C++ sqrt function returning truncated (wrong) output

Question

I have a doubt on the function sqrt of math.h

I was debugging some code, and I figured out that my function was not working properly, as the square root of 2/3 was always returning zero.

I tried to isolate the problem by just writing down some square root calculations in a separate file, and the function isn't returning the correct values.

Am I missing something?

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

using namespace std;
int main(){
        cout << sqrt(2/3) << endl;
        cout << sqrt(16/2) << endl;
        cout << sqrt(9/2) << endl;
        return 0;

}

This is the output I receive:

0
2.82843
2

When the correct output should be:

0.81650
2.82843
2.12132

Thank you in advance.

@YSC if you can find it, that would be really helpful. Couldn't find anything beside someone saying that sqrt(3) was evalued as zero in gdb — Michele Fattoruso, Nov 12 '18 at 17:17
@YSC more like `+Inf` dupes, to keep in spirit with IEEE 754 — SergeyA, Nov 12 '18 at 17:17
Does https://stackoverflow.com/questions/9764677/why-is-1-2x-different-from-0-5x?noredirect=1&lq=1 answers your question? — YSC, Nov 12 '18 at 17:17
Thank you both for the comment, I feel ashamed that i fell for such a silly mistake :\ — Michele Fattoruso, Nov 12 '18 at 17:19
@SergeyA May be, it's hair-splitting but 16/2 results in 8 and 9/2 results in 4. — Scheff's Cat, Nov 12 '18 at 17:20
@Scheff is is not hair splitting, but very valid comment. I obviously got carried away. Thx for pointing out, I have removed it. — SergeyA, Nov 12 '18 at 17:21
@MicheleFattoruso there is nothing to be ashamed of. The intricacies of arithmetic are one of the most counter-intuitive parts of C/C++. — SergeyA, Nov 12 '18 at 17:22

score 3 · Accepted Answer · answered Nov 12 '18 at 17:19

1,2,4,8,9 are integers constants, and the arithmetic result of integers will always be an integer.

Therefore, you should work with double constants, so you should try:

cout << sqrt(2.0/3.0) << endl;
cout << sqrt(16.0/2.0) << endl;
cout << sqrt(9.0/2.0) << endl;

C++ sqrt function returning truncated (wrong) output

1 Answers1