The question I have is a bit abstract, but I'll attempt to be clear in my statement. (This is something of a "rubber duck effect" post, so I'll be thankful if just typing it out gets me somewhere. Replies, however, would be brilliant!)
I have old fortran code that I can't change (at least not yet) so I'm stuck with its awkward output.
I'm using perl to post-process poorly annotated ascii output files, which, as you might imagine, are a very specialized mixture of text and numbers. "Ah, the perfect perl objective," you say. Yes, it is. But what I've come up with is pretty terrible coding, I've recently concluded.
My question is about the generic structure that folks prefer to use to achieve such an objective. As I've said, I'm not happy with the one I've chosen.
Here's some pseudocode of the structure that I've arrived at:
flag1 = 0;
flag2 = 0;
while (<INPUT>) {
if (cond1) {
do something [like parse and set a header];
flag1 = 1;
} else {
next;
}
if (flag1 == 1 && cond2) {
do something else [like process a block of data];
} else {
next;
}
}
The objective of the above code is to be able to break the processing into blocks corresponding to the poorly partitioned ascii file -- there isn't much by way of "tags" in the file, so the conditions (cond1, cond2, etc.) are involved. The purpose of having the flags set is, among other reasons, to track the progress of the code through the file.
It occurs to me now that a better structure might be
while (<INPUT>) {
do stuff;
}
while (<INPUT>) {
do other stuff;
}
In any event, if my rambling inspires any thoughts I'd appreciate hearing them.
Thank you
Your original structure is perfectly fine. You're building a state machine, and doing it in a completely reasonable way that can't really be made any more idiomatic.
The only thing you can possibly do if you wish is to modularize the code a wee bit:
our %state = (last => 0, current => 0, next => 0);
our %extra_flags = ();
sub cond1($line) { return $next_state } # Returns 0 if cond==false
sub cond2($line) { return $next_state } # Returns 0 if cond==false
our %conditions = (
0 => \&cond1
1 => \&cond2 # flag1 is set
);
while (<INPUT>) {
my $state = $state->{current};
if ($state->{next} = $conditions{$state}->($_, $state)) {
$do_stuff{$state}->{$next_state}->($line);
$state->{last} = $state->{current};
$state->{current} = $state->{next};
next;
}
}
If the file does indeed lend itself to being processed in multiple loops, that would be a much clearer way to do it than emulating that with conditionals, IMO.
If not, even if there are just a few exceptions to code around, it's probably better to stick with the original approach you describe.
Related
I have a really quick question. I have a program with a lot of functions that are run from main. Is it best practice to have the functions first and then the call from main, or the other way around?
For example:
sub myFunction {
#Do something
}
my $stuff = myFunction();
Or:
my $stuff = myFunction();
sub myFunction {
#Do something
}
Sorry for any ignorance, I do not have any formal training and I have seen it done both ways online. Thanks
I recommend placing your code at the bottom.
Issue 1
The latter snippet poor because myFunction is in scope of $stuff, but it shouldn't be. That's easy to fix though.
{
my $stuff = myFunction();
}
sub myFunction {
#Do something
}
Ok, so that's not a big issue since I place all top-level code in a block, even if it comes at the end. It looks cleaner to me that way, and it makes it easier to transform into a sub from which I can return.
sub myFunction {
#Do something
}
sub main {
return 0 if is_nothing_to_do();
my $stuff = myFunction();
...
return 0;
}
exit(main(parse_args));
Issue 2
Many languages require that you declare your subs before you call them. That's rarely needed in Perl, though there are a couple of scenarios where it is required. Subs with prototypes is one of those. If you wanted to place your code at the top, you would need to add declarations even before that.
sub myFunction(&#);
{
my $stuff = myFunction { ... } ...;
}
sub myFunction(&#) {
#Do something
}
You probably never have to do that since all but some rare uses of prototypes is discouraged, and the other scenarios are even rarer.
Issue 3
You might accidentally skip initialization code by placing your top-level code before your subroutines.
Compare:
print my_counter(), "\n"; # Warns, then prints a blank line
...
{
my $counter = 1;
sub my_counter {
return $counter++;
}
}
...
and
...
{
my $counter = 1;
sub my_counter {
return $counter++;
}
}
...
print my_counter(), "\n"; # Prints 1
Issue 4
Many languages require that you declare your subs before you call them, so more people will be more familiar with having the top-level code at the bottom.
It doesn't matter, so long as you're able to find the code that you need to find. I typically like to set up my code like this:
use strict;
use warnings;
exit main();
sub main {
do_this();
dont_do_that();
cant_you_read_the_signs();
return 0;
}
sub do_this {
....
