Hello again dotowwxo,

map and grep are two very useful functions that Perl provides for operating on lists. Whereas grep acts as a filter, map acts as a transformer which changes each list element in some way. For example, in this one-liner:

14:20 >perl -MData::Dump -wE "dd map { $_ * 2 } 1 .. 5;"
(2, 4, 6, 8, 10)

17:20 >
[download]

the integers 1, 2, 3, 4, and 5 are doubled.

By contrast, here’s an example using grep to filter out odd numbers:

17:20 >perl -MData::Dump -wE "dd grep { $_ % 2 == 0 } 1 .. 10;"
(2, 4, 6, 8, 10)

17:26 >
[download]

In both map and grep, each element in the input list is represented in the block by Perl’s default variable $_. So, the expression map { <./1848_SoPW/$_/*\.log> } @dirs takes each directory name in @dirs and globs it in the expression <./1848_SoPW/$_/*\.log>. The result in this case is that map outputs a list of all the names of log files in the given directories.

Hope that helps,

See also How can i debug compound map/grep statements just using print? about chaining and debugging map and grep.

Best regards, Karl