=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.17 $, $Date: 1997/03/25 18:16:24 $)
+perlfaq4 - Data Manipulation ($Revision: 1.19 $, $Date: 1997/04/24 22:43:57 $)
=head1 DESCRIPTION
=head1 Data: Numbers
+=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
+
+Internally, your computer represents floating-point numbers in binary.
+Floating-point numbers read in from a file, or appearing as literals
+in your program, are converted from their decimal floating-point
+representation (eg, 19.95) to the internal binary representation.
+
+However, 19.95 can't be precisely represented as a binary
+floating-point number, just like 1/3 can't be exactly represented as a
+decimal floating-point number. The computer's binary representation
+of 19.95, therefore, isn't exactly 19.95.
+
+When a floating-point number gets printed, the binary floating-point
+representation is converted back to decimal. These decimal numbers
+are displayed in either the format you specify with printf(), or the
+current output format for numbers (see L<perlvar/"$#"> if you use
+print. C<$#> has a different default value in Perl5 than it did in
+Perl4. Changing C<$#> yourself is deprecated.
+
+This affects B<all> computer languages that represent decimal
+floating-point numbers in binary, not just Perl. Perl provides
+arbitrary-precision decimal numbers with the Math::BigFloat module
+(part of the standard Perl distribution), but mathematical operations
+are consequently slower.
+
+To get rid of the superfluous digits, just use a format (eg,
+C<printf("%.2f", 19.95)>) to get the required precision.
+
=head2 Why isn't my octal data interpreted correctly?
Perl only understands octal and hex numbers as such when they occur
ceil(), floor(), and a number of other mathematical and trigonometric
functions.
-The Math::Complex module (part of the standard perl distribution)
-defines a number of mathematical functions that can also work on real
-numbers. It's not as efficient as the POSIX library, but the POSIX
-library can't work with complex numbers.
+In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex
+module. With 5.004, the Math::Trig module (part of the standard perl
+distribution) implements the trigonometric functions. Internally it
+uses the Math::Complex module and some functions can break out from
+the real axis into the complex plane, for example the inverse sine of
+2.
Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely. In these
print "That yields ${\($n + 5)} widgets\n";
+See also "How can I expand variables in text strings?" in this section
+of the FAQ.
+
=head2 How do I find matching/nesting anything?
This isn't something that can be tackled in one regular expression, no
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);
+The paragraphs you give to Text::Wrap may not contain embedded
+newlines. Text::Wrap doesn't justify the lines (flush-right).
+
=head2 How can I access/change the first N letters of a string?
There are many ways. If you just want to grab a copy, use
$string = "ThisXlineXhasXsomeXx'sXinXit":
$count = ($string =~ tr/X//);
- print "There are $count X characters in the string";
+ print "There are $count X charcters in the string";
This is fine if you are just looking for a single character. However,
if you are trying to count multiple character substrings within a
$line =~ s/\b(\w)/\U$1/g;
+This has the strange effect of turning "C<don't do it>" into "C<Don'T
+Do It>". Sometimes you might want this, instead (Suggested by Brian
+Foy E<lt>comdog@computerdog.comE<gt>):
+
+ $string =~ s/ (
+ (^\w) #at the beginning of the line
+ | # or
+ (\s\w) #preceded by whitespace
+ )
+ /\U$1/xg;
+ $string =~ /([\w']+)/\u\L$1/g;
+
To make the whole line upper case:
$line = uc($line);
}gx;
push(@new, undef) if substr($text,-1,1) eq ',';
+If you want to represent quotation marks inside a
+quotation-mark-delimited field, escape them with backslashes (eg,
+C<"like \"this\""). Unescaping them is a task addressed earlier in
+this section.
+
Alternatively, the Text::ParseWords module (part of the standard perl
distribution) lets you say:
Which is bizarre enough that you'll probably actually need an EEG
afterwards. :-)
+See also "How do I expand function calls in a string?" in this section
+of the FAQ.
+
=head2 What's wrong with always quoting "$vars"?
The problem is that those double-quotes force stringification,
@sorted = map { $_->[0] }
sort { $a->[1] cmp $b->[1] }
- map { [ $_, uc((/\d+\s*(\S+) )[0] ] } @data;
+ map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data;
If you need to sort on several fields, the following paradigm is useful.
Using C<keys %hash> in a scalar context returns the number of keys in
the hash I<and> resets the iterator associated with the hash. You may
need to do this if you use C<last> to exit a loop early so that when you
-reenter it, the hash iterator has been reset.
+re-enter it, the hash iterator has been reset.
=head2 How can I get the unique keys from two hashes?
Use the Tie::IxHash from CPAN.
+ use Tie::IxHash;
+ tie(%myhash, Tie::IxHash);
+ for ($i=0; $i<20; $i++) {
+ $myhash{$i} = 2*$i;
+ }
+ @keys = keys %myhash;
+ # @keys = (0,1,2,3,...)
+
=head2 Why does passing a subroutine an undefined element in a hash create it?
If you say something like:
I<not> cause that key to be forever there. This is different than
awk's behavior.
-=head2 How can I make the Perl equivalent of a C structure/C++ class/hash
-or array of hashes or arrays?
+=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
Use references (documented in L<perlref>). Examples of complex data
structures are given in L<perldsc> and L<perllol>. Examples of
Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved. See L<perlfaq> for distribution information.
+
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