问题
I'm trying to port some Java code, which requires arithmetic and logical bit shifts, to ABAP. As far as I know, ABAP only supports the bitwise NOT, AND, OR and XOR operations.
Does anyone know another way to implement these kind of shifts with ABAP? Is there perhaps a way to get the same result as the shifts, by using just the NOT, AND, OR and XOR operations?
回答1:
Disclaimer: I am not specifically familiar with ABAP, hence this answer is given on a more general level.
Assuming that what you said is true (ABAP doesn't support shifts, which I somewhat doubt), you can use multiplications and divisions instead.
Logical shift left (LSHL)
Can be expressed in terms of multiplication:
x LSHL n = x * 2^n
For example given x=9, n=2:
9 LSHL 2 = 9 * 2^2 = 36
Logical shift right (LSHR)
Can be expressed with (truncating) division:
x LSHR n = x / 2^n
Given x=9, n=2:
9 LSHR 2 = 9 / 2^2 = 2.25 -> 2 (truncation)
Arithmetic shift left (here: "ASHL")
If you wish to perform arithmetic shifts (=preserve sign), we need to further refine the expressions to preserve the sign bit.
Assuming we know that we are dealing with a 32-bit signed integer, where the highest bit is used to represent the sign:
x ASHL n = ((x AND (2^31-1)) * 2^n) + (x AND 2^31)
Example: Shifting Integer.MAX_VALUE to left by one in Java
As an example of how this works, let us consider that we want to shift Java's Integer.MAX_VALUE to left by one. Logical shift left can be represented as *2. Consider the following program:
int maxval = (int)(Integer.MAX_VALUE);
System.out.println("max value : 0" + Integer.toBinaryString(maxval));
System.out.println("sign bit : " + Integer.toBinaryString(maxval+1));
System.out.println("max val<<1: " + Integer.toBinaryString(maxval<<1));
System.out.println("max val*2 : " + Integer.toBinaryString(maxval*2));
The program's output:
max value : 01111111111111111111111111111111 (2147483647)
sign bit : 10000000000000000000000000000000 (-2147483648)
max val<<1: 11111111111111111111111111111110 (-2)
max val*2 : 11111111111111111111111111111110 (-2)
The result is negative due that the highest bit in integer is used to represent sign. We get the exact number of -2, because of the way negative numbers are represents in Java (for details, see for instance http://www.javabeat.net/qna/30-negative-numbers-and-binary-representation-in/).
回答2:
Edit: the updated code can now be found over here: github gist
来源：https://stackoverflow.com/questions/8349886/bit-shifts-with-abap