使用这个应该输出最小可能整数的程序陷入无限循环

Question

hope someone could help me this and where the error in my code is. I have to write a program for class that does not use arrays or vectors and am restricted in using only nested loops for the exercise. The program is supposed to run through a series of loops and then outputs the lowest possible integer solution to the problem (a^3 + b^3 = c^3 + d^3). I'm still learning c++ so please forgive any ignorance or sloppy coding. Also thanks in advance.

#include <iostream> 
#include <cmath> 
#include <cstdlib>
#include <cstdio> 


using namespace std; 

int a, b, c, d, n, a_pow, b_pow, c_pow, d_pow, exp_pow, smaller_int,   larger_int, my_num; 


int main()
{


while (n == exp_pow && n == my_num)
{
    for (a = 1; a < 20; a++)
    {
        a_pow = pow(a, 3);
        larger_int = (a_pow + b_pow);

        for (b = 1; b < 20; b++)
        {
            b_pow = pow(b, 3);
            smaller_int = (a_pow + b_pow);
            if (larger_int < smaller_int)
            {
                smaller_int = larger_int;
            } 
                for (c = 1; c < 20; c++)
                {
                    c_pow = pow(c, 3); 
                    larger_int = (c_pow + d_pow);
                    if (larger_int < smaller_int)
                    {
                        smaller_int = larger_int;
                    }
                        for (d = 1; d < 20; d++)
                        {
                            d_pow = pow(d, 3); 
                            larger_int = (c_pow + d_pow);
                            if (larger_int < smaller_int)
                            {
                                smaller_int = larger_int;
                                exp_pow = smaller_int; 
                            }
                        }

                }

        }
    }

}

cout << smaller_int << endl;    
return 0;
} 

Edit: I think I pretty much got it to work, not sure if the computation is right, but was able to produce an output from the code. Thank you to the ones that did provide some insight. Yes I was not updating the variables correctly while inside the loop. Now I just have to check my logic.

On a side note, down voted for asking a question??? Smh.

Answer 1

If you want to get technical, the while loop is what never ends.

It's because you never assign a value to a, b, c, or d before you do:

a_pow = pow(a, 3); 
b_pow = pow(b, 3); 
c_pow = pow(c, 3);
d_pow = pow(d, 3);

So a,b,c,d are all 0, and thus a_pow:d_pow are all also 0. So when you do this:

n = (a_pow + b_pow);   
exp_pow = (c_pow + d_pow) 

(and by the way you need a semi-colon at the end of the 2nd line there), both n and exp_pow = 0.

Since your loop never reassigns values to either n or ex_pow, this:

while (n == exp_pow)

is always true. I should add that there are a lot of other problems in the code, as well. But this does answer your actual question.