Programming Language Concepts Perl I/O

1. Programming Language ConceptsPerl I/O Adapted by Carl Reynolds from materials by Sean P. Strout PLC - Perl I/O (v2.1)

2. Input from Standard Input • The line input operator will return undef on EOF while (defined($line = <STDIN>)) { print “I saw $line”; } • Perl provides a scalar context shortcut to read one line of input at a time and evaluate the loop while (<STDIN>) { while(defined($_=<STDIN>)) { print “I saw $_”; print “I saw $_”; } }  PLC - Perl I/O (v2.1)

3. Input from Standard Input • In a list context, all lines of input must be read into memory before the looping operation can begin foreach (<STDIN>) { print “I saw $_”; } • For extremely large input files, the scalar context (while) is usually the more efficient programming approach. PLC - Perl I/O (v2.1)

4. The Diamond Operator • Use the diamond operator, <> , to make your programs work like standard Unix utilities: cat, grep, lpr, etc. % ./myprog.pl fred barney - betty while (defined($line = <>)) { chomp($line); print “It was $line that I saw!\n”; } • The diamond operator reads input, and stores it into $_ by default • The command line input files look like a single, continuous input stream file file stdin file PLC - Perl I/O (v2.1)

5. Invocation Arguments • The diamond operator finds its list of files by looking at the special array, @ARGV , which is a list of invocation arguments • The program name is stored in $0, not@ARGV • Read the input into $_ , 1 line at a time: while (<>) { chomp; print “I saw $_.”; } • (Use the Getopt::Std module to process Unix-style flags) • See Programming in Perl, Chapt. 7 ./myprog –f filename –o –r2 PLC - Perl I/O (v2.1)

6. Output to Standard Output • Use the print operator to take a list of values and send each item to standard output @array = qw/ fred barney betty /; print @array; # fredbarneybetty print “@array”; # fred barney betty • What happens if: @array = <STDIN>; print “@array”; • Your turn. Write a Perl script for the basic Unix cat and sort commands… print <>; print sort <>; PLC - Perl I/O (v2.1)

7. Output to Standard Output • “If it looks like a function call, it is a function call” print “Hello world!\n”; # ok print (“Hello world!\n”); # ok $result = print (2+3); # prints “5”, $result=1 $result = print (2+3)*4; # What happens here? • With no parentheses, print is a list operator • Otherwise it is a function call which only operates on the stuff inside the parentheses. PLC - Perl I/O (v2.1)

8. Formatted Output with printf • The printf operator takes a format string followed by a list of things to print $user = “Sarah Connor”; $days = 7; printf “Hello, %s; you expire in %d days”, $user, $days; #Hello, Sarah Connor; you expire in 7 days • Each conversion begins with a percent sign and ends with a letter • Use %g for expected number printing printf “%g %g %g\n”, 5/2, 51/17, 51**17; #2.5 3 1.0683e+29 PLC - Perl I/O (v2.1)

9. Formatted Output with printf • Use %d for decimal integer printing (truncates) printf “in %d days”, 17.85 # in 17 days • To print formatted columnar data, specify a field width printf “%6d\n”, 42 # 4 spaces + 42 printf “%2d\n”, 2e3 + 1.95 # 2001 • To interpolate a value into a string, use %s printf “%10s\n”, “wilma”; # 5 spaces + wilma printf “%-15s\n”, “dino”; # dino + 11 spaces (left-justified) PLC - Perl I/O (v2.1)

10. Formatted Output with printf • Use %f to control precision in floating-point rounds • Note: The . character counts as one character printf “%12f\n”, 6*7 + 2/3; # 3 spaces + 42.666667 printf “%12.3f\n”, 6*7 + 2/3; # 6 spaces + 42.667 printf “%12.0f\n”, 6*7 + 2/3; # 10 spaces + 43 • Use %perldoc perlfuncor look at your favorite C book for a complete list of conversions PLC - Perl I/O (v2.1)

