Quickly determine whether the power of Npower is 2

Today, I think about how to quickly determine the power of Npower 2. Here is a summary.

I. The fastest way:

(Number & Number -   1 ) =   0

Cause: Because the Npower of 2 is converted to binary 10 ...... 0 (except 0 ). And the number of digits-1 on the top. The result is 0. For example. The binary value of 8 is 1000, and the binary value of 8-1 = 7 is 111. The result of the two phases is 0. The calculation is as follows.

1000
& 0111
----- --
0000

 Code:

// Get a number from textbox
Long Number =   Long . Parse (txbnumber. Text );

///   <Summary>
/// Use "and"
///   </Summary>
Private   Void Useandmethod ()
{
If (Number = 1 )
{
Response. Write ( " No " );
Return ;
}
String Result = (Number & Number -   1 ). Tostring ();
If (Number & Number -   1 ) =   0 )
{
Response. Write ( " Yes " );
}
Else
{
Response. Write ( " No " );
}
}

 

2. Use mathematical functions (not as good as the first method)

// Get a number from textbox
Long Number =   Long . Parse (txbnumber. Text );

///   <Summary>
/// Calculation using math. Log Method
///   </Summary>
Private   Void Usemathlog ()
{
Datetime dtstart = Datetime. now;
Double Power = Math. Log (number, 2 );
Long Result =   0 ;
If (Int64.tryparse (power. tostring (), Out Result ))
{
Datetime dtend = Datetime. now;
Response. Write (number +   " Yes 2 "   + Power +   " Power, time spent is "   + Datediff (dtend, dtstart ));
}
Else
{
Datetime dtend = Datetime. now;
Response. Write (number +   " Not the Npower of 2, "   +   " The power of proximity is: "   + Power +   " , The time spent is "   + Datediff (dtend, dtstart ));
}
}

 

// Computing Time
Private   String Datediff (datetime datetimeend, datetime datetimestart)
{
String Datediff =   Null ;
Timespan ts1 =   New Timespan (datetimeend. ticks );
Timespan ts2 =   New Timespan (datetimestart. ticks );
Timespan TS = Ts1.subtract (ts2). duration ();
Datediff = TS. totalmilliseconds. tostring () +   " Millisecond " ;
Return Datediff;
}

 

There are otherAlgorithm. Only two are listed here.