I realize the question name doesn't really tell much but I didn't really know how to explain it shortly, so here's the long version.
First, here's my current code:
#! /usr/bin/perl
use strict;
use warnings;
my $input;
while (<>) {
$input .= $_;
}
$input =~ s/ |\n//g;
print "\n";
What I want to do is make a calculator, e.g. when a user does echo "8 * 5 + 21-15" | calculate it will calculate it correctly. So here's my thought progress. First I take the string as a whole and strip it of all whitespace characters. Then I wanted to index() it for the occurence of *, +, / or -. Then I wanted to add all the characters before any of those operators to a string and then (int) the string and then do the same to the part after the operator and then do the operation between them. But I don't actually have much of a clue on how to do this. Also, I'm very new to Perl (3 days experience) so please go slowly on me if possible.
Thanks a lot.
If you can accept that your calculator won't be able to handle parenthesis, use a regular expression to parse the string for you:
#!/usr/bin/env perl
use strict;
use warnings;
my #tokens = <STDIN> =~ /(\d+|\+|-|\*|\/)/g;
print "$_\n" for #tokens;
this will provide you with an array of tokens that you can work on, so
echo "8 * 5 + 21-15" | script.pl
will print
8
*
5
+
21
-
15
Now it's up to you to write some code that does the right calculations on the tokens. It isn't too hard if you don't try parsing parens, but if you do, you'll need to write a recursive parser, which is much harder.
Related
What is wrong specifically with this code? How can I correct it?
$x = "without any vowels after the first letter\n";
foreach $i (#x[1..]) {
if ($i =~ /[AEIOUaeiou]/) {
$x =~ tr/A E I O U a e i o u//d;
}
}
print "$x\n";
I tried [1..] to exclude the first letter. If it does not work, how else can I remove the first letter?
EDIT I edited code to make it syntactically (mostly) correct to convey their obvious original idea, except for the attempt to index into a string which isn't correct in Perl. (Clarifying that is a part of what I consider useful in this question.)
First, most of that is not Perl, or any programming language for that matter. I'd suggest to work through a Perl tutorial of your choice first, before trying to get solutions for specific problems. However, here's an answer since the problem itself is of enough interest in general.
Next, in Perl you can't directly index into a string, so you can't skip the first character(s) like that.
But you can separate that first character in the string and process the rest (removing vowels), of course. One way with regex†
use warnings;
use strict;
use feature 'say';
my $str = shift // 'out with ALL vowels after first';
$str =~ s/.\K(.*)/ $1 =~ tr{[aeiouAEIOU]}{}dr /e;
say $str; #--> ot wth LL vwls ftr frst
This relies on the /e modifier, which makes it so that the replacement side is evaluated as code, and so it runs an independent transliteration (tr) there, processing the captured substring.
Then we need the /r modifier in that embedded tr/regex, to return the new string instead of changing the old one in place -- what wouldn't be possible anyway as one can't change $1.
One can also use a regex insteda of tr, less efficient but with its many conveniences
$str =~ s/.\K(.*)/ $1 =~ s{[aeiou]}{}igr /e;
Now we can use far more sophisticated tools in that regex than in tr; in this case it's only the i flag, for case-insensitive.
If it were more than the one first character to keep change . to .{N}.
† Regex is not compulsory, of course. A more elementary take: split the string into its first character and the rest, then use tr on the rest
use warnings;
use strict;
use feature 'say';
my $str = shift // q(out with ALL vowels after first);
my ($result, $rest) = split //, $str, 2; # first char, rest of string
$result .= $rest =~ tr/aeiouAEIOU//dr; # prune $rest of vowels, append to $result
say $result;
Then put this in a little mini subroutine. To change the original string in place, instead of getting a new ($result) string, use it ($str) everywhere instead of $result.
I am not sure about how it compares efficiency wise but it may well fare well.
For the curiosity's sake, here it is in a single statement
$str = join '', map { length > 1 ? tr/aeiouAEIOU//dr : $_ } split //, $str, 2;
This specifically uses the fact that only the first (one) character need be skipped; that is easily made dynamical, as long as the criterion does involve the length of substrings.
More importantly, this assumes that the rest of the string is longer than 1 character. To drop that assumption change the criterion
use feature 'state';
$str = join '', map {
state $chr_cnt = 0;
++$chr_cnt > 1 ? tr/aeiouAEIOU//dr : $_
}
split //, $str, 2;
This also relies on leaving aside just one character. It uses a feature to keep a lexical value across executions, state.
