c - I have a continuous loop and I cannot figure out what is wrong with it -

i have continuous loop in c code , cannot figure out why acting way. feel missing noticable here can't see reason. here segment of code causing loop:

while (err > 0.0000001) {     if (err != 1.0) {         bab = ((2.0*bab) + input/(pow(bab, 2)))/3.0;         printf("the approximate cube root %.3lf\n", bab);         err = input - bab;     }     else {         bab = ((2.0*app) + input/(pow(app, 2)))/3.0;         printf("the approximate cube root %.3lf\n", bab);         err = input - bab;     } } 

any appreciated.

bab (sooner or later) approximately cube root of input. set

err = input - bab; 

and test

while (err > 0.0000001) 

so if input > 1 + 3*1e-7 (roughly), difference input - cube_root(input) larger threshold.

you want set

err = abs(input - bab*bab*bab); 

to control quality of approximation cube root of input.