(int x)
{
    int y;
    int ans = 0;
    while (x > 0)
    {
        y = x%10;
        ans += pow(y, 2.0);
        x = x/10;
        ans += pow(x, 2.0);
    }

    return ans;
 }

Any suggestions?

Answer 1

Your logic is a little off in calculating a happy number. It's a cycle that keeps continuing until it reaches 1, or an infinite loop. A happy number reaches 1, while an unhappy number reaches 4 and loops forever.

bool is_happy(int x) //Let the function determine if the number is happy
{
    if (x <= 0) //Discrimination! Only positive numbers are allowed to experience joy
        return false;
    int result;
    while (x != 1) //If x == 1, it is a happy number
    {
        result = 0;
        while (x) //Until every digit has been summed
        {
            result += (x % 10) * (x % 10); //Square digit and add it to total
            x /= 10;
        }
        x = result;
        if (x == 4) //if x is 4, its a sad number
            return false;
    }
    return true;
}

You should use it like this:

for (int x = 2; x < 10; ++x)
{
    if (is_happy(x)) //Let the function do the logic, we just need true or false
        ++happy_number;
}

EDIT: You can see it work here.

Answer 2

I used Wikipedia. Happy number

The function named isHappy calculates whether or not the argument is happy. I am not sure if it works right if the argument is a negative integer.

You asked:

How to make a program that finds the number of happy numbers between 1 and 1 million

Function int happyNumbersBetween1_1000000() returns with the number of happy numbers between 1 and 1 000 000.

#include <iostream>

int happy(int i){
    int j=0;
    while(i!=0){
        j+=(i%10)*(i%10);
        i-=(i%10);
        i/=10;
    }
    return j;
}

bool isHappy(int i){
    switch(i){
        case 1: return true;
        case 4: return false;
        default: return isHappy(happy(i));
    }
}

int happyNumbersBetween1_1000000(){
    int j=0;
    for(int i=1; i<=1000000; ++i)
        if(isHappy(i)){
            ++j;
           // std::cout<<i<<std::endl;
        }
    return j;
}

int main()
{
    for(int i=1; i<100; ++i)
        if(isHappy(i))
            std::cout<<i<<" ";
    std::cout<<std::endl;

    std::cout<<happyNumbersBetween1_1000000()<<std::endl;

    return 0;
}