What's wrong with long? Why is subtracting 1 automatically?

问题

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

using namespace std;

int main() {
    int t, c1, c2, res;
    string str1, str2;

    cin >> t;

    for (int i = 0; i < t; i++) {
        c1 = c2 = res = 0;
        cin >> str1 >> str2;
        c1 = count(str1.begin(), str1.end(), '1');
        c2 = count(str2.begin(), str2.end(), '1');
        cout << (long)(((pow(10, c1) - 1) / 9) * ((pow(10, c2) - 1) / 9)) << '\n';
    }
}

For input:

1
11111 11111

Output is:

123454321

but, here is the problem,

For input:

1
10101 10100

Output is:

Also, for

1
11000 11000

Output:

I'm not getting why long is subtracting 1 from the final answer if the length of answer is less than 4?

Note: Here, input string is of length <= 10^5

回答1:

Some pow implementations return inaccurate results even when exact results are representable. Microsoft’s pow implementation is notorious for this.

In (long)(((pow(10, c1) - 1) / 9) * ((pow(10, c2) - 1) / 9)), pow is returning a value slightly less than the correct value, such as returning 99999.999999999985448084771633148193359375 instead of 100000. This results in the rest of the arithmetic producing a value slightly less than 1221 instead of 1221. When that is converted to long, the fraction is truncated, producing 1220.

Ideally, enough people would complain to Microsoft about this, or other publishers of bad pow routines, that they would fix it.

Given that the problem exists, you can work around it by rounding the pow result to the nearest integer (as with the round function) or by writing your own power routine to suit your purposes.

来源：https://stackoverflow.com/questions/51869997/whats-wrong-with-long-why-is-subtracting-1-automatically

标签

c++

casting

floating-point