Output of a << with -ve no?

Question

Possible Duplicate:
Left shifting with a negative shift count

Consider my code follows

int main()
{
    int k=1;
    k=k<<-1;
    printf("%d",k);
}

o/p

-2147483648

Why the output is like this.I know negative no's are stored in 2's complement but here is there any use of this concept of -2s complement.Kindly give me some idea.

what do you expect? you are shifting with a negative index, not really sense — logoff, Oct 15 '12 at 12:11
That can't be your complete code. For one you don't actually store the result of the expression `k<<-1` anywhere. — Some programmer dude, Oct 15 '12 at 12:14

ouah · Answer 1 · 2012-10-15T12:16:56.477

3

k << -1; // Before the OP edit

and

k = k<<-1;

These statements are undefined behavior in C.

Using a negative value in the right operand of the bitwise left or right shift operator is undefined behavior in C (see C99, 6.5.7).

edited Oct 15 '12 at 12:16

answered Oct 15 '12 at 12:11

ouah

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Thanks for answer ,Is the standard says this. – pradipta Oct 15 '12 at 12:12
@pradipta See http://stackoverflow.com/questions/4945703/left-shifting-with-a-negative-shift-count for a cite from Standard – halex Oct 15 '12 at 12:13

score 2 · Answer 2 · answered Oct 15 '12 at 12:18

2

The reason for the -2147483648 result - if you're testing this on an x86 processor - is that the shift count gets clamped to 31 (see "Intel Architecture Software Developer’s Manual Volume 2: Instruction Set Reference"). So you're getting 1<<31, which equals 0x80000000, which when printed as a signed 32-bit integer becomes -2147483648.

answered Oct 15 '12 at 12:18

Michael

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Thanks for the answer ,but how -1 is becoming 31 here. – pradipta Oct 15 '12 at 12:22
-1 is 0xFFFFFFFF (32-bit). So if you do a bitwise AND between -1 and 31 you get 31. – Michael Oct 15 '12 at 12:25
Thanks but how it is getting AND with 31. – pradipta Oct 15 '12 at 12:29
That's the fastest way of clamping the value to 31; only use the lower 5 bits. As Intel's manual states: "The count is masked to 5 bits, which limits the count range to 0 to 31.". – Michael Oct 15 '12 at 12:32

score 1 · Answer 3 · edited May 23 '17 at 12:27

1

The result of a shift operation is undefined if the second operand is negative.

http://msdn.microsoft.com/en-us/library/f96c63ed%28v=vs.80%29.aspx
Why am I getting strange results bit-shifting by a negative value?

edited May 23 '17 at 12:27

Community

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answered Oct 15 '12 at 12:12

Jeyaram

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Output of a << with -ve no?

3 Answers3