Perl Refreshments ⌘

Arrays #9 - Splice

No this is not a joke. We can slice and splice arrays (amongst other things).

Splicing is when you want to join one thing to itself or something else, like splicing one rope to another.

Splicing with arrays is used when you want to add some items of one array into another array.

splice REMOVES elements designated by OFFSET and LIMIT below, and replaces them with LIST if any.

Formally here is the syntax:

splice (ARRAY, OFFSET, LENGTH, LIST)
splice (ARRAY, OFFSET, LENGTH)
splice (ARRAY, OFFSET)

if OFFSET is negative splice counts back from the end of the array
if LENGTH is negative it removes elements from OFFSET forward
if OFFSET & LENGTH are in list context splice returns the removed elements
but if they are in scalar context, the last element removed is returned
if the number of new elements <> number of old elements the array will grow or shrink correspondingly
if LENGTH is omitted splice removes everything from OFFSET onward
if OFFSET is omitted the array is emptied
if both OFFSET and LENGTH are missing everything in the array is removed

As you can see this seems to be a very powerful tool, and as with many tools, it must be used with caution.

Sticking with our cities example, let's see what we can do. First we'll start with an array of some Ontario cities (because I live in Ontario). Then we'll make another array of Québec cities.

my @on=(
	"Toronto",
	"Windsor",
	"Hamilton",
	"London",
	"Ottawa",
	"Sudbury",
	"Timmins",
	"Perth",
);
print qq{\@on:\n} . join("\n",@on) . "\n\n";

@on:
Toronto
Windsor
Hamilton
London
Ottawa
Sudbury
Timmins
Perth

my @pq=(
	"Montreal",
	"Kipawa",
	"Val d'Or",
	"Québec",
	"Chicoutimi",
	"Gaspé",
	"Ivujivik",
	"Matagami",
);
print qq{\@pq:\n} . join("\n",@pq) . "\n\n";

@pq:
Montreal
Kipawa
Val d'Or
Québec
Chicoutimi
Gaspé
Ivujivik
Matagami

Now we will splice the Québec array into the Ontario array beginning at index #2 ...

splice (@on,2,1,@pq);
print qq{\@on\n} . join("|",@on) . "\n\n";

@on
Toronto|Windsor|Montreal|Kipawa|Val d'Or|Québec|Chicoutimi|Gaspé|Ivujivik|Matagami|London|Ottawa|Sudbury|Timmins|Perth

Notice that ONLY @on[2] (Hamilton) has disappeared and been replaced by all of @pq, then the rest of @on.

This is a result of the 2,1 constraint in the splice command.