What is the simplest way to calculate the amount of even numbers in a range of unsigned integers?
An example: if range is (0,4) then it should return 3.
I'm having hard time to think of any simple way. The only solution I came up involved couple of if-statements. Is there a simple line of code that can do this without if-statements or ternary operator.
public static int countEvens(int first, int last)
{
int count = 0;
for(int i = first; i <= last; i++)
count += i%2 == 0 ? 1 : 0;
return count;
}
Will this work?
你需要
last / 2 - (first + 1) / 2 + 1
Here is one approach:
The number of evens from 0 to the first number is:
num_evens_first = (first/2 + 1)
The number of evens from 0 to the last number is:
num_evens_last = (last/2 + 1)
The number of evens in the range would be the difference between these two number PLUS 1 if the first number itself is even. Putting this altogether, you can use this formula:
num_evens_last - num_evens_first + (first + 1)%2
Or all at one time:
(last/2 + 1) - (first/2 + 1) + (first + 1)%2
Simplified:
last/2 - first/2 + (first + 1)%2
If the amount of numbers in your range is even, you have n / 2 even numbers in it.
If it is odd, you have n / 2 rounded down if first
is odd and n / 2 rounded up if first
is even.
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