}
...
Putting your main code in an actual function or block called "main" helps keep you from polluting the program with globals.
I am writing a function in perl which will kill a process given its PID.
sub ShutPidForWindows()
{
require Win32::Process;
$PID = 1234;
$count = 0;
$ReturnStatus = 0;
$ExitCode = 0 ;
if ($PID == 0)
{
return ($ReturnStatus);
}
Win32::Process::KillProcess($PID, $ExitCode);
print "PID = ".$PID."\n";
print "Return Code = ".$ExitCode."\n";
if ($ExitCode)
{
$ReturnStatus = 1;
}
else
{
$ReturnStatus = 2;
}
return ($ReturnStatus);
}
when this function is executed it always returns 2. Even though the process 1234 does not exists.
The o/p I get is:
PID = 1234
Return Code = 0
Perl Doc says that ExitCode will be populated by the exit code returned by the process. Then ExitCode should be 1.
Am I doing anything wrong?
The problem is that you are using require instead of use to load the module. Sometimes this is OK, but you should always follow the examples in the module's documentation.
You must also always use strict and use warnings at the top of every Perl program you write. This will make it necessary to declare all of your variables, which should be done as close as possible to their first point of use. These measures will reveal many errors that you may otherwise overlook, and is especially important when you are asking others for help with your code.
If you examine $^E after the call to Win32::Process::KillProcess, you might see a value like
The parameter is incorrect
which should tell you that you did something wrong.
According to the perl manual for for last (http://perldoc.perl.org/functions/last.html), last can't be used to break out of do {} loops, but it doesn't mention an alternative. The script I'm maintaining has this structure:
do {
...
if (...)
{
...
last;
}
} while (...);
and I'm pretty sure he wants to go to the end of the loop, but its actually exiting the current subroutine, so I need to either change the last or refactor the whole loop if there is a better way that someone can recommend.
Wrap the do "loop" in a bare block (which is a loop):
{
do {
...
if (...)
{
...
last;
}
} while (...);
}
This works for last and redo, but not next; for that place the bare block inside the do block:
do {{
...
if (...)
{
...
next;
}
...
}} while (...);
do BLOCK while (EXPR) is funny in that do is not really a loop structure. So, last, next, and redo are not supposed to be used there. Get rid of the last and adjust the EXPR to evaluate false when that situation is found.
Also, turn on strict, which should give you at least a warning here.
Never a fan of do/while loops in Perl. the do isn't really a loop which is why last won't break out of it. In our old Pascal daze you couldn't exit a loop in the middle because that would be wrong according to the sage Niklaus "One entrance/one exit" Wirth. Therefore, we had to create an exit flag. In Perl it'd look something like this:
my $endFlag = 0;
do {
...
if (...)
{
...
$endFlag = 1;
}
} while ((...) and (not $endFlag));
Now, you can see while Pascal never caught on.
Why not just use a while loop?
while (...) {
...
if (...) {
last;
}
}
You might have to change your logic slightly to accommodate the fact that your test is at the beginning instead of end of your loop, but that should be trivial.
By the way, you actually CAN break out of a Pascal loop if you're using Delphi, and Delphi DID catch on for a little while until Microsoft wised up and came out with the .net languages.
# "http://perldoc.perl.org/functions/last.html":
last cannot be used to exit a block that returns a value such as eval {} , sub {} or do {} , and should not be used to exit a grep() or map() operation.
So, use a boolean in the 'while()' and set it where you have 'last'...
Late to the party - I've been messing with for(;;) recently. In my rudimentary testing, for conditional expressions A and B, what you want to do with:
do {
last if A;
} while(B);
can be accomplished as:
for(;; B || last) {
last if A;
}
A bit ugly, but perhaps not more so than the other workarounds :) . An example:
my $i=1;
for(;; $i<=3 || last) {
print "$i ";
++$i;
}
Outputs 1 2 3. And you can combine the increment if you want:
my $i=1;
for(;; ++$i, $i<=3 || last) {
print "$i ";
}
(using || because it has higher precedence than ,)
sub DirectoryExists {
my $param = shift;
# Remove first element of the array
shift #{$param};
# Loop through each directory to see if it exists
foreach my $directory (#{$param}) {
unless (-e $directory && -d $directory) {
return 0;
}
}
# True
return 1;
}
Is there any way to optimize this code?
Is there any good way to optimize this code?
That algorithm is pretty efficient, because it stops at the first item but you might want to give List::Util::first a try.
use List::Util qw<first>;
#...
return defined first { -e && -d } #$param;
The only major optimization would be that it runs in the C-layer. It's also a pretty recognizable idiom in Perl, and so despite the golf look, the purpose is to "speak perl", not to golf.