A more generic solution, which uses the property of substr to be possible to write to
substr($str, 1) =~ tr/aeiouAEIOU//d;
Here it's much cleaner and simpler to relax the limitation to the first character: just change that 1 in order to skip more characters. The tricky -- unexpected -- part here may be that normally builtins can't be written to like that, they aren't lvalue subroutines
The algorithm for solution of the problem is in your question
add letter to a string if it isn't vowel
add letter to the string if it is first vowel in the input string
use strict;
use warnings;
my $x = "without any vowels after the first letter\n";
my($o,$count) = ('',0);
print 'IN: ' . $x;
for ( split('',$x) ) {
$o .= $_ unless $count != 0 and /[aeiou]/i;
$count++ if /[aeiou]/i;
}
print 'OUT: ' . $o;
Output
IN: without any vowels after the first letter
OUT: witht ny vwls ftr th frst lttr
Addendum: OP's clarification of the problem
look at each word in the sentence
if a word starts from vowel then delete all vowels but first one
if a word starts from none vowel then delete all vowels
use strict;
use warnings;
use feature 'say';
my $x = 'I like apples more than oranges';
my #o;
say 'IN: ' . $x;
for ( split(' ', $x) ) {
if ( /^[aeiou]/i ) {
s/.\K(.*)/$1 =~ tr|aeiouAEIOU||dr/e;
} else {
tr|aeiouAEIOU||d;
}
#o = (#o,$_);
}
say 'OUT: ' . join(' ', #o);
Output
IN: I like apples more than oranges
OUT: I lk appls mr thn orngs
Or in perlish style
use strict;
use warnings;
use feature 'say';
my $x = "I like apples more than oranges";
say 'IN: ' . $x;
say 'OUT: ' . join(' ', map { s/.\K(.*)/$1 =~ tr|aeiouAEIOU||dr/e && $_ } split('[ ]+', $x));
Output
IN: I like apples more than oranges
OUT: I lk appls mr thn orngs
I am starting up with Perl and confused on how to render unicode characters given a hex string variable.
#!/usr/bin/perl
use warnings;
foreach my $i (0..10000) {
my $hex = sprintf("%X", $i);
print("unicode of $i is \x{$hex}\n");
}
print("\x{2620}\n");
print("\x{BEEF}\n");
Gives me the warning: Illegal hexadecimal digit '$' ignored at perl.pl line 9.
and no value prints for \x{$hex}
Both chr($num) and pack('W', $num) produce a string consisting of the single character with the specified value, just like "\x{XXXX}" does.
As such, you can use
print("unicode of $i is ".chr(hex($hex))."\n");
or just
print("unicode of $i is ".chr($i)."\n");
Note that your program makes no sense without
use open ':std', ':encoding(UTF-8)';
Yup. You can't do that. No variable interpolation allowed in the middle of a \x like that. You can use chr() to get that character though.
Randal's answer is correct. For more info, you might want to read perluniintro.
From there, you can find, for example:
At run-time you can use:
use charnames ();
my $hebrew_alef_from_name
= charnames::string_vianame("HEBREW LETTER ALEF");
my $hebrew_alef_from_code_point = charnames::string_vianame("U+05D0");
I'm trying to edit the numbers in a string and put it back in the same place as they have been before.
Example:
$string = "struct:{thin:[[75518103,75518217],[75518338,75518363],[75532810,75533910],],thick:[[75518363,75518424],[75521257,75521463],],}";
I need to edit the numbers, but want to keep the rest of the string at it is. Additionally the number of brackets can vary.
Until now I split the string at "," with
#array = split (',',$string);
and extracted the numbers for editing with
foreach (#array) {
$_ =~ s/\D//g;
$_ = $number - $_;
}
now I want to put the numbers back in their original place in the string, but I don't know how.
Somehow I hope there is a better way to edit the numbers in the string without splitting it and extracting the numbers. Hope you can help me
You could use a regular expression substitution with the /e flag, search for long numbers and run Perl code in the substitution part.
use strict;
use warnings;
use feature 'say';
my $number = 100_000_000;
my $string = "struct:{thin:[[75518103,75518217],[75518338,75518363],[75532810,75533910],],thick:[[75518363,75518424],[75521257,75521463],],}";
$string =~ s/(\d+)/{$number - $1}/eg;
say $string;
__END__
struct:{thin:[[24481897,24481783],[24481662,24481637],[24467190,24466090],],thick:[[24481637,24481576],[24478743,24478537],],}
If there are no other numbers in the string, that would work. In case there is more logic involved, you can also move it into a subroutine and just call that in the substitution.
sub replace {
return $_ % 2 ? $_ * 2 : $_ / 4;
}
$string =~ s/(\d+)/{replace($1)}/eg;
You might also need to revise the search pattern to be a bit more precise.
