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Asked: 2026-06-12T18:14:17+00:00

I am working on bit shifts, and I’ve run into this problem. I have

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I am working on bit shifts, and I’ve run into this problem.
I have two int:

int x = 1;
int y = 2;

What is the difference between:

 x = x << (31 + 1);

and

 y = y << 31;

I thought the result would be the same (namely that x and y would both equal 1) , but they aren’t…..I don’t understand why. 2 is just 1 with a “1” bit moved one space to the left.

Thanks!

I mean when we can’t shift-left anymore, don’t we wrap around to the beginning?

EDIT:
Let me clarify what I THINK is going on:

We start with x = 1, so that’s:

 00000000 00000000 00000000 00000001

We then left shift that by 31 +1 (or 32). That gives us:

 00000000 00000000 00000000 00000001

which is also 1.

Then we do y = 2, so that’s

 00000000 00000000 00000000 00000010

We left shift that by 31. That also gives us:

 00000000 00000000 00000000 00000001

Therefore, we get x = y = 1. I know this is wrong, but can anyone explain why???

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1 Answer

  1. Editorial Team

    You are confusing shifting with rotating:

    • Shifting means shifting all bits and filling the empty spot with a 0
      or a 1 (depending on value and/or signedness).

    • Rotating is shifting all bits and filling the empty spot with the bit
      that “fell out” in the end.

    AFAIK C does not support rotating, but only shifting (probably because of platform dependencies?). x86 assembler implements both shift and rotate operations.

    A explanation better than I can give here, can be found here: http://en.wikibooks.org/wiki/X86_Assembly/Shift_and_Rotate

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