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1. What Is Perl?
2. Course Requisites and Goals
3. Perl References & Resources
4. State of Perl
5. Taste of Perl
6. Storing & Running Perl Programs
7. The Elements
8. Literals & Operators
    9. Loops and I/O
10. Grade Book Example
11. Pipe I/O and System Calls
12. Matching
13. Parsing
14. Simple CGI
15. Testing Perl Programs
16. Common Goofs

9. Loops and I/O

Example: Command Line Values and Iterative Loops

    print "$#ARGV is the subscript of the ",
        "last command argument.\n";
    # Iterate on numeric subscript 0 to $#ARGV:
    for ($i=0; $i <= $#ARGV; $i++) {
        print "Argument $i is $ARGV[$i].\n";
    # A variation on the preceding loop:
    foreach $item (@ARGV) {
        print "The word is:  $item.\n";
    # A similar variation, using the
    # "Default Scalar Variable" $_ :
    foreach (@ARGV) {
        print "Say:  $_.\n";
    > perl Gooood morning, Columbia!
    2 is the subscript of the last command argument.
    Argument 0 is Gooood.
    Argument 1 is morning,.
    Argument 2 is Columbia!.
    The word is:  Gooood.
    The word is:  morning,.
    The word is:  Columbia!.
    Say:  Gooood.
    Say:  morning,.
    Say:  Columbia!.

Example: Standard I/O

    print STDOUT "Tell me something: ";
    while ($input = <STDIN>) {
            print STDOUT "You said, quote: $input endquote\n";
            chop $input;
            print STDOUT "Without the newline:  $input endquote\n";
            if ($input eq '') { print STDERR "Null input!\n"; }
            print STDOUT "Tell me more, or ^D to end:\n";
    print STDOUT "That's all!\n";
Note 1: The while statement's condition is an assignment statement: assign the next record from standard input to the variable $input. On end of file, this will assign not a null value but an "undefined" value. An undefined value in the context of a condition evaluates to "false". So the "while ($input = <STDIN>)" does three things: gets a record, assigns it to $input, and tests whether $input is undefined. In other contexts, Perl treats an undefined variable as null or zero. Thus, if $i is not initialized, $i++ sets $i to 1. Perl Paradox Number Three: Side effects can yield an elegant face or a pain in the rear.

Note 2: Data records are by default terminated by a newline character "\n" which in the above example is included as the last character of variable $input. The "chop" function removes the last character of its argument. Perl 5 introduces a "chomp" function that removes the last characters of a variable only if they are the currently defined end-of-record sequence, which is defined in the special variable $/.


    > perl
    Tell me something: I'm warm.
    You said, quote: I'm warm.
    Without the newline:  I'm warm. endquote
    Tell me more, or ^D to end:
    Can I have some water?
    You said, quote: Can I have some water?
    Without the newline:  Can I have some water? endquote
    Tell me more, or ^D to end:

    You said, quote:
    Without the newline:   endquote
    Null input!
    Tell me more, or ^D to end:
    That's all!

Example: Perls, A Perl Shell, Calculator, & Learning Tool

    for (;;) {
	print '(',join(', ',@ReSuLt),') ?';
	last unless $InPuT = <STDIN>;
	$? = ''; $@ = ''; $! = ''; 
	@ReSuLt = eval $InPuT;
	if ($?) { print 'status=',$?,' ' }
	if ($@) { print 'errmsg=',$@,' ' }
	if ($!) { print 'errno=',$!+0,': ',$!,' ' }
This reads a line from the terminal and executes it as a Perl program. The "for (;;) {...}" makes an endless loop. The "last unless" line might be equivalently specified:
    $InPuT = <STDIN>;               # Get line from standard input.
    if (! defined($InPuT)) {last;}  # If no line, leave the loop.
The "eval" function in Perl evaluates a string as a Perl program. "$@" is the Perl error message from the last "eval" or "do".


    () ?Howdy
    (Howdy) ?2+5
    (7) ?sqrt(2)
    (1.4142135623730951) ?$x
    () ?$x = sqrt(2)
    (1.4142135623730951) ?$x + 5
    (6.4142135623730949) ?1/0
    errmsg=Illegal division by constant zero in file (eval) at line 2, next 2 tokens "0;"
    () ?system 'date'
    Fri Sep 27 10:02:43 CDT 1996
    (0) ?$x = `date`
    (Fri Sep 27 10:02:52 CDT 1996
    ) ?chop $x
    ) ?$x
    (Fri Sep 27 10:02:52 CDT 1996) ?@y = split(/ /,$x)
    (Fri, Sep, 27, 10:02:52, CDT, 1996) ?@y[1,2,5]
    (Sep, 27, 1996) ?localtime()
    (37, 4, 10, 27, 8, 96, 5, 270, 1) ?
    () ?foreach (1..10) {print sqrt(),' '}
    1 1.4142135623730951 1.7320508075688772 2 2.2360679774997898 
    2.4494897427831779 2.6457513110645907 2.8284271247461903 3 3.1622776601683795    
    () ?exit

Example: File I/O

    # Function:  Reverse each line of a file

    # 1: Get command line values:
    if ($#ARGV !=1) {
        die "Usage:  $0 inputfile outputfile\n";
    ($infile,$outfile) = @ARGV;
    if (! -r $infile) {
        die "Can't read input $infile\n";
    if (! -f $infile) {
        die "Input $infile is not a plain file\n";

    # 2: Validate files
    # Or statements "||" short-circuit, so that if an early part
    # evaluates as true, Perl doesn't bother to evaluate the rest.
    # Here, if the file opens successfully, we don't abort:
    open(INPUT,"<$infile") ||
        die "Can't input $infile $!";
    if ( -e $outfile) {
        print STDERR "Output file $outfile exists!\n";
        until ($ans eq 'r' || $ans eq 'a' || $ans eq 'e' ) {
            print STDERR "replace, append, or exit? ";
            $ans = getc(STDIN);
        if ($ans eq 'e') {exit}
    if ($ans eq 'a') {$mode='>>'}
    else {$mode='>'}
    open(OUTPUT,"$mode$outfile") ||
        die "Can't output $outfile $!";

    # 3: Read input, reverse each line, output it.
    while (<INPUT>) {
        chop $_;
        $_ = reverse $_;
        print OUTPUT $_,"\n";

    # 4: Done!
    close INPUT,OUTPUT;

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