How to calculate 3rd root of a number in c++

Question

I am trying to calculate the 3rd root of a number.

For example if n=8->2; if n=27->3;

The pow function works well on square root (x^0.5) but it does not work on 3rd root (x^1/3), why is that?

#include <iostream>
#include <math.h>

using namespace std;

int main() {
    int e = 0.3;
    double k;
    cout << "Enter k:" << endl;
    cin >> k;
    k = pow(k, e);
    cout << "The result of k^1/3 " << k << endl;
    return 0;
}

You've declared e as an int, and assigned it the value 0.3 (3/10)(which will be cast down to 0) rather than 1/3 — Ben, Mar 02 '16 at 21:38
http://stackoverflow.com/questions/18103769/how-can-i-obtain-the-cube-root-in-c — Md. Shohan Hossain, Mar 02 '16 at 21:39
This is not a dup. OP knows to use the `pow` function. He's just having trouble figuring out how. — dbush, Mar 02 '16 at 21:44
Actually even `pow(k, 1.0 / 3.0)` wouldn't calculate a cube root. Close though. — harold, Mar 02 '16 at 21:46
Issues of (1/3) aside, why not just use the `cbrt` function? — Brett Hale, Mar 03 '16 at 01:00
Does this answer your question? [finding cube root in C++?](https://stackoverflow.com/questions/4269069/finding-cube-root-in-c) — phuclv, Nov 30 '19 at 15:56

score 5 · Answer 1 · answered Mar 02 '16 at 21:46

Two issues:

You declare e as an int but try to store a double value in it, so it gets truncated. You need to declare e as double to properly store the value.
The value you're using for e is not correct. 0.3 is not the same as 1.0/3.0, so your results will be off. Use 1.0/3.0 for this value instead.

score 2 · Answer 2 · answered Mar 02 '16 at 21:49

You've declared the exponent as an integer so it will always be assigned as a zero. declare it as a double.

inside your main try

double e = 1.0/3.0;
double k;
cout << "Enter k:" << endl;
cin >> k;
k = pow(k, e);
cout << "The result of k^1/3 " << k << endl;
return 0;

score 2 · Answer 3 · answered Mar 02 '16 at 21:53

You have to use double here:

double e = 1./3.;

instead of int.

#include <iostream>
#include <math.h>
using namespace std;

int main() {
    double e = 1./3.; // <- this line is changed!
    double k;
    cout << "Enter k:" << endl;
    cin >> k;
    k = pow(k, e);
    cout << "The result of k^1/3 " <<k << endl;
    return 0;
}

How to calculate 3rd root of a number in c++

3 Answers3