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# C - logical vs arithmetic right shift

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7 replies to this topic

### #1StubornAH  Members

Posted 27 September 2005 - 06:24 AM

I have an assignment to do a logical shift n spots to the right. On my WinXP machine using Dev-C++ >> is a logical shift. On my school's Linux machine, or maybe there is a flag in the compiler?, it does an arithmetic shift. C is not taught in this class, just general programming. I have search dozens of C websites and they all just tell me that >> does a right shift. How do I tell C I want a logical shift or an arithmetic shift? Thanks.

### #2Tinyn  Members

Posted 27 September 2005 - 06:26 AM

>>>

### #3doynax  Members

Posted 27 September 2005 - 06:27 AM

In general C compilers perform logical right shifts on unsigned data types and arithmetic right shifts (those that maintain the sign) on signed data types.
However it's something of a trick question since the compiler isn't required to support arithmetic ones at all and may choose to use logical ones exclusively.

It's also interesting to know that a division by two is not necessarily (and almost always in practice) equivalent to a two's complement arithmetic right shift. The division rounds towards zero (although it doesn't have to) while the shift does not.
signed a = -10;unsigned b = a;a >>= 1; // aritmetic (probably)b >>= 1; // logical

Quote:
 Original post by Tinyn>>>

That's actually a Java operator.

BTW does anyone know what the standard says about one's completement machines and arithmetic right shifts? You'd think that the natural implementation, unlike the two's complement version, would round towards zero instead.

### #4Tinyn  Members

Posted 27 September 2005 - 06:30 AM

Huh, I coulda sworn it was in C++ too.

### #5MaulingMonkey  Members

Posted 27 September 2005 - 06:33 AM

Quote:
 Original post by TinynHuh, I coulda sworn it was in C++ too.

Nope. MSDN Operator List

Posted 27 September 2005 - 06:59 AM

You could also use some inline asm. iirc, shl sal shr sar

### #7ZQJ  Members

Posted 27 September 2005 - 07:33 AM

Do any modern CPUs actually use one's complement?

### #8doynax  Members

Posted 27 September 2005 - 07:39 AM

Quote:
 Original post by ZQJDo any modern CPUs actually use one's complement?
None that I know of (aside IEEE floats which kind of uses it internally). They do introduce some interesting exceptions in the language however. So they're great when you want to say 'well.. that's technically incorrect' =)

I wouldn't be surprised if a few old beasts were still up and running (even actively developed for) though..

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