update the octal situation in perlfaq4.pod

[p5sagit/p5-mst-13.2.git] / pod / perlfaq4.pod

diff --git a/pod/perlfaq4.pod b/pod/perlfaq4.pod
index 87fd66a..714bc08 100644 (file)
--- a/pod/perlfaq4.pod
+++ b/pod/perlfaq4.pod
@@ -39,7 +39,7 @@ arbitrary-precision decimal numbers with the Math::BigFloat module
 (part of the standard Perl distribution), but mathematical operations
 are consequently slower.
 
-If precision is important, such as when dealing with money, its good
+If precision is important, such as when dealing with money, it's good
 to work with integers and then divide at the last possible moment.
 For example, work in pennies (1995) instead of dollars and cents
 (19.95) and divide by 100 at the end.
@@ -51,19 +51,36 @@ See L<perlop/"Floating-point Arithmetic">.
 =head2 Why isn't my octal data interpreted correctly?
 
 Perl only understands octal and hex numbers as such when they occur
-as literals in your program.  If they are read in from somewhere and
-assigned, no automatic conversion takes place.  You must explicitly
-use oct() or hex() if you want the values converted.  oct() interprets
+as literals in your program.  Octal literals in perl must start with 
+a leading "0" and hexadecimal literals must start with a leading "0x".
+If they are read in from somewhere and assigned, no automatic 
+conversion takes place.  You must explicitly use oct() or hex() if you 
+want the values converted to decimal.  oct() interprets
 both hex ("0x350") numbers and octal ones ("0350" or even without the
 leading "0", like "377"), while hex() only converts hexadecimal ones,
 with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
+The inverse mapping from decimal to octal can be done with either the
+"%o" or "%O" sprintf() formats.  To get from decimal to hex try either 
+the "%x" or the "%X" formats to sprintf().
 
 This problem shows up most often when people try using chmod(), mkdir(),
-umask(), or sysopen(), which all want permissions in octal.
+umask(), or sysopen(), which by widespread tradition typically take 
+permissions in octal.
 
-    chmod(644,  $file);        # WRONG -- perl -w catches this
+    chmod(644,  $file);        # WRONG
     chmod(0644, $file);        # right
 
+Note the mistake in the first line was specifying the decimal literal 
+644, rather than the intended octal literal 0644.  The problem can
+be seen with:
+
+    printf(0"%o",644); # prints 01204
+
+Surely you had not intended C<chmod(01204, $file);> - did you?  If you
+want to use numeric literals as arguments to chmod() et al. then please
+try to express them as octal constants, that is with a leading zero and 
+with the following digits restricted to the set 0..7.
+
 =head2 Does Perl have a round() function?  What about ceil() and floor()?  Trig functions?
 
 Remember that int() merely truncates toward 0.  For rounding to a
@@ -1018,7 +1035,7 @@ Like (d), but @in contains only small positive integers:
 
 But perhaps you should have been using a hash all along, eh?
 
-=head2 How can I tell whether a list or array contains a certain element?
+=head2 How can I tell whether a certain element is contained in a list or array?
 
 Hearing the word "in" is an I<in>dication that you probably should have
 used a hash, not a list or array, to store your data.  Hashes are