Re: How can i debug compound map/grep statements just using print?

[roboticus]

karlgoethebler:

Before I resort to using print statements, I try to read the statement bit by bit to see if I'm going to get what I want.

$result = grep { $_ == 1 } ...a list...

Ok, so grep is going to read a list and keep the elements equal to 1. We're assigning a list to a scalar, so $result is ultimately going to be the number of things that are equal to 1. Is that what you're looking for? Judging by your code, that seems to be the case, so far so good.

That bit of code is receiving a list of values from comparisons:

   ..new list.. = map  { $time - $_ >= $amount } ..a list..

which will return a list of 1 and '', that looks good, too.

Eventually, I either find the bug, or I just don't see it. So then I resort to my next technique. I alter the code a bit, to look like this:

my @t =
#$result =
#       grep { $_ == 1 }
#       map  { $time - $_ >= $amount }
#       map  { ( $_->stat )[9] }  # e.g i'm interested in this...
#       grep { $_->name =~ /.+\.$suffix$/ } # ...and this
       io($dir)->all;
die "<", join(", ", @t), ">\n";

Now when I run it, I can see the list we're starting from. If it looks fine, then I uncomment the last commented line, and try again:

my @t =
#$result =
#       grep { $_ == 1 }
#       map  { $time - $_ >= $amount }
#       map  { ( $_->stat )[9] }  # e.g i'm interested in this...
       grep { $_->name =~ /.+\.$suffix$/ } # ...and this
       io($dir)->all;
die "<", join(", ", @t), ">\n";

By working my way backwards through the filters, I can see where the list looks wrong, and make suitable fixes. Then, since I'm in this cycle anyway, I go ahead and complete the process, until I the list run through all filters. If it looks good, then I simply uncomment the assignment to $result, and delete the my @t = and die lines.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

[karlgoethebier]

Yes, but my intent was more naive. I searched for a equivalent using map/grep for saying:

for(@things) {print if DEBUG}