perlfaq4

Current Perl documentation can be found at perldoc.perl.org.

Here is our local, out-dated (pre-5.6) version:

Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
Why isn't my octal data interpreted correctly?
Does perl have a round function? What about ceil() and floor()? Trig functions?
How do I convert bits into ints?
How do I multiply matrices?
How do I perform an operation on a series of integers?
How can I output Roman numerals?
Why aren't my random numbers random?
Why aren't my random numbers random?

How do I find the week-of-the-year/day-of-the-year?
How can I compare two dates and find the difference?
How can I take a string and turn it into epoch seconds?
How can I find the Julian Day?
Does Perl have a year 2000 problem? Is Perl Y2K compliant?

How do I validate input?
How do I unescape a string?
How do I remove consecutive pairs of characters?
How do I expand function calls in a string?
How do I find matching/nesting anything?
How do I reverse a string?
How do I expand tabs in a string?
How do I reformat a paragraph?
How can I access/change the first N letters of a string?
How do I change the Nth occurrence of something?
How can I count the number of occurrences of a substring within a string?
How do I capitalize all the words on one line?
How do I strip blank space from the beginning/end of a string?
How do I extract selected columns from a string?
How do I find the soundex value of a string?
How can I expand variables in text strings?
What's wrong with always quoting "$vars"?
Why don't my <<HERE documents work?

What is the difference between $array[1] and @array[1]?
How can I extract just the unique elements of an array?
How can I tell whether a list or array contains a certain element?
How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
How do I find the first array element for which a condition is true?
How do I handle linked lists?
How do I handle circular lists?
How do I shuffle an array randomly?
How do I process/modify each element of an array?
How do I select a random element from an array?
How do I permute N elements of a list?
How do I sort an array by (anything)?
How do I manipulate arrays of bits?
Why does defined() return true on empty arrays and hashes?
How do I process an entire hash?

What happens if I add or remove keys from a hash while iterating over it?
How do I look up a hash element by value?
How can I know how many entries are in a hash?
How do I sort a hash (optionally by value instead of key)?
How can I always keep my hash sorted?
What's the difference between "delete" and "undef" with hashes?
Why don't my tied hashes make the defined/exists distinction?
How do I reset an each() operation part-way through?
How can I get the unique keys from two hashes?
How can I store a multidimensional array in a DBM file?
How can I make my hash remember the order I put elements into it?
Why does passing a subroutine an undefined element in a hash create it?
How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
How can I use a reference as a hash key?
How can I use a reference as a hash key?

How do I handle binary data correctly?
How do I determine whether a scalar is a number/whole/integer/float?
How do I keep persistent data across program calls?
How do I print out or copy a recursive data structure?
How do I define methods for every class/object?
How do I verify a credit card checksum?