Using map and grep

Kevin Eye

Buffalo Perl Mongers

grep

• grep BLOCK LIST
  • Evaluates the BLOCK for each element of LIST (locally setting "$_" to each element) and returns the list value consisting of those elements for which the expression evaluated to true.
    
    
  • filter out comments
    @foo = grep {!/^#/} @bar
  • how many comments in @bar
    $num_comments = grep {!/^#/} @bar
  • does @bar have any comments
    $has_comments = grep {!/^#/} @bar
  • first comment in @bar
    ($first_comment) = grep {!/^#/} @bar

map

• map BLOCK LIST
  • Evaluates the BLOCK for each element of LIST (locally setting "$_" to each element) and returns the list value composed of the results of each such evaluation.
  • Evaluates BLOCK in list context, so each element of LIST may produce zero, one, or more elements in the returned value.
    
    
  • translate numbers to characters
    @chars = map { chr } @nums
  • double every value in @nums
    @nums = map { $_ * 2 } @nums
  • make a hash
    %hash = map { getkey($_) => $_ } @values
  • is 'something' in @unindexed_array?
    %index = map { $_ => 1 } @unindexed_array;
    exists $index{'something'}

Putting it together

open FH, 'names.txt';
@names = <FH>;

foreach $name (@names) {
    next unless $name =~ /\w/; # filter blank lines
    @firstlast = split / /, $name;
    $lastfirst = join ', ', reverse @firstlast;
    push @rnames, $lastfirst;
    }

@sorted = sort @rnames;

foreach $name (@sorted) {
    print $name;
    }

Putting it together

open FH, 'names.txt';
@names = grep { /\w/ } <FH>;

foreach $name (@names) {

    @firstlast = split / /, $name;
    $lastfirst = join ', ', reverse @firstlast;
    push @rnames, $lastfirst;
    }

@sorted = sort @rnames;

foreach $name (@sorted) {
    print $name;
    }

Putting it together

open FH, 'names.txt';
@names = grep { /\w/ } <FH>;

foreach $name (@names) {



    push @rnames, join ', ', reverse split / /, $name;
    }

@sorted = sort @rnames;

foreach $name (@sorted) {
    print $name;
    }

Putting it together

open FH, 'names.txt';
@names = grep { /\w/ } <FH>;





@rnames = map { join ', ', reverse split / / } @names;


@sorted = sort @rnames;

foreach $name (@sorted) {
    print $name;
    }

Putting it together

open FH, 'names.txt';
@names = grep { /\w/ } <FH>;





@rnames = map { join ', ', reverse split / / } @names;


@sorted = sort @rnames;


print for @sorted;

Putting it together

open FH, 'names.txt';
@names = grep { /\w/ } <FH>;
@rnames = map { join ', ', reverse split / / } @names;
@sorted = sort @rnames;
print for @sorted;

Putting it together

open FH, 'names.txt';
print for sort
          map { join ', ', reverse split / / }
          grep { /\w/ }
          <FH>;