The built-in Math.Pow() function in .NET raises a double base to a double exponent and returns a double result.
What's the best way to do the same with integers?
Added: It seems that one can just cast Math.Pow() result to (int), but will this always produce the correct number and no rounding errors?
A pretty fast one might be something like this:
int IntPow(int x, uint pow)
{
int ret = 1;
while ( pow != 0 )
{
if ( (pow & 1) == 1 )
ret *= x;
x *= x;
pow >>= 1;
}
return ret;
}
Note that this does not allow negative powers. I'll leave that as an exercise to you. :)
Added: Oh yes, almost forgot - also add overflow/underflow checking, or you might be in for a few nasty surprises down the road.
Using the math in John Cook's blog link,
public static long IntPower(int x, short power)
{
if (power == 0) return 1;
if (power == 1) return x;
// ----------------------
int n = 15;
while ((power <<= 1) >= 0) n--;
long tmp = x;
while (--n > 0)
tmp = tmp * tmp *
(((power <<= 1) < 0)? x : 1);
return tmp;
}
to address objection that the code will not work if you change the type of power, well... leaving aside the point that anyone who changes code they don't understand and then uses it without testing.....
but to address the issue, this version protects the foolish from that mistake... (But not from a myriad of others they might make) NOTE: not tested.
public static long IntPower(int x, short power)
{
if (power == 0) return 1;
if (power == 1) return x;
// ----------------------
int n =
power.GetType() == typeof(short)? 15:
power.GetType() == typeof(int)? 31:
power.GetType() == typeof(long)? 63: 0;
long tmp = x;
while (--n > 0)
tmp = tmp * tmp *
(((power <<= 1) < 0)? x : 1);
return tmp;
}
Also try this recursive equivalent (slower of course):
public static long IntPower(long x, int power)
{
return (power == 0) ? x :
((power & 0x1) == 0 ? x : 1) *
IntPower(x, power >> 1);
}
How about:
public static long IntPow(long a, long b)
{
long result = 1;
for (long i = 0; i < b; i++)
result *= a;
return result;
}
Use double version, check for overflow (over max int or max long) and cast to int or long?
Another way is:
int Pow(int value, int pow) {
var result = value;
while (pow-- > 1)
result *= value;
return pow == 0 ? result : pow == -1 ? 1 : throw new ArgumentOutOfRangeException(nameof(pow));
}