=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.15 $)
+perlfaq4 - Data Manipulation ($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
You have to keep track. For example, let's say you want
to change the fifth occurrence of "whoever" or "whomever"
-into "whosoever", case insensitively.
+into "whosoever" or "whomsoever", case insensitively.
$count = 0;
s{((whom?)ever)}{
=head2 How do I capitalize all the words on one line?
To make the first letter of each word upper case:
+
$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);
To force each word to be lower case, with the first letter upper case:
+
$line =~ s/(\w+)/\u\L$1/g;
=head2 How can I split a [character] delimited string except when inside
}gx;
push(@new, undef) if substr($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.
If you mean how many keys, then all you have to do is
take the scalar sense of the keys() function:
- $num_keys = scalar keys %hash;
+ $num_keys = scalar keys %hash;
In void context it just resets the iterator, which is faster
for tied hashes.
Use the Tie::IxHash from CPAN.
+ use Tie::IxHash;
+ tie(%myhash, Tie::IxHash);
+ for ($i=0; $i<20; $i++) {
+ $myhash{$i} = 2*$i;
+ }
+ @keys = keys %myhash;
+ # @keys = (0,1,2,3,...)
+
=head2 Why does passing a subroutine an undefined element in a hash create it?
If you say something like:
I<not> cause that key to be forever there. This is different than
awk's behavior.
-=head2 How can I make the Perl equivalent of a C structure/C++ class/hash
-or array of hashes or arrays?
+=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
Use references (documented in L<perlref>). Examples of complex data
structures are given in L<perldsc> and L<perllol>. Examples of
If you're concerned about 8-bit ASCII data, then see L<perllocale>.
-If you want to deal with multi-byte characters, however, there are
+If you want to deal with multibyte characters, however, there are
some gotchas. See the section on Regular Expressions.
=head2 How do I determine whether a scalar is a number/whole/integer/float?
warn "has nondigits" if /\D/;
warn "not a whole number" unless /^\d+$/;
warn "not an integer" unless /^-?\d+$/; # reject +3
- warn "not an integer" unless /^[+-]?\d+$/;
+ warn "not an integer" unless /^[+-]?\d+$/;
warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
warn "not a C float"
Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved. See L<perlfaq> for distribution information.
+
projects / p5sagit/p5-mst-13.2.git / blobdiff