How do I perform a signed right shift(>>) in c?

Question

how do I perform a signed right shift in c? like -25 >> 3.

I tried like this: 10011001 >> 3 == 00010011

But the result is -4.

Here you can find good answers related to your question: https://stackoverflow.com/questions/7622/are-the-shift-operators-arithmetic-or-logical-in-c — Jose, Jan 31 '19 at 14:30
-25 is not 10011001 on a 2's complement system (i.e. pretty much all of them). It is 11100111. — interjay, Jan 31 '19 at 14:44
`10011001` base 2 is 99. Example "I tried like this: 10011001 >> 3 == 00010011" is unclear. — chux - Reinstate Monica, Jan 31 '19 at 15:23

Osiris · Accepted Answer · 2019-01-31T14:53:38.083

According to the standard 6.5.7p5

The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a signed type and a nonnegative value, the value of the result is the integral part of the quotient of E1 /2^E2. If E1 has a signed type and a negative value, the resulting value is implementation-defined.

The result is implementation-defined, and you should look up how it is defined there.

Assuming twos complement and signed right shift, -25 is 11100111, shifting it by 3 will lead to 11111100 which is -4.

jwismar · Answer 2 · 2019-01-31T15:02:30.107

0

That is a signed right shift. In signed shifting, a negative number (identified by the '1' in the highest bit) is often filled from the left. (Although this depends on the specific compiler. It looks like this is what your compiler is doing.) 11100111 --> 11110011 --> 11111001 --> 11111100

What it sounds like you're looking for is an unsigned shift. If you use (signed char)(((unsigned char)-25)>>3) you probably will get the results you're looking for.

edited Jan 31 '19 at 15:02

answered Jan 31 '19 at 14:31

jwismar

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OP's example would correspond to `unsigned uint8_t` rather than `unsigned short`. – interjay Jan 31 '19 at 14:37
I got -4 in my computer, however, the decimal of 11110011 is 115 – chen Jan 31 '19 at 14:38
@chen if it's cast back to a signed char, I suspect it will be -4. – jwismar Jan 31 '19 at 14:41
@interjay. you're right. char, not short. will fix. but `unsigned uint8_t` is a little redundant. :) – jwismar Jan 31 '19 at 14:42
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How right shifts of negative values operate is implementation-defined in C; the C standard does not mandate sign extension. – Eric Postpischil Jan 31 '19 at 14:49
@EricPostpischil Fair enough. It sure sounds like the implementation he's using is using sign extension, though. I'll add a clarification in my response. – jwismar Jan 31 '19 at 14:58

score 0 · Answer 3 · answered Jan 31 '19 at 14:55

As the right shift of the signed integer is implementation defined from my experience the result depends on the availability of the arithmetic shift on the target machine. Because nowadays is rather hard to find any computers or uC without it is almost always the result of the arithmetic shift right

From Wikipedia:

How do I perform a signed right shift(>>) in c?

3 Answers3