Filtering and altering Perl lists with grep and map

In Perl, you can process whole lists (arrays) in single operations - and that's very efficient at run time as it avoids the needs for loops, and the needed for Perl's Virtual Machine to go back to the byte code (in effect source) for each member.

The map function applies an opertion to each member of a list, and returns a list of the same length, with each element duely mapped. The input to the transform function is $_ - so with embedded calls to functions like uc there's no need to specify it. But if you're going to take 1 off each member then you'll find $_ turn up in the code.

The grep function applies an operation to each member of a list, and returns the member unaltered for inclusion in the output list if the result is a true value. Otherwise, the input member is not included in the output list. So the output list from grep is going to be shorter than the incoming list, but the members themselves won't be modified.

@numbers = (10,30,40,33,45,32,43,22);

 

# Subtract 1 of each member of @numbers

@numbers = map($_-1,@numbers);

# Return a list of all numbers that are possible days of the month

@digit = grep( $_ <=31 ,@numbers);

 

print ("@digit\n");

 

@people = ("tim","steve","JASPER","DaVeY");

 

# Force all input members to lower case, then capitalise the first letter

@people = map(ucfirst lc,@people);

print "@people\n";

(written 2007-08-23)

Associated topics are indexed under

P208 - Perl - Lists
  [3939] Lots of ways of doing the same thing in Perl - list iteration - (2012-12-03)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)
  [3870] Writing more maintainable Perl - naming fields from your data records - (2012-09-25)
  [3669] Stepping through a list (or an array) in reverse order - (2012-03-23)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3400] $ is atomic and % and @ are molecular - Perl - (2011-08-20)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [2813] Iterating over a Perl list and changing all items - (2010-06-15)
  [2484] Finding text and what surrounds it - contextual grep - (2009-10-30)
  [2295] The dog is not in trouble - (2009-07-17)
  [2226] Revision / Summary of lists - Perl - (2009-06-10)
  [2067] Perl - lists do so much more than arrays - (2009-03-05)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [1917] Out of memory during array extend - Perl - (2008-12-02)
  [1828] Perl - map to process every member of a list (array) - (2008-10-09)
  [1703] Perl ... adding to a list - end, middle, start - (2008-07-09)
  [1304] Last elements in a Perl or Python list - (2007-08-16)
  [968] Perl - a list or a hash? - (2006-12-06)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [773] Breaking bread - (2006-06-22)
  [762] Huge data files - what happened earlier? - (2006-06-15)
  [622] Queues and barrel rolls in Perl - (2006-02-24)
  [560] The fencepost problem - (2006-01-10)
  [463] Splitting the difference - (2005-10-13)
  [355] Context in Perl - (2005-06-22)
  [240] Conventional restraints removed - (2005-03-09)
  [230] Course sizes - beware of marketing statistics - (2005-02-27)
  [140] Comparison Chart for Perl programmers - list functions - (2004-12-04)
  [28] Perl for breakfast - (2004-08-25)

P669 - Perl - Data Munging
  [3764] Shell, Awk, Perl of Python? - (2012-06-14)
  [3707] Converting codons via Amino Acids to Proteins in Perl - (2012-04-25)
  [3335] Practical Extraction and Reporting - (2011-06-26)
  [2702] First and last match with Regular Expressions - (2010-04-02)
  [2129] Nothing beats Perl to solve a data manipulation requirement quickly - (2009-04-14)
  [1947] Perl substitute - the e modifier - (2008-12-16)
  [1509] Extracting information from a file of records - (2008-01-16)
  [597] Storing a regular expression in a perl variable - (2006-02-09)

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