Modulo issue C++

Question

Hi all,

why is it not possible to use code like this? (All integers)

...
cin >> number;
cin >> i;
cout << number % pow(10,i);
...

This returns a bunch of errors depending on compiler, something about mismatch int, double, bool.... but I use only integers... I must declare new variable first and fill it with divisor like:

x = pow(10,i);
cout << number % x;

This works but why can't I use it directly? I have only 16GB RAM and need to save every integer declared :-)

Thanks

Answer 1

You can't use it directly like that because std::pow() always returns a floating point type, not an integral type:

https://en.cppreference.com/w/cpp/numeric/math/pow

However, the built-in remainder operator requires both operands to be integral types. An explicit cast would do the job, i.e.

number % static_cast< int >( pow( 10, i ) );

or in C-style:

number % ( int ) pow( 10, i );

When assigning the return value to an integer variable, this conversion is done implicitely. But I don't see why you would have to worry about one integer with 16GB of RAM (I'd tend to think the compiler would potentially optimize it away anyway).

Answer 2

Thank you Shadow. Now it is clear. Regarding the RAM, I tried to joke a little. Never mind...