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

6.2.4 Increment and Decrement Operators
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"Increment" and "decrement operators" increase or decrease the value of
a variable by one.  An assignment operator can do the same thing, so the
increment operators add no power to the 'awk' language; however, they
are convenient abbreviations for very common operations.

   The operator used for adding one is written '++'.  It can be used to
increment a variable either before or after taking its value.  To
"pre-increment" a variable 'v', write '++v'.  This adds one to the value
of 'v'--that new value is also the value of the expression.  (The
assignment expression 'v += 1' is completely equivalent.)  Writing the
'++' after the variable specifies "post-increment".  This increments the
variable value just the same; the difference is that the value of the
increment expression itself is the variable's _old_ value.  Thus, if
'foo' has the value four, then the expression 'foo++' has the value
four, but it changes the value of 'foo' to five.  In other words, the
operator returns the old value of the variable, but with the side effect
of incrementing it.

   The post-increment 'foo++' is nearly the same as writing '(foo += 1)
- 1'.  It is not perfectly equivalent because all numbers in 'awk' are
floating point--in floating point, 'foo + 1 - 1' does not necessarily
equal 'foo'.  But the difference is minute as long as you stick to
numbers that are fairly small (less than 10e12).

   Fields and array elements are incremented just like variables.  (Use
'$(i++)' when you want to do a field reference and a variable increment
at the same time.  The parentheses are necessary because of the
precedence of the field reference operator '$'.)

   The decrement operator '--' works just like '++', except that it
subtracts one instead of adding it.  As with '++', it can be used before
the lvalue to pre-decrement or after it to post-decrement.  Following is
a summary of increment and decrement expressions:

'++LVALUE'
     Increment LVALUE, returning the new value as the value of the
     expression.

'LVALUE++'
     Increment LVALUE, returning the _old_ value of LVALUE as the value
     of the expression.

'--LVALUE'
     Decrement LVALUE, returning the new value as the value of the
     expression.  (This expression is like '++LVALUE', but instead of
     adding, it subtracts.)

'LVALUE--'
     Decrement LVALUE, returning the _old_ value of LVALUE as the value
     of the expression.  (This expression is like 'LVALUE++', but
     instead of adding, it subtracts.)

                       Operator Evaluation Order

     Doctor, it hurts when I do this!
     Then don't do that!
                           -- _Groucho Marx_

What happens for something like the following?

     b = 6
     print b += b++

Or something even stranger?

     b = 6
     b += ++b + b++
     print b

   In other words, when do the various side effects prescribed by the
postfix operators ('b++') take effect?  When side effects happen is
"implementation-defined".  In other words, it is up to the particular
version of 'awk'.  The result for the first example may be 12 or 13, and
for the second, it may be 22 or 23.

   In short, doing things like this is not recommended and definitely
not anything that you can rely upon for portability.  You should avoid
such things in your own programs.