The squared values (real_comb_quad, imaginary_comb_quad) are both "long" variables. I get 9 digits long numbers and via sprintf it is no problem to get them on the screen via UART. But when I then try to calculate the magnitude (row3), my output shows me "NAN".
My variable "magnitude" is "long double", so I choose %Lf as the argument for sprintf. I wrote a code with the same calculations but with small numbers and it is working fine. So is it a problem because of my huge values?
Fixed the problem, it was neither to way I labeled the variables nor the sqrt(). And also not the size of my values. Somehow the program can not multiple the values. The fault was by multiplying real_comb*real_comb.
By changing this code to magnitude=pow(real_comb,2) it worked. I think calculating the power of 2 or just multiplying with the same value should be same mathematical operation.
Pay attention that calculating pow(n,2) takes ~5000 CPU ticks, while n*n is only about 100 ticks. Check output precision, you may be getting 'long' instead of 'long long' and that's why sprintf() was able to handle it.