Scripting Languages Chapter 6 I/O Basics

Input from STDIN We’ve been doing so with $line = chomp($line); Same as chomp($line= ); line input op gives you an undef value when you reach eof – handy for dropping out of loops while (defined($line = )) { print “I saw $line”; }

Shortcut We’re reading the input into a variable – checking to see if it’s reached the eof (meaning its defined) – if its defined we run the while loop. Inside the loop – we’ll see each line one after another. Since this is something that is done quite often – Perl has a shortcut. while ( ) { print “I saw $_”; } This only works if you put a line-input operator in the condition of the while loop.

List context The previous examples were evaluated in scalar context Now for list context: foreach ( ) { print “I saw $_”; } gives you all the remaining lines as a list – each element of the list is one line

What’s the difference A while loop in Perl reads a line of input – puts it into a variable – runs the body of the loop. Then it goes back to fine another line of input. A foreach loop --- the line-input operator is being used in a list context – foreach needs a list to iterate through – so it has to read all the input before the loop starts running. For large files – its best to use the while loop for better performance.

Input from Diamond Operator is another way to read input it uses invocation arguments to accept input invocation arguments are command line arguments Exp: –$./myprogram.pl x y z –means to run myprogram and it should process file x followed by file y and then file z –If you give it no invocation args – program should process the standard input stream or –x means stdin as well.

Benefits you can choose where the program gets its input at run time exp: you won’t have to rewrite the program to use it in a pipeline ( more later). makes it easy for you to write your programs that work like standard Unix utilities – even on non-Unix machines. portability of code

Exp: while ( defined($line = <>)){ chomp ($line); print “It was $line that I saw \n”; } run this with x, y, z invocation args – It was [a line from file x] that I saw -- eof then y – then z -- no break with using the <> its as if all the files are merged into one.

Same Shortcut may also use the same shortcut: while (<>){ chomp; print “It was $_ that I saw!\n”; } Typically used for all of your input – mistake to use it in more than one place in your program.

Unix Invocation Arguments Typically, the diamond operator isn’t looking at the invocation args – its works with array. It is a special array – preset by Perl interpreter to be a list of command line args. When program is stuffed full of the list of invocation args. Use it like any other array – shift things off or foreach through it. If list is empty -- <> looks uses STDIN stream – otherwise it uses contents of that array

Tinker Exp: process three specific files – regardless of user choice at = qw! Monday Tuesday Wednesday !; while (<>) { chomp; print “It is $_ \n”; }

Output to Standard Output print operator takes a list of values and sends them – as a string – to stdout – one after another. doesn’t add any extra characters before, after or in between by default. perlvar man page gives you more information about changing defaults if you want spaces between items and a newline at the end – you have to add them

Output - STDOUT $name = “Bob Barker”; print “Hello there, $name, did you know that 4*5 is “, 4*5, “?\n”; difference between printing an array and interpolating an array: #print a list of items print #print a string (containing and interpolated array)

Print Statment First print statement -- one item after another no spaces Second statement will print exactly one item, which is the string you get by into the empty string – the contents of the array separated by spaces.

Print Cont’d But what is a list of unchomped lines of input? Contain trailing newline character First print statement – Monday Tuesday Wednesday Second one: Monday Tuesday Wednesay it’s interpolating the array so it puts spaces between elements – this is what happens when you put an array in double quotes

So …. So – if your strings contain newlines no newlines print

Print Cont’d print is looking for a list of strings to print its arguments are evaluated in list context <> will return a list of lines in list context so: print ; # similar to the cat command print sort <>; #similar to Unix sort command

Revisit Parenthesis print (“Hello, World\n”); print “Hello, World\n”; if the invocation of print looks like a function call, then it is a function call print (5*4); #prints 20 but also contains a return value – true / false usually always succeeds unless I/O error

Return Value Cont’d print (5*4) +3; error prints 20 but return value is 1 – is 4 – logic error. If it looks like a function – it is a function applies to all Perl functions

Formatted Output with printf C’s printf is similar to Perl’s printf take a format string followed b a list of things to print the format string is a fill-in-the-blanks template with the desired form of output. printf “Hello, %s; your password expires in %d days!\n”, $user, $days_to_expire;

printf conversions begin with % should be the same number of items in the list as there are conversions don’t match – don’t work common conversions: –%s – string, %g – auto chooses int, fp or exp –%d – decimal point – value is truncated not rounded

printf Cont’d field width: –printf “%6d\n”, 42 # –printf “%2d\n”, 2e ; #2001 %s – string it interpolates the given value as a string printf “10%s\n”, “perl”; # perl negative field left justifies printf “-15%s\n”, “perl”; #perl

%f rounds off its output as needed even lets you request number of digits after the decimal pt. printf “%12f\n”, 6*7 + 2/3; # printf “%12.3f\n”, 6*7 + 2/3; # printf “%12.0f\n”, 6*7 + 2/3; # to print a real % sign use %

Arrays and printf You won’t use an array as an argument to printf = qw( soap shampoo conditioner); my $format = “The items are: \n”. (“%10s\n” printf This uses the x operator to replicate the given string a number of times given That’s 3 in this case – look at our array values output prints item on its own line – right justified in a ten character column We once in a list context and once in a scalar context

/exercises Perform 2, 3 exercises on page 97