Assignment Expressions
======================

   An "assignment" is an expression that stores a new value into a
variable.  For example, let's assign the value 1 to the variable `z':

     z = 1

   After this expression is executed, the variable `z' has the value 1.
Whatever old value `z' had before the assignment is forgotten.

   Assignments can store string values also.  For example, this would
store the value `"this food is good"' in the variable `message':

     thing = "food"
     predicate = "good"
     message = "this " thing " is " predicate

(This also illustrates concatenation of strings.)

   The `=' sign is called an "assignment operator".  It is the simplest
assignment operator because the value of the right-hand operand is
stored unchanged.

   Most operators (addition, concatenation, and so on) have no effect
except to compute a value.  If you ignore the value, you might as well
not use the operator.  An assignment operator is different; it does
produce a value, but even if you ignore the value, the assignment still
makes itself felt through the alteration of the variable.  We call this
a "side effect".

   The left-hand operand of an assignment need not be a variable (
see Variables.); it can also be a field (*note Changing the Contents of a
Field: Changing Fields.) or an array element (see Arrays in `awk': Arrays.).  These are all called "lvalues", which means they can appear
on the left-hand side of an assignment operator.  The right-hand
operand may be any expression; it produces the new value which the
assignment stores in the specified variable, field or array element.

   It is important to note that variables do *not* have permanent types.
The type of a variable is simply the type of whatever value it happens
to hold at the moment.  In the following program fragment, the variable
`foo' has a numeric value at first, and a string value later on:

     foo = 1
     print foo
     foo = "bar"
     print foo

When the second assignment gives `foo' a string value, the fact that it
previously had a numeric value is forgotten.

   An assignment is an expression, so it has a value: the same value
that is assigned.  Thus, `z = 1' as an expression has the value 1.  One
consequence of this is that you can write multiple assignments together:

     x = y = z = 0

stores the value 0 in all three variables.  It does this because the
value of `z = 0', which is 0, is stored into `y', and then the value of
`y = z = 0', which is 0, is stored into `x'.

   You can use an assignment anywhere an expression is called for.  For
example, it is valid to write `x != (y = 1)' to set `y' to 1 and then
test whether `x' equals 1.  But this style tends to make programs hard
to read; except in a one-shot program, you should rewrite it to get rid
of such nesting of assignments.  This is never very hard.

   Aside from `=', there are several other assignment operators that do
arithmetic with the old value of the variable.  For example, the
operator `+=' computes a new value by adding the right-hand value to
the old value of the variable.  Thus, the following assignment adds 5
to the value of `foo':

     foo += 5

This is precisely equivalent to the following:

     foo = foo + 5

Use whichever one makes the meaning of your program clearer.

   Here is a table of the arithmetic assignment operators.  In each
case, the right-hand operand is an expression whose value is converted
to a number.

`LVALUE += INCREMENT'
     Adds INCREMENT to the value of LVALUE to make the new value of
     LVALUE.

`LVALUE -= DECREMENT'
     Subtracts DECREMENT from the value of LVALUE.

`LVALUE *= COEFFICIENT'
     Multiplies the value of LVALUE by COEFFICIENT.

`LVALUE /= QUOTIENT'
     Divides the value of LVALUE by QUOTIENT.

`LVALUE %= MODULUS'
     Sets LVALUE to its remainder by MODULUS.

`LVALUE ^= POWER'
`LVALUE **= POWER'
     Raises LVALUE to the power POWER.  (Only the `^=' operator is
     specified by POSIX.)