Exponent not working properly in C

Question

When i run the following code

/*Program to find the greatest common divisor of two nonnegative integer
 values*/


#include <stdio.h>

int main(void){
    printf("    n  |  n^2\n");
    printf("-----------------\n");
    for(int n = 1; n<11; n++){
        int nSquared = n^2;
        printf("%i          %i\n",n,nSquared);
    }
}

The table that gets returned to the terminal displays as follows

    n  |  n^2
-----------------
1          3
2          0
3          1
4          6
5          7
6          4
7          5
8          10
9          11
10          8

why does the "n^2" side generate the wrong numbers? And is there a way to write superscripts and subscripts in C, so I do not have to display "n^2" and can display that side of the column as "n²" instead?

Your question about writing superscripts and subscripts is unrelated to your question about exponentiation. I suggest you submit that as a separate question. (It's not a simple matter, and it's likely to depend on your locale settings.) — Keith Thompson, Apr 16 '14 at 17:54
haha.. the first time i see someone expecting `^` to be exponentiation in C. — bolov, Apr 16 '14 at 17:58
@bolov It seems that you are new to this site. This is probably the third one that I've seen during the day. It seems that many have this assignment due. — devnull, Apr 16 '14 at 18:01

score 6 · Accepted Answer · answered Apr 16 '14 at 17:52

6

Use pow function from math.h.

^ is the bitwise exclusive OR operator and has to nothing to do with a power function.

answered Apr 16 '14 at 17:52

ouah

142,963
15
272
331

score 2 · Answer 2 · answered Apr 16 '14 at 17:52

2

The ^ is the XOR operation. You'd either want to use the math.h function "pow", or write your own.

answered Apr 16 '14 at 17:52

HungryBeagle

262
1
13

1

`pow` is overkill for squaring a number, and it's subject to floating-point rounding errors. Just write `int nSquared = n*n;` – Keith Thompson Apr 16 '14 at 17:53
@OMGtechy I don't think that's correct. `n << 1` isn't the same as `n * n`. – Whymarrh Apr 16 '14 at 18:11
@Whymarrh I was getting mixed up with something else, thanks for pointing it out. – OMGtechy Apr 16 '14 at 18:16

score 0 · Answer 3 · answered Apr 16 '14 at 17:57

^ is the bitwise xor operator. You should use the pow function declared in the math.h header.

#include <stdio.h>
#include <math.h>

int main(void) {
    printf("    n  |  n^2\n");
    printf("-----------------\n");
    for(int n = 1; n < 11; n++){
        int nSquared = pow(n, 2); // calculate n raised to 2
        printf("%i          %i\n", n, nSquared);
    }
    return 0;
}

Include the math library by the flag -lm for gcc compilation.

score 0 · Answer 4 · answered Apr 21 '14 at 19:36

As others have pointed out, the problem is that ^ is the bitwise xor operator. C has no exponentiation operator.

You're being advised to use the pow() function to compute the square of an int value.

That's likely to work (if you're careful), but it's not the best approach. The pow function takes two double arguments and returns a double result. (There are powf and powl functions that operator on float and long double, respectively.) That means that pow has to be able to handle arbitrary floating-point exponents; for example, pow(2.0, 1.0/3.0) will give you an approximation of the cube root of two.

Like many floating-point operations, pow is subject to the possibility of rounding errors. It's possible that pow(3.0, 2.0) will yield a result that's just slightly less than 9.0; converting that to int will give you 8 rather than 9. And even if you manage to avoid that problem, converting from integer to floating-point, performing an expensive operation, and then converting back to integer is massive overkill. (The implementation might optimize calls to pow with integer exponents, but I wouldn't count on that.)

It's been said (with slight exaggeration) that premature optimization is the root of all evil, and the time spent doing the extra computations is not likely to be noticeable. But in this case there's a way to do what you want that's both simpler and more efficient. Rather than

int nSquared = n^2;

which is incorrect, or

int nSquared = pow(n, 2);

which is inefficient and possibly unreliable, just write:

int nSquared = n * n;

Exponent not working properly in C

4 Answers4