#######################################################
#
# display-notes related to Intro to Perl, Class 1
#
#######################################################

# last modified: 4-6-03


#-------------------------------------------------------------
# what version of Perl are you running?

# on a UNIX system, you should be able to find out by typing:

perl --version

# note that redwood is running 5.005 (!!), and sorrel is
# running 5.8.0 --- we'll be using sorrel for this course!


#---------------------------------------------------------
# under what kind of name should you save a Perl program?

    # under any name you wish;

    # in text's opinion --- including NO extension/suffix is preferable;

        # (BUT --- note that some non-UNIX versions require .plx as
          an extension/suffix)

#----------------------------------------------------------
# how do you make a file executable?

    # varies --- but, in UNIX:

    # chmod 700 filename
        # IF you do not anyone else to read/write/execute it;

    # chmod 755 filename
        # IF you'd like others to be able to read and execute it, also;

#------------------------------------------------------------
# what do you put IN this file?

#--------------------------------------------------------------
# a TYPICAL Perl program should start with:

#!/usr/bin/perl

    # ...on a UNIX system, where the perl executable is stored (or
      has a symbolic link to) /usr/bin/perl, as it OFTEN does;

    # (as it must on redwood and sorrel, for this DOES work in both!)

    # non-UNIX-note: text suggests that it is traditional/useful to
      use:

#!perl

      ...in a Perl script on a non-UNIX system, if at all possible;
      this clues in a future maintenance programmer about what's IN 
      there...!

#-------------------------------------------------------------------
# see hello1
#-------------------------------------------------------------------

# by the way, Perl statements should end with semicolons....

#-------------------------------------------------------------------
# a COMMENT

# everything from # to the end of the line is ignored

# (there are no multi-line comments in Perl...)

#---------------------------------------------------------------------
# scalar - a single value

#--------------------------------------------------------------------
# numbers

# all are stored underneath as floating point numbers
# (as C double values)

# large integers: CAN use underscores to make 'em more readable!
# 1_000_000 for 1 million

# integers and floats: are just what you think they are...

# see hello2 

#-------------------------------------------------------------------
# operations

# the usual!

# also % - remainder/modulus

# also ** - exponentiation

# also ++, --

# also +=, -=, everything =

#--------------------------------------------------------------------
# strings

# single-quoted string literals

# pretty much wysiwyg... (what you see is what you get)

# EXCEPT: type \' to get a single quote

#         type \\ to get a single backspace

'hello'
'Hello world!'
'Jane\'s Airplances'
'c:\\personal'

# double-quoted string literals

# now backslash has its usual power (\n is newline, \t is tab,
# see p. 24 for more)

# AND: when you put a variable inside, its VALUE appears int the string!

# (more on that in a minute...)

# string concatenation: the character .

# "hello" . 'world'

# see hello3

# string repetition: lowecase x

"fred" x 3

# see hello3

#---------------------------------------------------------------
# Perl is not picky about many things....

# ...converts between numbers and strings as needed!

"12" * 6

"12fred34" * "   3"  evaluates to 36

"fred" * 3

#---------------------------------------------------------------
# warnings:

perl -w name_of_program

OR inside script:

#!/usr/bin/perl -w

#----------------------------------------------------------------
# scalar variables

# a name starting with a $ (dollar sign)

$fred =5;

# see hello4

# remember: to see variable's VALUE inside a string: use DOUBLE quotes
# to use the variable name itself in a string: use SINGLE quotes

# but, yes, ++, --, +=, etc. do work with variables.

# what if you want a variable's value to be "surrounded" by non-blank
# characters??

print "I want $freds right now!\n";

# above will NOT work --- Perl thinks it is an undefined variable
# $freds

# put {} around the variable name to let Perl know THAT's the name:

print "I want ${fred}s right now!\n"; 

#-----------------------------------------------------------
# operator precedence in Perl - see p. 32

#-----------------------------------------------------------
# comparison

# for numbers: just what you think 
# note: == for equal,
#       != for not equal

# for strings: NOT the above!!!!!
# lt le eq ge gt ne

$fred = "apple";
$ann = "pear";
$fred lt $ann

# boolean operators: && (and) || (or)

#----------------------------------------------------------
# boolean values

# the following are all considered to be false
undef 0 '' (the empty string) '0'

# just about eveything else is considered to be true

#-----------------------------------------------------------
# if

if (cond)
{
    action1;
}
else
{
    action2;
}

# there is an elsif that you can put in here...

#-------------------------------------------------------
# while

while (cond)
{
    action;
}

# for is there, too and DOES behave like you expect.

#--------------------------------------------------------
# user input

$fred = <STDIN>;

...reads the next line of input into variable $fred
... NOTE: this NORMALLY includes a newline character!!!

if you don't WANT it --- you chomp it off!

chomp($fred);   # the newline is chopped off, IF it exists.

you COULD say:
print "type something";
$fred = <STDIN>;
chomp($fred);

print "type something";
chomp($fred= <STDIN>);
print $fred x 3;