JunctionsTreatmentofanemptylist
There are two schools of thought for how to evaluate a junction on an empty list:
• Reduction to an identity (boolean)
• Result is undefined (three-valued)
In the first case, the result of the junction applied to the empty list is determined by a mathematical
reduction to an identity depending on whether the underlying comparison is "or" or "and". Conceptually:
"any are true" "all are true"
-------------- --------------
2 elements: A || B || 0 A && B && 1
1 element: A || 0 A && 1
0 elements: 0 1
In the second case, three-value logic is desired, in which a junction applied to an empty list returns
"undef" rather than true or false
Junctions with a "_u" suffix implement three-valued logic. Those without are boolean.
allBLOCKLISTall_uBLOCKLIST
Returns a true value if all items in LIST meet the criterion given through BLOCK. Sets $_ for each item
in LIST in turn:
print "All values are non-negative"
if all { $_ >= 0 } ($x, $y, $z);
For an empty LIST, "all" returns true (i.e. no values failed the condition) and "all_u" returns "undef".
Thus, "all_u(@list)" is equivalent to "@list ? all(@list) : undef".
Note: because Perl treats "undef" as false, you must check the return value of "all_u" with "defined" or
you will get the opposite result of what you expect.
anyBLOCKLISTany_uBLOCKLIST
Returns a true value if any item in LIST meets the criterion given through BLOCK. Sets $_ for each item
in LIST in turn:
print "At least one non-negative value"
if any { $_ >= 0 } ($x, $y, $z);
For an empty LIST, "any" returns false and "any_u" returns "undef".
Thus, "any_u(@list)" is equivalent to "@list ? any(@list) : undef".
noneBLOCKLISTnone_uBLOCKLIST
Logically the negation of "any". Returns a true value if no item in LIST meets the criterion given
through BLOCK. Sets $_ for each item in LIST in turn:
print "No non-negative values"
if none { $_ >= 0 } ($x, $y, $z);
For an empty LIST, "none" returns true (i.e. no values failed the condition) and "none_u" returns
"undef".
Thus, "none_u(@list)" is equivalent to "@list ? none(@list) : undef".
Note: because Perl treats "undef" as false, you must check the return value of "none_u" with "defined" or
you will get the opposite result of what you expect.
notallBLOCKLISTnotall_uBLOCKLIST
Logically the negation of "all". Returns a true value if not all items in LIST meet the criterion given
through BLOCK. Sets $_ for each item in LIST in turn:
print "Not all values are non-negative"
if notall { $_ >= 0 } ($x, $y, $z);
For an empty LIST, "notall" returns false and "notall_u" returns "undef".
Thus, "notall_u(@list)" is equivalent to "@list ? notall(@list) : undef".
oneBLOCKLISTone_uBLOCKLIST
Returns a true value if precisely one item in LIST meets the criterion given through BLOCK. Sets $_ for
each item in LIST in turn:
print "Precisely one value defined"
if one { defined($_) } @list;
Returns false otherwise.
For an empty LIST, "one" returns false and "one_u" returns "undef".
The expression "one BLOCK LIST" is almost equivalent to "1 == true BLOCK LIST", except for short-cutting.
Evaluation of BLOCK will immediately stop at the second true value.
TransformationapplyBLOCKLIST
Applies BLOCK to each item in LIST and returns a list of the values after BLOCK has been applied. In
scalar context, the last element is returned. This function is similar to "map" but will not modify the
elements of the input list:
my @list = (1 .. 4);
my @mult = apply { $_ *= 2 } @list;
print "\@list = @list\n";
print "\@mult = @mult\n";
__END__
@list = 1 2 3 4
@mult = 2 4 6 8
Think of it as syntactic sugar for
for (my @mult = @list) { $_ *= 2 }
insert_afterBLOCKVALUELIST
Inserts VALUE after the first item in LIST for which the criterion in BLOCK is true. Sets $_ for each
item in LIST in turn.
my @list = qw/This is a list/;
insert_after { $_ eq "a" } "longer" => @list;
print "@list";
__END__
This is a longer list
insert_after_stringSTRINGVALUELIST
Inserts VALUE after the first item in LIST which is equal to STRING.
my @list = qw/This is a list/;
insert_after_string "a", "longer" => @list;
print "@list";
__END__
This is a longer list
pairwiseBLOCKARRAY1ARRAY2
Evaluates BLOCK for each pair of elements in ARRAY1 and ARRAY2 and returns a new list consisting of
BLOCK's return values. The two elements are set to $a and $b. Note that those two are aliases to the
original value so changing them will modify the input arrays.
