这是什么意思？

Question

hi guys can you explain what does that greater than sign do in the follwing code

 if (header->mode > forceMode)
      {
        *rate >>= (header->mode - forceMode); //What does this mean >>=
         *granule_frame_size <<= (header->mode - forceMode); //What does this mean <<=
      }

Answer 1

While all the answers above are correct, it's possible you might still not understand it. You will need to understand binary.

Imagine you have the expression

7 << 2

That means "Convert 7 into binary, then shift all the bits left 2 times."

So 7 is 00000111

shifted left twice (and inserting zeros) yeilds:

00011100

This is equal to 28.

So if the variable A is 7, and B is 2, then:

A <<= B;

Leaves A equal to 28;

Answer 2

>>= is the right-shift assignment operator. x >>= y is equivalent to x = x >> y (with the caveat that @bdonlan points out below).

Answer 3

The << and >> operators are traditionally thought of bitwise because that's how they're implemented, but as far as the C language is concerned, they are arithmetic operators (not bitwise operators) equivalent to multiplication and division by the corresponding power of 2. That is, x<<y is x*2^y and x>>y is x/2^y (with integer truncation), where by ^ I mean "raised to the power" and not "xor". They are only defined for positive values of x , and like all arithmetic operators, << has undefined behavior for signed operands if the result overflows.