path: root/source/lang/cpp/compoundbitwise.rst

.. highlight:: cpp

.. _lang-compoundbitwise:

Compound Bitwise Operators (``&=``, ``|=``, ``^=``)
===================================================

The compound bitwise operators perform their calculations at the
bit level of variables. They are often used to clear and set
specific bits of a variable.

See the :ref:`bitwise math tutorial <lang-bitwisemath>` for more
information on bitwise operators.

.. contents:: Contents
   :local:

.. _lang-compoundbitwise-and:

Compound bitwise AND (``&=``)
-----------------------------

The compound bitwise AND operator ``&=`` is often used with a variable
and a constant to force particular bits in a variable to be zero. This
is often referred to in programming guides as "clearing" or
"resetting" bits.  In a program, writing the line ``x &= y;`` is
equivalent to writing ``x = x & y;``.  That is, the value of ``x``
after the line will be equal to its old value bitwise ANDed with the
value of ``y``::

    x &= y;   // equivalent to x = x & y;

You can use any integer variable for ``x`` (i.e., any variable of type
``int``, ``char``, ``byte``, ``long long``, etc.).  You can use either
an integer variable or any :ref:`integer value
<lang-constants-integers>` (like ``3`` or ``0x20``) for ``y``.

Before doing an example of ``&=``, let's first review the Bitwise AND
(``&``) operator::

       0  0  1  1    operand1
       0  1  0  1    operand2
       ----------
       0  0  0  1    (operand1 & operand2) = result

As shown above, bits that are "bitwise ANDed" with 0 become 0, while
bits that are "bitwise ANDed" with 1 are left unchanged.  So, if ``b``
is a ``byte`` variable, then ``b & B00000000`` equals zero, and ``b &
B11111111`` equals ``b``.

.. _lang-compoundbitwise-binconst:

.. note:: The above uses :ref:`binary constants
   <lang-constants-integers-bin>`\ .  The numbers are still the same
   value in other representations, they just might not be as easy to
   understand.

   Normally, in C and C++ code, :ref:`hexadecimal
   <lang-constants-integers-hex>` or :ref:`octal
   <lang-constants-integers-oct>` are used when we're interested in
   an integer's bits, rather than its value as a number.

   While hexadecimal and octal literals might be harder to understand
   at first, you should really take the time to learn them.  They're
   part of C, C++, and many other programming languages, while binary
   constants are available only for compatibility with Arduino.

   Also, ``B00000000`` is shown for clarity, but zero in any number
   format is zero.

So, to clear (set to zero) bits 0 and 1 of a one-byte variable, while
leaving the rest of the variable's bits unchanged, use the compound
bitwise AND operator ``&=`` with the constant ``B11111100``
(hexadecimal ``0xFC``\ )::

       1  0  1  0  1  0  1  0    variable
       1  1  1  1  1  1  0  0    mask
       ----------------------
       1  0  1  0  1  0  0  0
       ^^^^^^^^^^^^^^^^  ^^^^
          unchanged      cleared


Here is the same representation with the variable's bits replaced
with the symbol ``x``\ ::

       x  x  x  x  x  x  x  x    variable
       1  1  1  1  1  1  0  0    mask
       ----------------------
       x  x  x  x  x  x  0  0
       ^^^^^^^^^^^^^^^^  ^^^^
          unchanged       cleared


So, using a byte variable ``b``\ , if we say::

    b =  B10101010; // B10101010 == 0xAA
    b &= B11111100; // B11111100 == 0xFC

then we will have ::

    b == B10101000; // B10101000 == 0xA8

.. _lang-compoundbitwise-or:

Compound bitwise OR (``|=``)
----------------------------

The compound bitwise OR operator ``|=`` is often used with a variable
and a constant to "set" (set to 1) particular bits in a variable.  In
a program, writing the line ``x |= y;`` is equivalent to writing ``x =
x | y;``.  That is, the value of ``x`` after the line will be equal to
its old value bitwise ORed with the value of ``y``::

    x |= y;   // equivalent to x = x | y;

You can use any integer variable for ``x`` (i.e., any variable of type
``int``, ``char``, ``long long`` etc.).  You can use either an integer
variable or any integer value (like ``3`` or ``0x20``) for ``y``.
(This works the same way as :ref:`compound bitwise AND
<lang-compoundbitwise-and>`\ , ``&=``).