@a = (1 .. 5);
@b = (11 .. 15);
@x = pairwise { $a + $b } @a, @b; # returns 12, 14, 16, 18, 20
# mesh with pairwise
@a = qw/a b c/;
@b = qw/1 2 3/;
@x = pairwise { ($a, $b) } @a, @b; # returns a, 1, b, 2, c, 3
meshARRAY1ARRAY2[ARRAY3...]zipARRAY1ARRAY2[ARRAY3...]
Returns a list consisting of the first elements of each array, then the second, then the third, etc,
until all arrays are exhausted.
Examples:
@x = qw/a b c d/;
@y = qw/1 2 3 4/;
@z = mesh @x, @y; # returns a, 1, b, 2, c, 3, d, 4
@a = ('x');
@b = ('1', '2');
@c = qw/zip zap zot/;
@d = mesh @a, @b, @c; # x, 1, zip, undef, 2, zap, undef, undef, zot
"zip" is an alias for "mesh".
zip6zip_unflatten
Returns a list of arrays consisting of the first elements of each array, then the second, then the third,
etc, until all arrays are exhausted.
@x = qw/a b c d/;
@y = qw/1 2 3 4/;
@z = zip6 @x, @y; # returns [a, 1], [b, 2], [c, 3], [d, 4]
@a = ('x');
@b = ('1', '2');
@c = qw/zip zap zot/;
@d = zip6 @a, @b, @c; # [x, 1, zip], [undef, 2, zap], [undef, undef, zot]
"zip_unflatten" is an alias for "zip6".
listcmpARRAY0ARRAY1[ARRAY2...]
Returns an associative list of elements and every id of the list it was found in. Allows easy
implementation of @a & @b, @a | @b, @a ^ @b and so on. Undefined entries in any given array are skipped.
my @a = qw(one two three four five six seven eight nine ten eleven twelve thirteen);
my @b = qw(two three five seven eleven thirteen seventeen);
my @c = qw(one one two three five eight thirteen twentyone);
my %cmp = listcmp @a, @b, @c; # returns (one => [0, 2], two => [0, 1, 2], three => [0, 1, 2], four => [0], ...)
my @seq = (1, 2, 3);
my @prim = (undef, 2, 3, 5);
my @fib = (1, 1, 2);
my %cmp = listcmp @seq, @prim, @fib;
# returns ( 1 => [0, 2], 2 => [0, 1, 2], 3 => [0, 1], 5 => [1] )
arrayifyLIST[,LIST[,LIST...]]
Returns a list consisting of each element of given arrays. Recursive arrays are flattened, too.
@a = (1, [[2], 3], 4, [5], 6, [7], 8, 9);
@l = arrayify @a; # returns 1, 2, 3, 4, 5, 6, 7, 8, 9
uniqLISTdistinctLIST
Returns a new list by stripping duplicate values in LIST by comparing the values as hash keys, except
that undef is considered separate from ''. The order of elements in the returned list is the same as in
LIST. In scalar context, returns the number of unique elements in LIST.
my @x = uniq 1, 1, 2, 2, 3, 5, 3, 4; # returns 1 2 3 5 4
my $x = uniq 1, 1, 2, 2, 3, 5, 3, 4; # returns 5
# returns "Mike", "Michael", "Richard", "Rick"
my @n = distinct "Mike", "Michael", "Richard", "Rick", "Michael", "Rick"
# returns "A8", "", undef, "A5", "S1"
my @s = distinct "A8", "", undef, "A5", "S1", "A5", "A8"
# returns "Giulia", "Giulietta", undef, "", 156, "GTA", "GTV", 159, "Brera", "4C"
my @w = uniq "Giulia", "Giulietta", undef, "", 156, "GTA", "GTV", 159, "Brera", "4C", "Giulietta", "Giulia"
"distinct" is an alias for "uniq".
