x, xx and X

Word familyB1

Three operators that look alike — one repeats a string, one repeats a list element, and one combines two lists.

The three members of this family all multiply something, and all three are spelled with the letter x. Each one gives you something bigger than the one before: a string, a list, a list of combinations.

The family

x — repeats a string

x takes a string on the left and a number on the right, and gives you one string with the left side repeated that many times.

say 'ha' x 3;
say '-' x 20;
say ('=' x 5) ~ ' end';
hahaha
--------------------
===== end

The result is always a string, even when the left side is a number: 3 x 2 is the string 33.

say 3 x 2;
say (3 x 2).WHAT;
33
(Str)

xx — repeats an element in a list

xx gives you a list instead of a string. Each repetition is a separate element.

say 'ha' xx 3;
say ('ha' xx 3).join(', ');
my @row = 0 xx 4;
say @row;
(ha ha ha)
ha, ha, ha
[0 0 0 0]

xx does something that x does not: it runs the left side again for every element. If the left side changes something, you see the change.

my $n = 0;
say $n++ xx 3;
(0 1 2)

That is why [0 xx 3] xx 2 gives you two separate rows, not the same row twice. Changing one row does not change the other.

my @grid = [0 xx 3] xx 2;
@grid[0][1] = 5;
say @grid;
[[0 5 0] [0 0 0]]

X — combines two lists

X is the cross product. It takes two lists and gives you every pair you can make with one element from the left and one from the right.

say <S M L> X <red blue>;
say (<S M L> X <red blue>).elems;
((S red) (S blue) (M red) (M blue) (L red) (L blue))
6

Three sizes and two colours make six pairs. You can put another operator right after the X, and it is applied to each pair instead of making a list of two. With ~, the pairs are joined into strings:

say <S M L> X~ <-red -blue>;
say (1, 2) X* (10, 100);
(S-red S-blue M-red M-blue L-red L-blue)
(10 100 20 200)

Side by side

Operator Left side You get Example
x a string one longer string 'ab' x 2 → abab
xx anything a list of copies 'ab' xx 2 → (ab ab)
X a list a list of all combinations <a b> X <1 2> → ((a 1) (a 2) (b 1) (b 2))

Which one?

Which operator would you use for each task?

  1. Print a line of 30 stars under a heading.
  2. Start a list of seven zeros, one for each day of the week.
  3. Make the names of all chess squares from a1 to h8.
  4. Indent a line by four spaces.
Answers
  1. x: say '*' x 30.
  2. xx: my @days = 0 xx 7.
  3. X~: ('a'..'h') X~ (1..8) gives a1 a2 … a8 b1 … h8.
  4. x: ' ' x 4 ~ $line.