Re: searching a whole array without using a loop
[EMAIL PROTECTED] (John W. Krahn) wrote in
news:[EMAIL PROTECTED]:
If all the elements are unique then use a hash.
if ( exists $hash{ 'my string' } ) {
# do something
}
The most efficient way to determine if an element exists in an array
is to use a for loop and exit early:
my $found = 0;
for ( @array ) {
if ( $_ eq 'my string' ) {
$found = 1;
last;
}
}
If you need to determine the number of matches you can use grep:
my $count = grep $_ eq 'my string', @array;
John
OK, to be more specific, I'm using Net::Telnet:Cisco. When logged onto
a device I'm using the show version command, and storing the output in
an array. Each element of the array will therefore contain lots of
rubbish. I just want to match certain keyword(s) to determine device
type. I guess a for loop is the closest I'm going to get. I just
thought there would be a specific function for this.
So then I wonder - which is more efficient. My earlier join example,
or a for loop?
Thanks for all the input
Darren
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Re: searching a whole array without using a loop
Darren Birkett wrote:
I just want to match certain keyword(s) to determine device type.
I guess a for loop is the closest I'm going to get. I just thought
there would be a specific function for this.
Isn't grep() specific enough?
Initially you said:
$line = join(' ',@myarray);
if ($line =~ /my string/) {
some code
}
The equivalent using grep() would be:
if ( grep /my string/, @myarray ) {
some code
}
So then I wonder - which is more efficient. My earlier join
example, or a for loop?
To get the answer to such a question, do a benchmark with help of the
Benchmark module.
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Re: searching a whole array without using a loop
On Aug 24, 2004, at 7:48 AM, Darren Birkett wrote:
OK, to be more specific, I'm using Net::Telnet:Cisco. When logged onto
a device I'm using the show version command, and storing the output
in
an array. Each element of the array will therefore contain lots of
rubbish. I just want to match certain keyword(s) to determine device
type.
That sounds like a Regular Expression, naturally.
I guess a for loop is the closest I'm going to get. I just thought
there would be a specific function for this.
If you need only to find the first occurrence, John's suggestion is
perfect. You could also use:
use List::Util qw/first/;
if (first { /test/ } @list) {
# ...
}
If you need all entries that match grep() is the right choice, in my
opinion.
So then I wonder - which is more efficient. My earlier join example,
or a for loop?
Does it matter? Will you have 10 million or more entries in this list?
Do you need the answer in microseconds? Will the results be used in
the guidance system of a nuclear missile?
Silly questions, I know, but often so is worrying about performance.
With reasonable data on modern hardware, both will likely be faster
than you blink. That's generally good enough, don't you think? My
rule for optimization is, Speed it up when it gets too slow.
Let me ask you a question: Which method is easier for you to
understand? The answer to that is a much more noble pursuit, I think.
To me, join() is for strings and we're dealing with a list. Because of
that, I would use list tools: for, grep(), first(), etc. That doesn't
make me right though. That's just what makes sense to me, so that's
what I would use.
Hope that helps.
James
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Re: searching a whole array without using a loop
On Aug 24, 2004, at 9:14 AM, Gunnar Hjalmarsson wrote:
Isn't grep() specific enough?
Initially you said:
$line = join(' ',@myarray);
if ($line =~ /my string/) {
some code
}
The equivalent using grep() would be:
if ( grep /my string/, @myarray ) {
some code
}
Ordinarily, I wouldn't even mention this, but since the conversation is
about what is fastest...
The above is pretty much the textbook perfect example of an inefficient
use of grep(). The reason is that we just want to know if @myarray
contains ANY /my strings/, but grep() fetches them ALL. If the list
contains 10,000 entries, and the first is a /my string/, that's 9,999
lookups we didn't need.
I would simply substitute List::Utils' first() in the above example, to
fix this.
Though again, it probably isn't broken at all. (See my speed rant in
previous message.)
James
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Re: searching a whole array without using a loop
James Edward Gray II wrote:
Gunnar Hjalmarsson wrote:
if ( grep /my string/, @myarray ) {
some code
}
Ordinarily, I wouldn't even mention this, but since the
conversation is about what is fastest...
The OP was not about what is fastest at all. It was about whether
there is a better way than first joining the elements, without any
definition of better.
The above is pretty much the textbook perfect example of an
inefficient use of grep(). The reason is that we just want to know
if @myarray contains ANY /my strings/, but grep() fetches them ALL.
If the list contains 10,000 entries, and the first is a /my
string/, that's 9,999 lookups we didn't need.
I didn't claim that grep() provides a fast solution. But it does
provide a solution with a minimum of Perl code.
As regards efficiency, we were informed that the array contains the
output from some show version command, so it should be safe to
assume that we are not talking about a huge number of elements...
I would simply substitute List::Utils' first() in the above
example, to fix this.
Though again, it probably isn't broken at all. (See my speed
rant in previous message.)
Yeah, I read it, and you did object to you own arguments, didn't you? ;-)
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Re: searching a whole array without using a loop
Darren Birkett wrote:
OK, to be more specific, I'm using Net::Telnet:Cisco. When logged onto
a device I'm using the show version command, and storing the output in
an array. Each element of the array will therefore contain lots of
rubbish. I just want to match certain keyword(s) to determine device
type. I guess a for loop is the closest I'm going to get. I just
thought there would be a specific function for this.
So then I wonder - which is more efficient. My earlier join example,
or a for loop?
Ok, perhaps you need something like this:
my @output = grep /keyword1|keyword2/, $session-cmd('show version');
John
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searching a whole array without using a loop
Hi,
Is there a better way to search all elements of an array for a string
(without using a loop to test individual elements) than:
$line = join(' ',@myarray);
if ($line =~ /my string/) {
some code
}
I've seen map and grep used by some but also critisized by some. I thought
there might be a function to easily search all elements of an array at once
for a string but I can't find one.
Cheers
Darren
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Re: searching a whole array without using a loop
On Mon, 23 Aug 2004, Darren Birkett wrote:
Is there a better way to search all elements of an array for a string
(without using a loop to test individual elements) than:
$line = join(' ',@myarray);
if ($line =~ /my string/) {
some code
}
I've seen map and grep used by some but also critisized by some.
The tools you want for this are probably map and grep.
These commands are critisized by some, not entirely unfairly, because
code written with them can be hard to understand for people not familiar
with the constructs, and you can often get similar functionality out of
more verbose (and, to some, ipso facto clearer) code that doesn't use
these constructs.
But to hell with all that -- if map grep make sense to you, use them.
If you are worried about maintainability -- and you should be -- then
use comments to explain what you're doing.
Commands like these can make your code more compact (and, to some, ipso
facto clearer), more efficient, and should be immediately clear to
anyone that has put in the effort to become familiar with the language.
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Re: searching a whole array without using a loop
Darren Birkett wrote:
Hi,
Hello,
Is there a better way to search all elements of an array for a string
(without using a loop to test individual elements) than:
$line = join(' ',@myarray);
if ($line =~ /my string/) {
some code
}
I've seen map and grep used by some but also critisized by some. I thought
there might be a function to easily search all elements of an array at once
for a string but I can't find one.
If all the elements are unique then use a hash.
if ( exists $hash{ 'my string' } ) {
# do something
}
The most efficient way to determine if an element exists in an array is to use
a for loop and exit early:
my $found = 0;
for ( @array ) {
if ( $_ eq 'my string' ) {
$found = 1;
last;
}
}
If you need to determine the number of matches you can use grep:
my $count = grep $_ eq 'my string', @array;
John
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