why does the pow function seem to be glitching?

Question

For the below code:

int digsum(ll num) {  //function to calculate sum of digits
    if (num < 0)
        num = abs(num);
    int ans = 0;
    while (num != 0) {
        ans = ans + num % 10;
        num = num / 10;
    }
    return ans;
}

int main() {
    ios_base::sync_with_stdio(0); cin.tie(0);

    int a, b, c, cnt = 0;
    long long x;
    cin >> a >> b >> c;

    for (int i = 0; i <= 72; i++) {
        x = (b * (pow(i, a))) + c;
        if (i == digsum(x) && x < mod) {
            cout << x << " ";
        }
    }

    return 0;
}

In the case a,b,c = 3,2,8 respectively and i=19; pow(19,3) is supposed to calculate 19^3 but when I replace pow by (19x19x19), this specific case is getting satisfied, where as that wasn't the case with the pow function. Can someone explain what the problem is?

Also, what value did you expect to have computed ? And what you actually getting? — selbie, Dec 29 '20 at 05:39
When I run your program and enter `3 2 8`, it spits out `10 2008 13726`. What values did you expect it to print? — selbie, Dec 29 '20 at 05:53
but im not getting 13726 in the output, for some reason it is calculating x as 13725 for that case — sidb2k, Dec 29 '20 at 06:08

selbie · Accepted Answer · 2020-12-29T06:55:50.907

0

My psychic powers suggest that your standard library's implementation of pow is not precise. I recall a discussion on SO a while back on this topic. Remember, pow returns a floating point value. I can't repro it, but it's entirely possibly your invocation of pow(19,3) returns 6858.999999999 or similar due to the way it's optimized.

Indeed, this this page says as much:

Due to rounding errors in floating point numbers, the results of pow() may not be precise (even if you pass it integers or whole numbers).

Also, this question and answer suggests the same thing.

I wouldn't have suspected it, but there you go.

Consider doing this as a workaround:

long long power = nearbyint(pow(i,a));
x = b * power + c;

edited Dec 29 '20 at 06:55

answered Dec 29 '20 at 06:42

selbie

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but if pow returned a float and rounding off was the problem, printing `d` where `float d = pow(19, 3);` should display a floating point value, but that isn't the case; it justs displays 6859 – sidb2k Dec 29 '20 at 06:57
Because print functions round too. – selbie Dec 29 '20 at 07:09
what if I print using `cout<<(float)d;` – sidb2k Dec 29 '20 at 07:13
That's won't do anything different. But this will: `cout << std::setprecision(32) << modified;` – selbie Dec 29 '20 at 07:47

why does the pow function seem to be glitching?

1 Answers1