RT#49800 can be used to give feedback about this behavior.
singletonLIST
Returns a new list by stripping values in LIST occurring more than once by comparing the values as hash
keys, except that undef is considered separate from ''. The order of elements in the returned list is
the same as in LIST. In scalar context, returns the number of elements occurring only once in LIST.
my @x = singleton 1,1,2,2,3,4,5 # returns 3 4 5
duplicatesLIST
Returns a new list by stripping values in LIST occurring less than twice by comparing the values as hash
keys, except that undef is considered separate from ''. The order of elements in the returned list is
the same as in LIST. In scalar context, returns the number of elements occurring more than once in LIST.
my @y = duplicates 1,1,2,4,7,2,3,4,6,9; #returns 1,2,4
frequencyLIST
Returns an associative list of distinct values and the corresponding frequency.
my @f = frequency values %radio_nrw; # returns (
# 'Deutschlandfunk (DLF)' => 9, 'WDR 3' => 10,
# 'WDR 4' => 11, 'WDR 5' => 14, 'WDR Eins Live' => 14,
# 'Deutschlandradio Kultur' => 8,...)
occurrencesLIST
Returns a new list of frequencies and the corresponding values from LIST.
my @o = occurrences ((1) x 3, (2) x 4, (3) x 2, (4) x 7, (5) x 2, (6) x 4);
# @o = (undef, undef, [3, 5], [1], [2, 6], undef, undef, [4]);
modeLIST
Returns the modal value of LIST. In scalar context, just the modal value is returned, in list context all
probes occurring modal times are returned, too.
my @m = mode ((1) x 3, (2) x 4, (3) x 2, (4) x 7, (5) x 2, (6) x 4, (7) x 3, (8) x 7);
# @m = (7, 4, 8) - bimodal LIST
slideBLOCKLIST
The function "slide" operates on pairs of list elements like:
my @s = slide { "$a and $b" } (0..3);
# @s = ("0 and 1", "1 and 2", "2 and 3")
The idea behind this function is a kind of magnifying glass that is moved along a list and calls "BLOCK"
every time the next list item is reached.
PartitioningafterBLOCKLIST
Returns a list of the values of LIST after (and not including) the point where BLOCK returns a true
value. Sets $_ for each element in LIST in turn.
@x = after { $_ % 5 == 0 } (1..9); # returns 6, 7, 8, 9
after_inclBLOCKLIST
Same as "after" but also includes the element for which BLOCK is true.
beforeBLOCKLIST
Returns a list of values of LIST up to (and not including) the point where BLOCK returns a true value.
Sets $_ for each element in LIST in turn.
before_inclBLOCKLIST
Same as "before" but also includes the element for which BLOCK is true.
partBLOCKLIST
Partitions LIST based on the return value of BLOCK which denotes into which partition the current value
is put.
Returns a list of the partitions thusly created. Each partition created is a reference to an array.
my $i = 0;
my @part = part { $i++ % 2 } 1 .. 8; # returns [1, 3, 5, 7], [2, 4, 6, 8]
You can have a sparse list of partitions as well where non-set partitions will be undef:
my @part = part { 2 } 1 .. 10; # returns undef, undef, [ 1 .. 10 ]
Be careful with negative values, though:
my @part = part { -1 } 1 .. 10;
__END__
Modification of non-creatable array value attempted, subscript -1 ...
Negative values are only ok when they refer to a partition previously created:
my @idx = ( 0, 1, -1 );
my $i = 0;
my @part = part { $idx[$i++ % 3] } 1 .. 8; # [1, 4, 7], [2, 3, 5, 6, 8]
samplesCOUNTLIST
Returns a new list containing COUNT random samples from LIST. Is similar to "shuffle" in List::Util, but
stops after COUNT.
@r = samples 10, 1..10; # same as shuffle
@r2 = samples 5, 1..10; # gives 5 values from 1..10;
Iterationeach_arrayARRAY1ARRAY2...