11. Arrays and printf • Printing arrays of variable items via printfcan be achieved with a bit of craftiness • Generate the format string “on the fly” my @items = qw/ wilma dino pebbles /; my $format = “The items are: \n” . (“%10s\n” x @items); print “the format is <<$format>>\n”; # for debugging printf $format, @items; • But can the apprentice become the master? Try to shorten the 3 lines in blue to 1... printf “The items are: \n” . (“%10s\n” x @items), @items; PLC - Perl I/O (v2.1)

12. Test Your Understanding • Write a program that acts like cat, but reverses the order of the output lines (so call it ‘tac’). If you run yours as: ./my_tacfred barney betty The output should be all of file betty from last line to first, then barney and then fred, also from last line to first. print reverse <>; PLC - Perl I/O (v2.1)

13. Filehandles • Use the openoperator to connect your program to a file stream open CONFIG, “dino”; # implicit read open CONFIG, “<dino”; # explicit read open BEDROCK,“>fred”; # create open LOG, “>>logfile”; # append (or create) • Use the close operator when finished with a file handle close BEDROCK; # forces flush • Re-opening a file handle causes an automatic close Filehandle PLC - Perl I/O (v2.1)

14. Filehandles • Once created, the file handle can be read from using the diamond operator open PASSWD, “/etc/passwd”; while (<PASSWD>) { chomp; if (/^root:/) { # found root entry # hack hack hack } } • Use print or printf to write/append to a file handle print LOG “Captain’s log, stardate 3.14159\n”; Regular expression Filehandle PLC - Perl I/O (v2.1)

15. Filehandles • A common way to handle bad file streams: “Open this file or die!” open LOG, “>>logfile” or die “Cannot create logfile: $!”; Cannot create logfile: Permission denied at line 1. $!has the most recent system error message PLC - Perl I/O (v2.1)

16. File tests Does the file exist? -e $filename Is the file a text file? -T $filename How long ago (in days) was the file last modified? -M $filename How long ago (in days) was the file last accessed? -A $filename How big is the file (in bytes)? -s $filename Is this a directory? -d $filename foreach my $filename (@files) { if –s $filename > 100_000 and –A $filename > 90 { push @big_old_files, $filename; } } PLC - Perl I/O (v2.1)

17. Perl Standard modules provide methods for dealing with file names and directory structures in a system-independent way: • use File::Spec; # Standard Perl module • Common file path and directory operations, for example: • catfile() -- concatenate directory names and a filename to form a complete path ending with a filename $path = File::Spec->catfile( @directories, $filename ); • curdir() -- returns a string representation of the current directory path $curdir = File::Spec->curdir(); • updir() -- returns a string representation of the parent directory. $updir = File::Spec->updir(); • use File::Find; # Standard Perl module • Traverse a directory tree. • find() -- does a depth-first search over the given @directories in the order they are given. For each file or directory found, it calls the &wanted subroutine. find(\&wanted, @directories); PLC - Perl I/O (v2.1)

18. Globbing for files • Use the glob operator to search for qualifying files • my @all_files = glob "*"; • Like the ls command • my @pl_files = glob "*.pl"; • Like the command: ls *.pl • A glob will remind you of a regular expression, but regular expressions are different. PLC - Perl I/O (v2.1)

19. Getting Output from Other Programs • To execute a system command and read the input into a perl program, use the backtick (left apostrophe `) my $date = `date`; my @files = `ls –l`; PLC - Perl I/O (v2.1)

20. Revision History • Revision History • v1.00, 10/16/2003 11:29 AM, sps Initial revision. • v1.01, 1/21/2004 12:56 PM, sps Filehandle stuff added. • v1.02, 4/14/2004 9:12 AM, sps 20033 updates. • v1.03, 10/18/2004 9:17 AM, sps 20041 updates. -- v2.0, 1/17/2005, chr -- v2.1, 2/14/2007, chr added information re. Perl Standard Modules PLC - Perl I/O (v2.1)