Before doing an example of ``|=``, let's first review the Bitwise OR
(``|``) operator::

       0  0  1  1    operand1
       0  1  0  1    operand2
       ----------
       0  1  1  1    (operand1 | operand2) = result

Bits that are "bitwise ORed" with 0 are unchanged, while bits that are
"bitwise ORed" with 1 are set to 1.  So if ``b`` is a ``byte``
variable, then ``b | B00000000`` equals ``b``, and ``b & B11111111``
equals ``B11111111`` (here we've used binary constants; see the
:ref:`note <lang-compoundbitwise-binconst>` above).

So, to set bits 0 and 1 of a one-byte variable, while leaving the rest
of the variable unchanged, use the compound bitwise OR operator
(``|=``) with the constant ``B00000011`` (hexadecimal ``0x3``)::

       1  0  1  0  1  0  1  0    variable
       0  0  0  0  0  0  1  1    mask
       ----------------------
       1  0  1  0  1  0  1  1
       ^^^^^^^^^^^^^^^^  ^^^^
           unchanged      set

Here is the same representation with the variable's bits replaced with
the symbol ``x``::

       x  x  x  x  x  x  x  x    variable
       0  0  0  0  0  0  1  1    mask
       ----------------------
       x  x  x  x  x  x  1  1
       ^^^^^^^^^^^^^^^^  ^^^^
           unchanged      set

So, using a byte variable ``b``, if we say::

    b  = B10101010; // B10101010 == 0xAA
    b |= B00000011; // B00000011 == 0x3

then we will have ::

    b == B10101011; // B10101011 == 0xAB

.. _lang-compoundbitwise-xor:

Compound bitwise XOR (``^=``)
-----------------------------

The compound bitwise XOR operator ``^=`` is used with a variable and a
constant to "toggle" (change 0 to 1, and 1 to 0) particular bits in a
variable.  In a program, writing the line ``x ^= y;`` is equivalent to
writing ``x = x ^ y;``.  That is, the value of ``x`` after the line
will be equal to its old value bitwise XORed with the value of ``y``::

    x ^= y;   // equivalent to x = x ^ y;

You can use any integer variable for ``x`` (i.e., any variable of type
``int``, ``char``, ``long long``, etc.).  You can use either an
integer variable or any integer value (like ``3`` or ``0x20``) for
``y``.  (This works the same way as :ref:`&=
<lang-compoundbitwise-and>` and :ref:`\|= <lang-compoundbitwise-or>`;
in fact, these three operators all work the same in this way).

Before doing an example of ``^=``, let's first review the Bitwise
XOR operator, ``^``::

        0  0  1  1    operand1
        0  1  0  1    operand2
        ----------
        0  1  1  0    (operand1 ^ operand2) = result

One way to look at bitwise XOR is that each bit in the result is a 1
if the input bits are different, or 0 if they are the same.  Another
way to think about it is that the result bit will be 1 when *exactly*
one (no more, no less) of the input bits is 1; otherwise, it will be
zero.  This means that if you XOR a bit with 1, it will change (or
toggle) its value, while if you XOR a bit with 0, it stays the same.

So, to toggle bits 0 and 1 of a one-byte variable, while leaving the
rest of the variable unchanged, use the compound bitwise XOR operator
``^=`` with the constant ``B00000011`` (hexadecimal ``0x3``\ ; see
:ref:`note <lang-compoundbitwise-binconst>` above)::

       1  0  1  0  1  0  1  0    variable
       0  0  0  0  0  0  1  1    mask
       ----------------------
       1  0  1  0  1  0  1  1
       ^^^^^^^^^^^^^^^^  ^^^^
           unchanged     toggled

So, using a byte variable ``b``, if we say::

    b  = B10101010; // B10101010 == 0xAA
    b ^= B00000011; // B00000011 == 0x3

then we will have ::

    b == B10101001; // B10101001 == 0xA9

See Also
--------

- :ref:`Boolean operations <lang-boolean>` (``&&``, ``||``)
- :ref:`Bitwise operators <lang-bitwisemath>` (``&``, ``|``, ``^``, ``~``)

.. include:: cc-attribution.txt