Creates an array iterator to return the elements of the list of arrays ARRAY1, ARRAY2 throughout ARRAYn
in turn. That is, the first time it is called, it returns the first element of each array. The next
time, it returns the second elements. And so on, until all elements are exhausted.
This is useful for looping over more than one array at once:
my $ea = each_array(@a, @b, @c);
while ( my ($a, $b, $c) = $ea->() ) { .... }
The iterator returns the empty list when it reached the end of all arrays.
If the iterator is passed an argument of '"index"', then it returns the index of the last fetched set of
values, as a scalar.
each_arrayrefLIST
Like each_array, but the arguments are references to arrays, not the plain arrays.
natatimeEXPR,LIST
Creates an array iterator, for looping over an array in chunks of $n items at a time. (n at a time, get
it?). An example is probably a better explanation than I could give in words.
Example:
my @x = ('a' .. 'g');
my $it = natatime 3, @x;
while (my @vals = $it->())
{
print "@vals\n";
}
This prints
a b c
d e f
g
slideatatimeSTEP,WINDOW,LIST
Creates an array iterator, for looping over an array in chunks of "$windows-size" items at a time.
The idea behind this function is a kind of magnifying glass (finer controllable compared to "slide") that
is moved along a list.
Example:
my @x = ('a' .. 'g');
my $it = slideatatime 2, 3, @x;
while (my @vals = $it->())
{
print "@vals\n";
}
This prints
a b c
c d e
e f g
g
SearchingfirstvalBLOCKLISTfirst_valueBLOCKLIST
Returns the first element in LIST for which BLOCK evaluates to true. Each element of LIST is set to $_ in
turn. Returns "undef" if no such element has been found.
"first_value" is an alias for "firstval".
onlyvalBLOCKLISTonly_valueBLOCKLIST
Returns the only element in LIST for which BLOCK evaluates to true. Sets $_ for each item in LIST in
turn. Returns "undef" if no such element has been found.
"only_value" is an alias for "onlyval".
lastvalBLOCKLISTlast_valueBLOCKLIST
Returns the last value in LIST for which BLOCK evaluates to true. Each element of LIST is set to $_ in
turn. Returns "undef" if no such element has been found.
"last_value" is an alias for "lastval".
firstresBLOCKLISTfirst_resultBLOCKLIST
Returns the result of BLOCK for the first element in LIST for which BLOCK evaluates to true. Each element
of LIST is set to $_ in turn. Returns "undef" if no such element has been found.
"first_result" is an alias for "firstres".
onlyresBLOCKLISTonly_resultBLOCKLIST
Returns the result of BLOCK for the first element in LIST for which BLOCK evaluates to true. Sets $_ for
each item in LIST in turn. Returns "undef" if no such element has been found.
"only_result" is an alias for "onlyres".
lastresBLOCKLISTlast_resultBLOCKLIST
Returns the result of BLOCK for the last element in LIST for which BLOCK evaluates to true. Each element
of LIST is set to $_ in turn. Returns "undef" if no such element has been found.
"last_result" is an alias for "lastres".
indexesBLOCKLIST
Evaluates BLOCK for each element in LIST (assigned to $_) and returns a list of the indices of those
elements for which BLOCK returned a true value. This is just like "grep" only that it returns indices
instead of values:
@x = indexes { $_ % 2 == 0 } (1..10); # returns 1, 3, 5, 7, 9
firstidxBLOCKLISTfirst_indexBLOCKLIST
Returns the index of the first element in LIST for which the criterion in BLOCK is true. Sets $_ for each
item in LIST in turn:
my @list = (1, 4, 3, 2, 4, 6);
printf "item with index %i in list is 4", firstidx { $_ == 4 } @list;
__END__
item with index 1 in list is 4
Returns "-1" if no such item could be found.
"first_index" is an alias for "firstidx".
onlyidxBLOCKLISTonly_indexBLOCKLIST
Returns the index of the only element in LIST for which the criterion in BLOCK is true. Sets $_ for each
item in LIST in turn:
my @list = (1, 3, 4, 3, 2, 4);
printf "uniqe index of item 2 in list is %i", onlyidx { $_ == 2 } @list;
__END__
unique index of item 2 in list is 4
Returns "-1" if either no such item or more than one of these has been found.
