Finding the contiguous sequences of equal elements in a list Raku

There are an even number of elements in your input:

say elems <1 1 0 2 0 2 1 2 2 2 4 4 3 3>; # 14

Your grep block consumes two elements each time:

{$^a eq $^b}

So if you add or remove an element you'll get the error you're getting when the block is run on the single element left over at the end.

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There are many ways to solve your problem.

But you also asked about the option of allowing for overlapping so, for example, you get two (2 2) sub-lists when the sequence 2 2 2 is encountered. And, in a similar vein, you presumably want to see two matches, not zero, with input like:

<1 2 2 3 3 4>

So I'll focus on solutions that deal with those issues too.

Despite the narrowing of solution space to deal with the extra issues, there are still many ways to express solutions functionally.

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One way that just appends a bit more code to the end of yours:

my @s = <1 1 0 2 0 2 1 2 2 2 4 4 3 3>;
say grep {$^a eq $^b}, @s .rotor( 2 => -1 ) .flat

The .rotor method converts a list into a list of sub-lists, each of the same length. For example, say <1 2 3 4> .rotor: 2 displays ((1 2) (3 4)). If the length argument is a pair, then the key is the length and the value is an offset for starting the next pair. If the offset is negative you get sub-list overlap. Thus say <1 2 3 4> .rotor: 2 => -1 displays ((1 2) (2 3) (3 4)).

The .flat method "flattens" its invocant. For example, say ((1,2),(2,3),(3,4)) .flat displays (1 2 2 3 3 4).

A perhaps more readable way to write the above solution would be to omit the flat and use .[0] and .[1] to index into the sub-lists returned by rotor:

say @s .rotor( 2 => -1 ) .grep: { .[0] eq .[1] }

See also Elizabeth Mattijsen's comment for another variation that generalizes for any sub-list size.

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If you needed a more general coding pattern you might write something like:

say @s .pairs .map: { .value xx 2 if .key < @s - 1 and [eq] @s[.key,.key+1] }

The .pairs method on a list returns a list of pairs, each pair corresponding to each of the elements in its invocant list. The .key of each pair is the index of the element in the invocant list; the .value is the value of the element.

.value xx 2 could have been written .value, .value. (See xx.)

@s - 1 is the number of elements in @s minus 1.

The [eq] in [eq] list is a reduction.

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If you need text pattern matching to decide what constitutes contiguous equal elements you might convert the input list into a string, match against that using one of the match adverbs that generate a list of matches, then map from the resulting list of matches to your desired result. To match with overlaps (eg 2 2 2 results in ((2 2) (2 2)) use :ov:

say @s .Str .match( / (.) ' ' $0 /, :ov ) .map: { .[0].Str xx 2 }

TIMTOWDI!

Here's an iterative approach using gather/take.

say gather for <1 1 0 2 0 2 1 2 2 2 4 4 3 3> { 
    state $last = ''; 
    take ($last, $_) if $last == $_; 
    $last = $_; 
};

# ((1 1) (2 2) (2 2) (4 4) (3 3))