X-Git-Url: http://wagnertech.de/gitweb/gitweb.cgi/mfinanz.git/blobdiff_plain/00ad7b5631ca2557bd02a592b4598699e47edbbd..67978a2d5a19e3d2af10a476fb9e0306dda118fc:/doc/modules/README.List-UtilsBy?ds=inline diff --git a/doc/modules/README.List-UtilsBy b/doc/modules/README.List-UtilsBy new file mode 100644 index 000000000..efdceb310 --- /dev/null +++ b/doc/modules/README.List-UtilsBy @@ -0,0 +1,238 @@ +NAME + `List::UtilsBy' - higher-order list utility functions + +SYNOPSIS + use List::UtilsBy qw( nsort_by min_by ); + + use File::stat qw( stat ); + my @files_by_age = nsort_by { stat($_)->mtime } @files; + + my $shortest_name = min_by { length } @names; + +DESCRIPTION + This module provides a number of list utility functions, all of which + take an initial code block to control their behaviour. They are + variations on similar core perl or `List::Util' functions of similar + names, but which use the block to control their behaviour. For example, + the core Perl function `sort' takes a list of values and returns them, + sorted into order by their string value. The `sort_by' function sorts + them according to the string value returned by the extra function, when + given each value. + + my @names_sorted = sort @names; + + my @people_sorted = sort_by { $_->name } @people; + +FUNCTIONS + @vals = sort_by { KEYFUNC } @vals + Returns the list of values sorted according to the string values + returned by the `KEYFUNC' block or function. A typical use of this may + be to sort objects according to the string value of some accessor, such + as + + sort_by { $_->name } @people + + The key function is called in scalar context, being passed each value in + turn as both `$_' and the only argument in the parameters, `@_'. The + values are then sorted according to string comparisons on the values + returned. + + This is equivalent to + + sort { $a->name cmp $b->name } @people + + except that it guarantees the `name' accessor will be executed only once + per value. + + One interesting use-case is to sort strings which may have numbers + embedded in them "naturally", rather than lexically. + + sort_by { s/(\d+)/sprintf "%09d", $1/eg; $_ } @strings + + This sorts strings by generating sort keys which zero-pad the embedded + numbers to some level (9 digits in this case), helping to ensure the + lexical sort puts them in the correct order. + + @vals = nsort_by { KEYFUNC } @vals + Similar to `sort_by' but compares its key values numerically. + + @vals = rev_sort_by { KEYFUNC } @vals + @vals = rev_nsort_by { KEYFUNC } @vals + Similar to `sort_by' and `nsort_by' but returns the list in the reverse + order. Equivalent to + + @vals = reverse sort_by { KEYFUNC } @vals + + except that these functions are slightly more efficient because they + avoid the final `reverse' operation. + + $optimal = max_by { KEYFUNC } @vals + @optimal = max_by { KEYFUNC } @vals + Returns the (first) value from `@vals' that gives the numerically + largest result from the key function. + + my $tallest = max_by { $_->height } @people + + use File::stat qw( stat ); + my $newest = max_by { stat($_)->mtime } @files; + + In scalar context, the first maximal value is returned. In list context, + a list of all the maximal values is returned. This may be used to obtain + positions other than the first, if order is significant. + + If called on an empty list, an empty list is returned. + + For symmetry with the `nsort_by' function, this is also provided under + the name `nmax_by' since it behaves numerically. + + $optimal = min_by { KEYFUNC } @vals + @optimal = min_by { KEYFUNC } @vals + Similar to `max_by' but returns values which give the numerically + smallest result from the key function. Also provided as `nmin_by' + + @vals = uniq_by { KEYFUNC } @vals + Returns a list of the subset of values for which the key function block + returns unique values. The first value yielding a particular key is + chosen, subsequent values are rejected. + + my @some_fruit = uniq_by { $_->colour } @fruit; + + To select instead the last value per key, reverse the input list. If the + order of the results is significant, don't forget to reverse the result + as well: + + my @some_fruit = reverse uniq_by { $_->colour } reverse @fruit; + + %parts = partition_by { KEYFUNC } @vals + Returns a key/value list of ARRAY refs containing all the original + values distributed according to the result of the key function block. + Each value will be an ARRAY ref containing all the values which returned + the string from the key function, in their original order. + + my %balls_by_colour = partition_by { $_->colour } @balls; + + Because the values returned by the key function are used as hash keys, + they ought to either be strings, or at least well-behaved as strings + (such as numbers, or object references which overload stringification in + a suitable manner). + + %counts = count_by { KEYFUNC } @vals + Returns a key/value list of integers, giving the number of times the key + function block returned the key, for each value in the list. + + my %count_of_balls = count_by { $_->colour } @balls; + + Because the values returned by the key function are used as hash keys, + they ought to either be strings, or at least well-behaved as strings + (such as numbers, or object references which overload stringification in + a suitable manner). + + @vals = zip_by { ITEMFUNC } \@arr0, \@arr1, \@arr2,... + Returns a list of each of the values returned by the function block, + when invoked with values from across each each of the given ARRAY + references. Each value in the returned list will be the result of the + function having been invoked with arguments at that position, from + across each of the arrays given. + + my @transposition = zip_by { [ @_ ] } @matrix; + + my @names = zip_by { "$_[1], $_[0]" } \@firstnames, \@surnames; + + print zip_by { "$_[0] => $_[1]\n" } [ keys %hash ], [ values %hash ]; + + If some of the arrays are shorter than others, the function will behave + as if they had `undef' in the trailing positions. The following two + lines are equivalent: + + zip_by { f(@_) } [ 1, 2, 3 ], [ "a", "b" ] + f( 1, "a" ), f( 2, "b" ), f( 3, undef ) + + The item function is called by `map', so if it returns a list, the + entire list is included in the result. This can be useful for example, + for generating a hash from two separate lists of keys and values + + my %nums = zip_by { @_ } [qw( one two three )], [ 1, 2, 3 ]; + # %nums = ( one => 1, two => 2, three => 3 ) + + (A function having this behaviour is sometimes called `zipWith', e.g. in + Haskell, but that name would not fit the naming scheme used by this + module). + + $arr0, $arr1, $arr2, ... = unzip_by { ITEMFUNC } @vals + Returns a list of ARRAY references containing the values returned by the + function block, when invoked for each of the values given in the input + list. Each of the returned ARRAY references will contain the values + returned at that corresponding position by the function block. That is, + the first returned ARRAY reference will contain all the values returned + in the first position by the function block, the second will contain all + the values from the second position, and so on. + + my ( $firstnames, $lastnames ) = unzip_by { m/^(.*?) (.*)$/ } @names; + + If the function returns lists of differing lengths, the result will be + padded with `undef' in the missing elements. + + This function is an inverse of `zip_by', if given a corresponding + inverse function. + + @vals = extract_by { SELECTFUNC } @arr + Removes elements from the referenced array on which the selection + function returns true, and returns a list containing those elements. + This function is similar to `grep', except that it modifies the + referenced array to remove the selected values from it, leaving only the + unselected ones. + + my @red_balls = extract_by { $_->color eq "red" } @balls; + + # Now there are no red balls in the @balls array + + This function modifies a real array, unlike most of the other functions + in this module. Because of this, it requires a real array, not just a + list. + + This function is implemented by invoking `splice()' on the array, not by + constructing a new list and assigning it. One result of this is that + weak references will not be disturbed. + + extract_by { !defined $_ } @refs; + + will leave weak references weakened in the `@refs' array, whereas + + @refs = grep { defined $_ } @refs; + + will strengthen them all again. + + @vals = weighted_shuffle_by { WEIGHTFUNC } @vals + Returns the list of values shuffled into a random order. The + randomisation is not uniform, but weighted by the value returned by the + `WEIGHTFUNC'. The probabilty of each item being returned first will be + distributed with the distribution of the weights, and so on recursively + for the remaining items. + + @vals = bundle_by { BLOCKFUNC } $number, @vals + Similar to a regular `map' functional, returns a list of the values + returned by `BLOCKFUNC'. Values from the input list are given to the + block function in bundles of `$number'. + + If given a list of values whose length does not evenly divide by + `$number', the final call will be passed fewer elements than the others. + +TODO + * XS implementations + These functions are currently all written in pure perl. Some at + least, may benefit from having XS implementations to speed up their + logic. + + * Merge into List::Util or List::MoreUtils + This module shouldn't really exist. The functions should instead be + part of one of the existing modules that already contain many list + utility functions. Having Yet Another List Utilty Module just + worsens the problem. + + I have attempted to contact the authors of both of the above + modules, to no avail; therefore I decided it best to write and + release this code here anyway so that it is at least on CPAN. Once + there, we can then see how best to merge it into an existing module. + +AUTHOR + Paul Evans