File: gawk.info,  Node: Assignment Ops,  Next: Increment Ops,  Prev: Concatenation,  Up: All Operators

6.2.3 Assignment Expressions
----------------------------

An "assignment" is an expression that stores a (usually different) value
into a variable.  For example, let's assign the value one to the
variable 'z':

     z = 1

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

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

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

This also illustrates string concatenation.  The '=' sign is called an
"assignment operator".  It is the simplest assignment operator because
the value of the righthand operand is stored unchanged.  Most operators
(addition, concatenation, and so on) have no effect except to compute a
value.  If the value isn't used, there's no reason to use the operator.
An assignment operator is different; it does produce a value, but even
if you ignore it, the assignment still makes itself felt through the
alteration of the variable.  We call this a "side effect".

   The lefthand operand of an assignment need not be a variable (*note
Variables::); it can also be a field (*note Changing Fields::) or an
array element (*note Arrays::).  These are all called "lvalues", which
means they can appear on the lefthand side of an assignment operator.
The righthand operand may be any expression; it produces the new value
that the assignment stores in the specified variable, field, or array
element.  (Such values are called "rvalues".)

   It is important to note that variables do _not_ have permanent types.
A variable's type is simply the type of whatever value was last assigned
to it.  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.

   String values that do not begin with a digit have a numeric value of
zero.  After executing the following code, the value of 'foo' is five:

     foo = "a string"
     foo = foo + 5

     NOTE: Using a variable as a number and then later as a string can
     be confusing and is poor programming style.  The previous two
     examples illustrate how 'awk' works, _not_ how you should write
     your programs!

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

     x = y = z = 5

This example stores the value five in all three variables ('x', 'y', and
'z').  It does so because the value of 'z = 5', which is five, is stored
into 'y' and then the value of 'y = z = 5', which is five, is stored
into 'x'.

   Assignments may be used anywhere an expression is called for.  For
example, it is valid to write 'x != (y = 1)' to set 'y' to one, and then
test whether 'x' equals one.  But this style tends to make programs hard
to read; such nesting of assignments should be avoided, except perhaps
in a one-shot program.

   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 righthand value to the
old value of the variable.  Thus, the following assignment adds five to
the value of 'foo':

     foo += 5

This is equivalent to the following:

     foo = foo + 5

Use whichever makes the meaning of your program clearer.

   There are situations where using '+=' (or any assignment operator) is
_not_ the same as simply repeating the lefthand operand in the righthand
expression.  For example:

     # Thanks to Pat Rankin for this example
     BEGIN  {
         foo[rand()] += 5
         for (x in foo)
            print x, foo[x]

         bar[rand()] = bar[rand()] + 5
         for (x in bar)
            print x, bar[x]
     }

The indices of 'bar' are practically guaranteed to be different, because
'rand()' returns different values each time it is called.  (Arrays and
the 'rand()' function haven't been covered yet.  *Note Arrays::, and
*note Numeric Functions:: for more information.)  This example
illustrates an important fact about assignment operators: the lefthand
expression is only evaluated _once_.

   It is up to the implementation as to which expression is evaluated
first, the lefthand or the righthand.  Consider this example:

     i = 1
     a[i += 2] = i + 1

The value of 'a[3]' could be either two or four.

   *note Table 6.2: table-assign-ops. lists the arithmetic assignment
operators.  In each case, the righthand operand is an expression whose
value is converted to a number.


Operator               Effect
--------------------------------------------------------------------------
LVALUE '+='            Add INCREMENT to the value of LVALUE.
INCREMENT
LVALUE '-='            Subtract DECREMENT from the value of LVALUE.
DECREMENT
LVALUE '*='            Multiply the value of LVALUE by COEFFICIENT.
COEFFICIENT
LVALUE '/=' DIVISOR    Divide the value of LVALUE by DIVISOR.
LVALUE '%=' MODULUS    Set LVALUE to its remainder by MODULUS.
LVALUE '^=' POWER      Raise LVALUE to the power POWER.
LVALUE '**=' POWER     Raise LVALUE to the power POWER.  (c.e.)

Table 6.2: Arithmetic assignment operators

     NOTE: Only the '^=' operator is specified by POSIX. For maximum
     portability, do not use the '**=' operator.

      Syntactic Ambiguities Between '/=' and Regular Expressions

   There is a syntactic ambiguity between the '/=' assignment operator
and regexp constants whose first character is an '='.  (d.c.)  This is
most notable in some commercial 'awk' versions.  For example:

     $ awk /==/ /dev/null
     error-> awk: syntax error at source line 1
     error->  context is
     error->         >>> /= <<<
     error-> awk: bailing out at source line 1

A workaround is:

     awk '/[=]=/' /dev/null

   'gawk' does not have this problem; BWK 'awk' and 'mawk' also do not.