The `for' Statement
===================

   The `for' statement makes it more convenient to count iterations of a
loop.  The general form of the `for' statement looks like this:

     for (INITIALIZATION; CONDITION; INCREMENT)
       BODY

This statement starts by executing INITIALIZATION.  Then, as long as
CONDITION is true, it repeatedly executes BODY and then INCREMENT.
Typically INITIALIZATION sets a variable to either zero or one,
INCREMENT adds 1 to it, and CONDITION compares it against the desired
number of iterations.

   Here is an example of a `for' statement:

     awk '{ for (i = 1; i <= 3; i++)
               print $i
     }'

This prints the first three fields of each input record, one field per
line.

   In the `for' statement, BODY stands for any statement, but
INITIALIZATION, CONDITION and INCREMENT are just expressions.  You
cannot set more than one variable in the INITIALIZATION part unless you
use a multiple assignment statement such as `x = y = 0', which is
possible only if all the initial values are equal.  (But you can
initialize additional variables by writing their assignments as
separate statements preceding the `for' loop.)

   The same is true of the INCREMENT part; to increment additional
variables, you must write separate statements at the end of the loop.
The C compound expression, using C's comma operator, would be useful in
this context, but it is not supported in `awk'.

   Most often, INCREMENT is an increment expression, as in the example
above.  But this is not required; it can be any expression whatever.
For example, this statement prints all the powers of 2 between 1 and
100:

     for (i = 1; i <= 100; i *= 2)
       print i

   Any of the three expressions in the parentheses following the `for'
may be omitted if there is nothing to be done there.  Thus,
`for (;x > 0;)' is equivalent to `while (x > 0)'.  If the CONDITION is
omitted, it is treated as TRUE, effectively yielding an "infinite loop"
(i.e., a loop that will never terminate).

   In most cases, a `for' loop is an abbreviation for a `while' loop,
as shown here:

     INITIALIZATION
     while (CONDITION) {
       BODY
       INCREMENT
     }

The only exception is when the `continue' statement (*note The
`continue' Statement: Continue Statement.) is used inside the loop;
changing a `for' statement to a `while' statement in this way can
change the effect of the `continue' statement inside the loop.

   There is an alternate version of the `for' loop, for iterating over
all the indices of an array:

     for (i in array)
         DO SOMETHING WITH array[i]

See Arrays in `awk': Arrays, for more information on this version of
the `for' loop.

   The `awk' language has a `for' statement in addition to a `while'
statement because often a `for' loop is both less work to type and more
natural to think of.  Counting the number of iterations is very common
in loops.  It can be easier to think of this counting as part of
looping rather than as something to do inside the loop.

   The next section has more complicated examples of `for' loops.