# Thurs Sep 18

  * Questions?

  * Feedback on homework: coming.

  * Conditionals, Loops, Booleans : take 1.

# The IF statement 
if ( $ a == 7 ) {
  print "a is 7\n";
}

# Blocks:  { ... }
  * we don't need a ; after the block.
  * "my" variables defined in the block exist _only_ in the block.
  * inside the () is effectivly also inside the block.

# "Boolean" values in perl:         ("What is truth?")
  * (), 0, "", undefined  => false
  * anything else         => true

# numeric vs string comparison operators
  * <, <=, >= ==, !=     : numbers
  * lt, le, eq, ne       : strings

# ELSE and ELSIF  (yes, that's how it's spelled.)
my $DEBUG   = 1;     # true in perl
my $VERBOSE = 0;     # false in 
if ($DEBUG){
  do something here;
}
elsif ($VERBOSE){
  or this
}
else {
  default
}

#ONE LINER
# if the block is on one, perl let's you stick the "if" 
# on the end, like this:
print "hi" if $DEBUG;

# same as
if ($DEBUG){
  print "hi";
}

# ------------------

discuss boolean AND, OR, NOT operators.
also &&, ||, ! with tighter precedence.

  - logical operations
  - lazy evaluation
  - parens for grouping

$a = $bar or 7;    # compare these two in the debugger for various values of $bar.
$a = $bar || 7;

# --- looping

while (expression){
  # loop
}

# first "for" loop
foreach my $loop_variable (@list){
  # set $loop_variable to each item of the list in turn.
  # Changing it DOES change @list!
}

next;      # skip to next loop 
last;      # leave loop entirely

# second "for" loop
for (my $i=0; $i<10; $i++){
  # This is just like C.
}

# The 2 are actually the same.  And the variable is optional.
for (1..10){  print "Hi.\n"; }

# "range" operator  1..10 or even "a".."z".

# do primes program


# Common idioms

foreach my $i (1..10){   # but be aware it creates an array of 10 numbers.
}

my @list = ('one', 'two', 'three');
while (@list){
  my $word = shift @list;
  # do something with the $word
}


# or even
my @list = ('one', 'two', 'three');
while (my $word = shift @list){
  # when the list is empty, $word is undefined and therefore false.
}

# Tertiary operator ( $condition ? $if_true : $if_false )
print $alive ? "yup" : "nope";


# A single "line" can get pretty tricky.
# Can you see what the perl parse does with this one?

@list = split //, "Here we go.";
$x = shift @list, print defined $x ? "yup, x is $x\n" : "nope\n" for 1..20;