Change one letter in a string in Perl 6 - raku

Question

It should be very simple, but I can't find an instrument to do it without making a list with .comb.
I have a $string and an (0 < $index < $string.chars - 1). I need to make a $new_string, whose element with number $index will be changed, say, to 'A'.
my $string = 'abcde';
my $index = 0; # $new_string should be 'Abcde'
my $index = 3; # $new_string should be 'abcAe'

Answer 1

This is what I would recommend using:
my $string = 'abcde';
my $index = 0;
( my $new-string = $string ).substr-rw( $index, 1 ) = 'A';
say $string; # abcde
say $new-string; # Abcde
If you wanted to stay away from a mutating operation:
sub string-index-replace (
Str $in-str,
UInt $index,
Str $new where .chars == 1
){
( # the part of the string before the value to insert
$in-str.substr( 0, $index ) xx? $index
)
~
( # add spaces if the insert is beyond the end of the string
' ' x $index - $in-str.chars
)
~
$new
~
( # the part of the string after the insert
$in-str.substr( $index + 1 ) xx? ( $index < $in-str.chars)
)
}
say string-index-replace( 'abcde', $_, 'A' ) for ^10
Abcde
aAcde
abAde
abcAe
abcdA
abcdeA
abcde A
abcde A
abcde A
abcde A

Answer 2

To change just a single letter in the string, you can use the subst method on type Str:
my $string = 'abcde';
my $index = 0;
my $new_string = $string.subst: /./, 'A', :nth($index+1); # 'Abcde'
$index = 3;
$new_string = $string.subst: /./, 'A', :nth($index+1); # 'abcAe'
The "indexing" for :nth starts at 1. What :nth really means is "replace nth match alone". Because our regex only matches a single character, :nth acts like an index (even though it technically is not).