grep v map v sort

Most languages support lists and / or arrays - and that includes Perl. In Perl, though, you can use functions such as grep, map, sort and reverse to operate on lists as a whole rather than having to loop through members of the list cell by cell.

function	desciption of action	element count	elements altered?	Output element order
grep	Filters incoming elements and copies those which match a criteria to the output list	"n" elements in, 0 to "n" elements out	outgoing elements are exact copies of incoming elements	outgoing elements are in same order as incoming elements
map	Performs an operation on each incoming element and writes the result to the output list	"n" elements in, "n" elements out	outgoing elements are the result of an operation on incoming elements	outgoing elements are in same order as incoming elements
sort	Re-orders the incoming elements and writes the result to the output list	"n" elements in, "n" elements out	outgoing elements are exact copies of incoming elements	outgoing elements are in a different order to the incoming elements
reverse	Writes the incoming elements to the output list in reverse order	"n" elements in, "n" elements out	outgoing elements are exact copies of incoming elements	outgoing elements are "back to front" from the incoming elements

It's a common misconception that grep is used only to filter incoming members against a regular expression - it CAN be (and that's the most common use and how it got its name), but it can also be used to perform another task, based on each member of the incoming list in turn being put into $_. The following sample program shows two uses of grep and two of map to illustrate the comments just made and parts of the table above.


# antonia Perl XML PHP Tcl/Tk MySQL
# barbara Tcl/Tk ASP Ruby Java
# cherry Perl Java Ruby MySQL

open (FH,"requests.xyz") or die "No requests.xyz file\n";
@stuff = grep (/^.[aeiou]/,<FH>); # Only want lines with vowel as 2nd letter

@m1 = grep((split)>5,@stuff); # Filter to remove short lines
print @m1;
print "============================\n";
@m1 = map(uc,@m1); # Change all lines to all upper case
print @m1;
print "============================\n";
@m3 = map(length()."\n",@m1); # Produce a list of line lengths
print @m3;


I ran this against a test data file of 52 lines (I've pasted the first three lines into the sample code above) and here are the results - you'll see that the only lines left by grep are those which comprise over 5 space separated fields, and have a lower case vowel as the second letter.


[localhost:~/dplp] graham% perl nq
hazel PHP Python Perl Ruby ASP
leane PHP Python ASP Perl Java
margaret XML Perl Ruby MySQL Tcl/Tk
petra XML Tcl/Tk ASP Perl Ruby
xena Java Perl PHP ASP XML
barry Python XML Java Perl PHP
============================
HAZEL PHP PYTHON PERL RUBY ASP
LEANE PHP PYTHON ASP PERL JAVA
MARGARET XML PERL RUBY MYSQL TCL/TK
PETRA XML TCL/TK ASP PERL RUBY
XENA JAVA PERL PHP ASP XML
BARRY PYTHON XML JAVA PERL PHP
============================
31
31
36
31
28
31
[localhost:~/dplp] graham%


There are further examples of the use of functions such as grep (and push and pop and others too) available under our training note pages.
(written 2004-12-04 08:00:46)

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