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<TITLE>Developer.com - Online Reference Library - 0672311739:RED HAT LINUX 2ND EDITION:gawk Programming</TITLE>
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<!-- TITLE=RED HAT LINUX 2ND EDITION //-->
<!-- AUTHOR=DAVID PITTS ET AL //-->
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<!-- CHAPTER=27 //-->
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<A NAME="PAGENUM-568"><P>Page 568</P></A>
<P>break causes the current (innermost) loop to be exited. It behaves as if the conditional test
was performed immediately with a false result. None of the remaining code in the loop (after
the break statement) executes, and the loop ends. This is useful when you need to handle
some error or early end condition.
</P>
<P>continue causes the current loop to return to the conditional test. None of the remaining
code in the loop (after the continue statement) is executed, and the test is immediately
executed. This is most useful when there is code you want to skip (within the loop) temporarily.
The continue is different from the break because the loop is not forced to
end.
</P>
<H5><A NAME="ch27_ 30">
The for Statement
</A></H5>
<P>The for statement provides a looping construct that modifies values within the loop. It is
good for counting through a specific number of items.
</P>
<P>The for statement has two general forms—the following
</P>
<!-- CODE SNIP //-->
<PRE>for (loop = 0; loop < 10; loop++)
statement
</PRE>
<!-- END CODE SNIP //-->
<P>and
</P>
<!-- CODE SNIP //-->
<PRE>for (subscript in array)
statement
</PRE>
<!-- END CODE SNIP //-->
<P>The first form initializes the variable (loop =
0), performs the test (loop < 10), and then performs the loop contents
(statement). Then it modifies the variable (loop++) and tests again.
As long as the test is true, statement will execute.
</P>
<P>In the second form, statement is executed with
subscript being set to each of the subscripts in
array. This enables you to loop through an array even if you don't know the values of the
subscripts. This works well for multidimension arrays.
</P>
<P>statement can be one statement or multiple statements enclosed in curly braces. The
condition (loop < 10) is any valid test like those used with the
if statement or the pattern used to trigger actions.
</P>
<P>In general, you don't want to change the loop control variable
(loop or subscript) within the loop body. Let the
for statement do that for you, or you might get behavior that is difficult
to debug.
</P>
<P>For the first form, the modification of the variable can be any valid operation (including
calls to functions). In most cases, it is an increment or decrement.
</P>
<TABLE BGCOLOR=#FFFF99><TR><TD><B>TIP</B></TD></TR><TR><TD><BLOCKQUOTE>
This example showed the postfix increment. It doesn't matter whether you use the
postfix (loop++) or prefix (++loop) increment—the results will be the same. Just be consistent.
</BLOCKQUOTE></TD></TR></TABLE>
<A NAME="PAGENUM-569"><P>Page 569</P></A>
<P>The for loop is a good method of looping through data of an unknown size:
</P>
<!-- CODE SNIP //-->
<PRE>for (i=1; i<=NF; i++)
print $i
</PRE>
<!-- END CODE SNIP //-->
<P>Each field on the current record will be printed on its own line. As a programmer, I don't
know how many fields are on a particular record when I write the code. The variable
NF lets me know as the program runs.
</P>
<H5><A NAME="ch27_ 31">
The while Statement
</A></H5>
<P>The final loop structure is the while loop. It is the most general because it executes while
the condition is true. The general form is as follows:
</P>
<!-- CODE SNIP //-->
<PRE>while(condition)
statement
</PRE>
<!-- END CODE SNIP //-->
<P>statement can be one statement or multiple statements enclosed in curly braces.
condition is any valid test like those used with the
if statement or the pattern used to trigger actions.
</P>
<P>If the condition is false before the while is encountered, the contents of the loop will not
be executed. This is different from do, which always executes the loop contents at least once.
</P>
<P>In general, you must change the value of the variable in the condition within the loop. If
you don't, you will have a loop forever condition because the test result
(condition) would never change (and become false).
</P>
<H3><A NAME="ch27_ 32">
Advanced Input and Output
</A></H3>
<P>In addition to the simple input and output facilities provided by
awk, there are a number of advanced features you can take advantage of for more complicated processing.
</P>
<P>By default, awk automatically reads and loops through your program; you can alter this
behavior. You can force input to come from a different file, cause the loop to recycle early (read
the next record without performing any more actions), or even just read the next record. You
can even get data from the output of other commands.
</P>
<P>On the output side, you can format the output and send it to a file (other than the
standard output device) or as input to another command.
</P>
<H4><A NAME="ch27_ 33">
Input
</A></H4>
<P>You don't have to program the normal input loop process in
awk. It reads a record and then searches for pattern matches and the corresponding actions to execute. If there are
multiple files specified on the command line, they are processed in order. It is only if you want to
change this behavior that you have to do any special programming.
</P>
<A NAME="PAGENUM-570"><P>Page 570</P></A>
<H5><A NAME="ch27_ 34">
next and exit
</A></H5>
<P>The next command causes awk to read the next record and perform the pattern match
and corresponding action execution immediately. Normally, it executes all your code in any
actions with matching patterns. next causes any additional matching patterns to be ignored
for this record.
</P>
<P>The exit command in any action except for END behaves as if the end of file was reached.
Code execution in all pattern/actions is ceased, and the actions within the
END pattern are executed. exit appearing in the
END pattern is a special case—it causes the program to end.
</P>
<H5><A NAME="ch27_ 35">
getline
</A></H5>
<P>The getline statement is used to explicitly read a record. This is especially useful if you have
a data record that looks like two physical records. It performs the normal field splitting
(setting $0, the field variables, FNR, NF, and NR). It returns the value
1 if the read was successful and zero if it failed (end of file was reached). If you want to explicitly read through a file, you can
code something like the following:
</P>
<!-- CODE SNIP //-->
<PRE>{ while (getline == 1)
{
# process the inputted fields
}
}
</PRE>
<!-- END CODE SNIP //-->
<P>You can also have getline store the input data in a field instead of taking advantage of
the normal field processing by using the form getline
variable. When used this way, NF is set to zero, and
FNR and NR are incremented.
</P>
<h5><P>Input from a file
</P></h5>
<P>You can use getline to input data from a specific file instead of the ones listed on the
command line. The general form is getline <
"filename". When coded this way, getline
performs the normal field splitting (setting $0, the field variables, and
NF). If the file doesn't exist, getline returns -1; it returns
1 on success and 0 on failure.
</P>
<P>You can read the data from the specified file into a variable. You can also replace
filename with stdin or a variable that contains the filename.
</P>
<P>
<TABLE BGCOLOR="#FFFF99">
<TR><TD><B>
NOTE
</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
If you use getline < "filename" to read data into your program, neither
FNR nor NR is changed.
</BLOCKQUOTE></TD></TR>
</TABLE></CENTER>
</P>
<h5><P>Input from a Command
</P></h5>
<P>Another way of using the getline statement is to accept input from a UNIX command. If
you want to perform some processing for each person signed on the system (send him or her
a message, for instance), you can code something like the following:
</P>
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