I just found the evaluation modifier for regex! I now did it with
$string =~ s/([0-9]+)/$number-$1/eg;
and it worked!
I am generating simple SVG diagrams by using sed find and replace on some text input. However, I need a more sophisticated find and replace operation involving simple math, to alter certain X and Y values.
E.g. I need to multiply all Y values by a factor of 0.5 or 0.2 or 0.
Because of how I want this to work, it cannot be achieved with a transform operation within the SVG.*
I just need to be able to find, say, all instances of ([0-9.]*)VERT (in sed speak) and replace with the mathematical result of \1 multiplied by the constant I choose.
You could write a simple perl script. Not sure what determines $factor, but this should at least give you a running start.
#! /usr/bin/perl
my $factor=0.5;
while(my $line=<>) {
if ($line =~ /([0-9.]*)VERT/) {
my $num = $factor * $1;
$line =~ s/([0-9.]*)VERT/${num}VERT/;
}
print $line;
}
Usage: ./scriptname.pl <file_to_process.txt;
perl -ape 's/[0-9.]*(?=VERT)/$& * .5/e' file
hope this works +
I want to remove n characters from each line using PERL.
For example, I have the following input:
catbathatxx (length 11; 11%3=2 characters) (Remove 2 characters from this line)
mansunsonx (length 10; 10%3=1 character) (Remove 1 character from this line)
#!/usr/bin/perl -w
open FH, "input.txt";
#array=<FH>;
foreach $tmp(#array)
{
$b=length($tmp)%3;
my $c=substr($tmp, 0, length($tmp)-$b);
print "$c\n";
}
I want to output the final string (after the characters have been removed).
However, this program is not giving the correct result. Can you please guide me on what the mistake is?
Thanks a lot. Please let me know if there are any doubts/clarifications.
I am assuming trailing whitespace is not significant.
#!/usr/bin/env perl
use strict; use warnings;
use constant MULTIPLE_OF => 3;
while (my $line = <DATA>) {
$line =~ s/\s+\z//;
next unless my $length = length $line;
my $chars_to_remove = $length % MULTIPLE_OF;
$line =~ s/.{$chars_to_remove}\z//;
print $line, "\n";
}
__DATA__
catbathatxx
mansunsonx
0123456789
012345678
The \K regex sequence makes this a lot clearer; it was introduced in Perl v5.10.0.
The code looks like this
use 5.10.0;
use warnings;
for (qw/ catbathatxx mansunsonx /) {
(my $s = $_) =~ s/^ (?:...)* \K .* //x;
say $s;
}
output
catbathat
mansunson
In general you would want to post the result you are getting. That being said...
Each line in the file has a \n (or \r\n on windows) on the end of it that you're not accounting for. You need to chomp() the line.
Edit to add: My perl is getting rusty from non-use but if memory serves me correct you can actually chomp() the entire array after reading the file: chomp(#array)
You should use chomp() on your array, like this:
#array=<FH>;
chomp(#array);
perl -plwe 'chomp; $c = length($_) % 3; chop while $c--' < /tmp/zock.txt
Look up the options in perlrun. Note that line endings are characters, too. Get them out of the way using chomp; re-add them on output using the -l option. Use chop to efficiently remove characters from the end of a string.
Reading your code, you are trying to print just the first 'nx3' characters for the largest value of n for each line.
The following code does this using a simple regular expression.
For each line, it first removes the line ending, then greedy matches
as many .{3} as it can (. matches any character, {3} asks for exactly 3 of them).
The memory requirement of this approach (compared with using an array the size of your file) is fixed. Not too important if your file is small compared with your free memory, but sometimes files are gigabytes, and sometimes memory is very small.
It's always worth using variable names that reflect the purpose of the variable, rather than things like $a or #array. In this case I used only one variable, which I called $line.
It's also good practice to close files as soon as you have finished with them.
#!/usr/bin/perl
use strict;
use warnings; # This will apply warnings even if you use command perl to run it
open FH, '<', 'input.txt'; # Use three part file open - single quote where no interpolation required.
for my $line (<FH>){
chomp($line);
$line =~ s/((.{3})*).*/$1\n/;
print $line;
}
close FH;