perl - find row index in 2D array using grep

[peterchan75]

Hi All,
Need to search the first column of a 2D array and return the row index.

my @array = (['a','b','c'],['d','e','f'],['g','h','i']);
my @index = map { $_ -> [0] } @array;
my @result = grep { $index[$_] =~ /g/ } 0..$#index;
print @result;

Possible to grep without using map ?

 

[davidktw]

Hi All,
Need to search the first column of a 2D array and return the row index.

my @array = (['a','b','c'],['d','e','f'],['g','h','i']);
my @index = map { $_ -> [0] } @array;
my @result = grep { $index[$_] =~ /g/ } 0..$#index;
print @result;

Possible to grep without using map ?

use Data::Dumper;

my @array = (['a','b','c'],['d','e','f'],['g','h','i']);
my @index = map { $_ -> [0] } @array;
my @result = grep { $index[$_] =~ /g/ } 0..$#index;

my @result2 = grep { $array[$_][0] =~ /g/ } 0..$#array;

print Data::Dumper->Dump([\@array, \@result, \@result2], [qw(array result result2)]);

https://onlinegdb.com/pXWfsu5Xn

 

[peterchan75]

@davidktw,
I did try this method. I even tried this $_->[0].
It could be some other errors that masked this solution.
I googled and it found this post.

Thank you very much.

 

[davidktw]

@davidktw,
I did try this method. I even tried this $_->[0].
It could be some other errors that masked this solution.
I googled and it found this post.

Thank you very much.

Which post are you referring to?

Execute the code provided before and observe how the various ways of dereferencing perl scalar, array and references work.
It will be kinda confusing at first, but you will get use to it once you practise more
https://onlinegdb.com/NTD5pwcuG

 

[davidktw]

In case you are not aware, map can also be used like a grep

my @result2 = map { $array[$_][0] =~ /g/ ? $_ : () } 0..$#array;

https://onlinegdb.com/WF3S9ly5K
Perl is always a *fun* language to me.

 

[peterchan75]

In case you are not aware, map can also be used like a grep

my @result2 = map { $array[$_][0] =~ /g/ ? $_ : () } 0..$#array;

https://onlinegdb.com/WF3S9ly5K
Perl is always a *fun* language to me.

Can please help explain the ?, $_, : and ().
Let me guess. The "?" is to compare and the $_ refer to the @array. But the ":" and "()" beats me.

 

[davidktw]

Can please help explain the ?, $_, : and ().
Let me guess. The "?" is to compare and the $_ refer to the @array. But the ":" and "()" beats me.

$_ is alias to each element in the list passed into the map/grep function.

?: is commonly known as the ternary operator. It is found in a lot of other programming languages too.

<conditional expression> ? <then-expression> : <else-expression> can be written in its long form as

if (<conditional expression>) {
  <then-expression>
}
else {
  <else-expression>
}

In the case of map, if you have read its documentation https://perldoc.perl.org/functions/map, you would have seen this statement "To omit an element, return an empty list ()"

use Data::Dumper;

@all = map { $_ } (1..10);
print Dumper(\@all);
@none = map { () } (1..10);
print Dumper(\@none);
@even = map { $_ % 2 ? () : $_ } (1..10);
print Dumper(\@even);
@odd = map { $_ % 2 ? $_ : () } (1..10);
print Dumper(\@odd);

For fun, this is how obfuscating you can write perl

use Data::Dumper;

@all=map{$_}1..10;
print Dumper(\@all);
@even2=map{@{([$_],[])[$_%2]}}1..10;
print Dumper(\@even2);
@odd2=map{@{([],[$_])[$_%2]}}1..10;
print Dumper(\@odd2);