"only_index" is an alias for "onlyidx".
lastidxBLOCKLISTlast_indexBLOCKLIST
Returns the index of the last element in LIST for which the criterion in BLOCK is true. Sets $_ for each
item in LIST in turn:
my @list = (1, 4, 3, 2, 4, 6);
printf "item with index %i in list is 4", lastidx { $_ == 4 } @list;
__END__
item with index 4 in list is 4
Returns "-1" if no such item could be found.
"last_index" is an alias for "lastidx".
Sortingsort_byBLOCKLIST
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; $_ } @strings
This sorts strings by generating sort keys which zero-pad the embedded numbers to some level (9 digits in
this case), helping to ensure the lexical sort puts them in the correct order.
nsort_byBLOCKLIST
Similar to sort_by but compares its key values numerically.
qsortBLOCKARRAY
This sorts the given array inplace using the given compare code. Except for tiny compare code like "$a
<=> $b", qsort is much faster than Perl's "sort" depending on the version.
Compared 5.8 and 5.26:
my @rl;
for(my $i = 0; $i < 1E6; ++$i) { push @rl, rand(1E5) }
my $idx;
sub ext_cmp { $_[0] <=> $_[1] }
cmpthese( -60, {
'qsort' => sub {
my @qrl = @rl;
qsort { ext_cmp($a, $b) } @qrl;
$idx = bsearchidx { ext_cmp($_, $rl[0]) } @qrl
},
'reverse qsort' => sub {
my @qrl = @rl;
qsort { ext_cmp($b, $a) } @qrl;
$idx = bsearchidx { ext_cmp($rl[0], $_) } @qrl
},
'sort' => sub {
my @srl = @rl;
@srl = sort { ext_cmp($a, $b) } @srl;
$idx = bsearchidx { ext_cmp($_, $rl[0]) } @srl
},
'reverse sort' => sub {
my @srl = @rl;
@srl = sort { ext_cmp($b, $a) } @srl;
$idx = bsearchidx { ext_cmp($rl[0], $_) } @srl
},
});
5.8 results
s/iter reverse sort sort reverse qsort qsort
reverse sort 6.21 -- -0% -8% -10%
sort 6.19 0% -- -7% -10%
reverse qsort 5.73 8% 8% -- -2%
qsort 5.60 11% 11% 2% --
5.26 results
s/iter reverse sort sort reverse qsort qsort
reverse sort 4.54 -- -0% -96% -96%
sort 4.52 0% -- -96% -96%
reverse qsort 0.203 2139% 2131% -- -19%
qsort 0.164 2666% 2656% 24% --
Use it where external data sources might have to be compared (think of Unix::Statgrab "tables").
"qsort" is available from List::MoreUtils::XS only. It's insane to maintain a wrapper around Perl's sort
nor having a pure Perl implementation. One could create a flip-book in same speed as PP runs a qsort.
SearchinginsortedListsbsearchBLOCKLIST
Performs a binary search on LIST which must be a sorted list of values. BLOCK must return a negative
value if the current element (stored in $_) is smaller, a positive value if it is bigger and zero if it
matches.
Returns a boolean value in scalar context. In list context, it returns the element if it was found,
otherwise the empty list.
bsearchidxBLOCKLISTbsearch_indexBLOCKLIST
Performs a binary search on LIST which must be a sorted list of values. BLOCK must return a negative
value if the current element (stored in $_) is smaller, a positive value if it is bigger and zero if it
matches.
Returns the index of found element, otherwise "-1".
"bsearch_index" is an alias for "bsearchidx".
lower_boundBLOCKLIST
Returns the index of the first element in LIST which does not compare lessthanval. Technically it's the
first element in LIST which does not return a value below zero when passed to BLOCK.
