gawk: For Statement
7.4.4 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
The INITIALIZATION, CONDITION, and INCREMENT parts are arbitrary 'awk'
expressions, and BODY stands for any 'awk' statement.
The 'for' statement starts by executing INITIALIZATION. Then, as
long as the CONDITION is true, it repeatedly executes BODY and then
INCREMENT. Typically, INITIALIZATION sets a variable to either zero or
one, INCREMENT adds one to it, and CONDITION compares it against the
desired number of iterations. For example:
awk '
{
for (i = 1; i <= 3; i++)
print $i
}' inventory-shipped
This prints the first three fields of each input record, with one input
field per output line.
It isn't possible to set more than one variable in the INITIALIZATION
part without using a multiple assignment statement such as 'x = y = 0'.
This makes sense only if all the initial values are equal. (But it is
possible to initialize additional variables by writing their assignments
as separate statements preceding the 'for' loop.)
The same is true of the INCREMENT part. Incrementing additional
variables requires separate statements at the end of the loop. The C
compound expression, using C's comma operator, is useful in this
context, but it is not supported in 'awk'.
Most often, INCREMENT is an increment expression, as in the previous
example. But this is not required; it can be any expression whatsoever.
For example, the following statement prints all the powers of two
between 1 and 100:
for (i = 1; i <= 100; i *= 2)
print i
If there is nothing to be done, any of the three expressions in the
parentheses following the 'for' keyword may be omitted. 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 never terminates).
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 (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.
The 'awk' language has a 'for' statement in addition to a 'while'
statement because a 'for' loop is often 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.
There is an alternative version of the 'for' loop, for iterating over
all the indices of an array:
for (i in array)
DO SOMETHING WITH array[i]
Scanning an Array for more information on this version of the
'for' loop.