Advanced Perl Programming

use English; # Import the module file called English.pm

# Try deleting a non-existent file

unlink ('/tmp/foo');

if ($ERRNO) { # Use $ERRNO instead of $!

print $ERRNO, "\n"; # Prints "No such file or directory" }

(We'll cover packages and the use statement in Chapter 6.) I think these well-known names should have been in there from the very beginning, instead of having to memorize funny-looking variables and an accompanying list of mnemonics. Some argue that you can use the same scheme for other languages ("use Dutch;"), but considering that the other system calls are in English anyway, I think that there's no point providing specific aliases for a small subset of the things you have to remember.

3.2.3 Aliasing Problem: Variable Suicide

Aliases, combined with the fact that local doesn't really create new variables (it temporarily slaps a new value onto a global variable), often leads to weird values of variables that presumably haven't even been touched. Consider the following simple case:

$x = 10; foo(*x); sub foo {

local(*y) = @_;

print "Before value of y : $y \n";

local($x) = 100;

print "After value of y : $y \n"; }

This prints the following:

Before value of y : 10 After value of y : 100

Can you resolve the mystery? Clearly, $y has not been touched between the two print statements, but its value seems to have changed. Hint: it reflects that of $x.

Let's trace the events in sequence: $x = 10;

local *y = *x; print "before value"

# Assign a value to global $x

# function called

# Save global *y's values. Alias it to *x

# Because of the alias, $y is the same as $x,

# hence this prints 10

# IMPORTANT: local saves $x's value (10)

# and substitutes 100. Note that it does not

# create a new $x variable

# replaced by 100

# But *y is still aliased to *x. Therefore,

# $y now contains 100

local $x = 100;

print "after value";



	use English; # Import the module file called English.pm # Try deleting a non-existent file unlink ('/tmp/foo'); if ($ERRNO) { # Use $ERRNO instead of $! print $ERRNO, "\n"; # Prints "No such file or directory" } (We'll cover packages and the use statement in Chapter 6.) I think these well-known names should have been in there from the very beginning, instead of having to memorize funny-looking variables and an accompanying list of mnemonics. Some argue that you can use the same scheme for other languages ("use Dutch;"), but considering that the other system calls are in English anyway, I think that there's no point providing specific aliases for a small subset of the things you have to remember. 3.2.3 Aliasing Problem: Variable Suicide Aliases, combined with the fact that local doesn't really create new variables (it temporarily slaps a new value onto a global variable), often leads to weird values of variables that presumably haven't even been touched. Consider the following simple case: $x = 10; foo(x); sub foo { local(y) = @_; print "Before value of y : $y \n"; local($x) = 100; print "After value of y : $y \n"; } This prints the following: Before value of y : 10 After value of y : 100 Can you resolve the mystery? Clearly, $y has not been touched between the two print statements, but its value seems to have changed. Hint: it reflects that of $x.

	Let's trace the events in sequence: $x = 10; local y = x; print "before value"	# Assign a value to global $x # function called # Save global y's values. Alias it to x # Because of the alias, $y is the same as $x, # hence this prints 10 # IMPORTANT: local saves $x's value (10) # and substitutes 100. Note that it does not # create a new $x variable # replaced by 100 # But y is still aliased to x. Therefore, # $y now contains 100
	local $x = 100;
	print "after value";