@ids = (1, 1, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 6, 7, 7, 7, 8, 8, 9, 9, 9, 9, 9, 11, 13, 13, 13, 17);
$lb = lower_bound { $_ <=> 2 } @ids; # returns 2
$lb = lower_bound { $_ <=> 4 } @ids; # returns 10
lower_bound has a complexity of O(log n).
upper_boundBLOCKLIST
Returns the index of the first element in LIST which does not compare greaterthanval. Technically it's
the first element in LIST which does not return a value below or equal to zero when passed to BLOCK.
@ids = (1, 1, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 6, 7, 7, 7, 8, 8, 9, 9, 9, 9, 9, 11, 13, 13, 13, 17);
$lb = upper_bound { $_ <=> 2 } @ids; # returns 4
$lb = upper_bound { $_ <=> 4 } @ids; # returns 14
upper_bound has a complexity of O(log n).
equal_rangeBLOCKLIST
Returns a pair of indices containing the lower_bound and the upper_bound.
OperationsonsortedListsbinsertBLOCKITEMLISTbsearch_insertBLOCKITEMLIST
Performs a binary search on LIST which must be a sorted list of values. BLOCK must return a negative
value if the current element (stored in $_) is smaller, a positive value if it is bigger and zero if it
matches.
ITEM is inserted at the index where the ITEM should be placed (based on above search). That means, it's
inserted before the next bigger element.
@l = (2,3,5,7);
binsert { $_ <=> 4 } 4, @l; # @l = (2,3,4,5,7)
binsert { $_ <=> 6 } 42, @l; # @l = (2,3,4,42,7)
You take care that the inserted element matches the compare result.
bremoveBLOCKLISTbsearch_removeBLOCKLIST
Performs a binary search on LIST which must be a sorted list of values. BLOCK must return a negative
value if the current element (stored in $_) is smaller, a positive value if it is bigger and zero if it
matches.
The item at the found position is removed and returned.
@l = (2,3,4,5,7);
bremove { $_ <=> 4 }, @l; # @l = (2,3,5,7);
CountingandcalculationtrueBLOCKLIST
Counts the number of elements in LIST for which the criterion in BLOCK is true. Sets $_ for each item
in LIST in turn:
printf "%i item(s) are defined", true { defined($_) } @list;
falseBLOCKLIST
Counts the number of elements in LIST for which the criterion in BLOCK is false. Sets $_ for each item
in LIST in turn:
printf "%i item(s) are not defined", false { defined($_) } @list;
reduce_0BLOCKLIST
Reduce LIST by calling BLOCK in scalar context for each element of LIST. $a contains the progressional
result and is initialized with 0. $b contains the current processed element of LIST and $_ contains the
index of the element in $b.
The idea behind reduce_0 is summation (addition of a sequence of numbers).
reduce_1BLOCKLIST
Reduce LIST by calling BLOCK in scalar context for each element of LIST. $a contains the progressional
result and is initialized with 1. $b contains the current processed element of LIST and $_ contains the
index of the element in $b.
The idea behind reduce_1 is product of a sequence of numbers.
reduce_uBLOCKLIST
Reduce LIST by calling BLOCK in scalar context for each element of LIST. $a contains the progressional
result and is uninitialized. $b contains the current processed element of LIST and $_ contains the index
of the element in $b.
This function has been added if one might need the extra of the index value but need an individual
initialization.
Usewithcaution: In most cases "reduce" in List::Util will do the job better.
minmaxLIST
Calculates the minimum and maximum of LIST and returns a two element list with the first element being
the minimum and the second the maximum. Returns the empty list if LIST was empty.
The "minmax" algorithm differs from a naive iteration over the list where each element is compared to two
values being the so far calculated min and max value in that it only requires 3n/2 - 2 comparisons. Thus
it is the most efficient possible algorithm.
However, the Perl implementation of it has some overhead simply due to the fact that there are more lines
of Perl code involved. Therefore, LIST needs to be fairly big in order for "minmax" to win over a naive
implementation. This limitation does not apply to the XS version.
minmaxstrLIST
Computes the minimum and maximum of LIST using string compare and returns a two element list with the
first element being the minimum and the second the maximum. Returns the empty list if LIST was empty.
The implementation is similar to "minmax".
Install Now
PNG Icons