List::MoreUtils::any would give you a similar effect and as well, it's a better fit to what you're trying to express: you're asking if any in the array are directories. (a hint though, stack parameter passing is slightly to significantly faster than constructing a reference and passing it--at least in my tests.)
Anyway, here's what it looks like:
return any { -e && -d } #$param;
Means to return true if any satisfy that expression. any often runs in the C-layer, if the module could load its XS version. Otherwise it's "Pure Perl" and probably runs similar to yours.
However, I'm pretty sure you don't have to test for both existence and directory. I'm pretty sure that if the file does not exist, it's not going to be seen as a directory. So, you could collapse it to one condition.
I would write that code as:
sub all_directories_exist {
my $param = shift;
# Remove first element of the array
shift #{$param};
for my $dir ( #{ $param } ) {
return unless -e $directory;
return unless -d _;
}
return 1;
}
I am guessing —although I haven't benchmarked it— one cannot get much faster than that.
Two points:
Do NOT return 0 to indicate failure. You will be surprised if your sub is called in list context.
Are you sure you want to modify the array pointed to by $param?
I have a Perl routine that manages error checking. There are about 10 different checks and some are nested, based on prior success. These are typically not exceptional cases where I would need to croak/die. Also, once an error occurs, there's no point in running through the rest of the checks.
However, I can't seem to think of a neat way to solve this issue except by using something analogous to the following horrid hack:
sub lots_of_checks
{
if(failcond)
{
goto failstate:
}
elsif(failcond2)
{
goto failstate;
}
#This continues on and on until...
return 1; #O happy day!
failstate:
return 0; #Dead...
}
What I would prefer to be able to do would be something like so:
do
{
if(failcond)
{
last;
}
#...
};
An empty return statement is a better way of returning false from a Perl sub than returning 0. The latter value will actually be true in list context:
sub lots_of_checks {
return if fail_condition_1;
return if fail_condition_2;
# ...
return 1;
}
Perhaps you want to have a look at the following articles about exception handling in perl5:
perl.com: Object Oriented Exception Handling in Perl
perlfoundation.com: Exception Handling in Perl
You absolutely can do what you prefer.
Check: {
last Check
if failcond1;
last Check
if failcond2;
success();
}
Why would you not use exceptions? Any case where the normal flow of the code should not be followed is an exception. Using "return" or "goto" is really the same thing, just more "not what you want".
(What you really want are continuations, which "return", "goto", "last", and "throw" are all special cases of. While Perl does not have full continuations, we do have escape continuations; see http://metacpan.org/pod/Continuation::Escape)
In your code example, you write:
do
{
if(failcond)
{
last;
}
#...
};
This is probably the same as:
eval {
if(failcond){
die 'failcond';
}
}
If you want to be tricky and ignore other exceptions:
my $magic = [];
eval {
if(failcond){
die $magic;
}
}
if ($# != $magic) {
die; # rethrow
}
Or, you can use the Continuation::Escape module mentioned above. But
there is no reason to ignore exceptions; it is perfectly acceptable
to use them this way.
Given your example, I'd write it this way:
sub lots_of_checks {
local $_ = shift; # You can use 'my' here in 5.10+
return if /condition1/;
return if /condition2/;
# etc.
return 1;
}
Note the bare return instead of return 0. This is usually better because it respects context; the value will be undef in scalar context and () (the empty list) in list context.
If you want to hold to a single-exit point (which is slightly un-Perlish), you can do it without resorting to goto. As the documentation for last states:
... a block by itself is semantically identical to a loop that executes once.
Thus "last" can be used to effect an early exit out of such a block.
sub lots_of_checks {
local $_ = shift;
my $all_clear;
{
last if /condition1/;
last if /condition2/;
# ...
$all_clear = 1; # only set if all checks pass
}
return unless $all_clear;
return 1;
}
If you want to keep your single in/single out structure, you can modify the other suggestions slightly to get:
sub lots_of_checks
{
goto failstate if failcond1;
goto failstate if failcond2;
# This continues on and on until...
return 1; # O happy day!
failstate:
# Any clean up code here.
return; # Dead...
}
IMO, Perl's use of the statement modifier form "return if EXPR" makes guard clauses more readable than they are in C. When you first see the line, you know that you have a guard clause. This feature is often denigrated, but in this case I am quite fond of it.
Using the goto with the statement modifier retains the clarity, and reduces clutter, while it preserves your single exit code style. I've used this form when I had complex clean up to do after failing validation for a routine.