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POWhy can't I use the map function to create a good hash from a simple data file in Perl?
 

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  1. POWhy can't I use the map function to create a good hash from a simple data file in Perl?
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    <p><strong>The post is updated. Please kindly jump to the Solution part, if you've already read the posted question. Thanks!</strong> </p> <p>Here's the minimized code to exhibit my problem:</p> <p>The input data file for test has been saved by Window's built-in Notepad as UTF-8 encoding. It has the following three lines:</p> <pre> abacus æbәkәs abalone æbәlәuni abandon әbændәn </pre> <p>The Perl script file has also been saved by Window's built-in Notepad as UTF-8 encoding. It contains the following code:</p> <pre><code>#!perl -w use Data::Dumper; use strict; use autodie; open my $in,'&lt;',"./hash_test.txt"; open my $out,'&gt;',"./hash_result.txt"; my %hash = map {split/\t/,$_,2} &lt;$in&gt;; print $out Dumper(\%hash),"\n"; print $out "$hash{abacus}"; print $out "$hash{abalone}"; print $out "$hash{abandon}"; </code></pre> <p>In the output, the hash table seems to be okay:</p> <pre> $VAR1 = { 'abalone' => 'æbәlәuni ', 'abandon' => 'әbændәn', 'abacus' => 'æbәkәs ' }; </pre> <p>But it is actually not, because I only get two values instead of three:</p> <pre> æbәlәuni әbændәn </pre> <p>Perl gives the following warning message:</p> <p><code>Use of uninitialized value $hash{"abacus"} in string at C:\test2.pl line 11, &lt;$i n&gt; line 3.</code></p> <p>where's the problem? Can someone kindly explain? Thanks.</p> <p><strong>The Solution</strong></p> <p>Millions of thanks to all of you guys :) Now finally the culprit is found and the problem becomes fixable :) As @Sinan insightfully pointed out, I'm now 100% sure that the culprit for causing the problem I described above is the two bytes of BOM, which Notepad added to my data file when it was saved as UTF-8 and which somehow Perl does not treat properly. Although many suggested that I should use "&lt;:utf8" and ">:utf8" to read and write files, the thing is these utf-8 configurations do not solve the problem. Instead they may cause some other problems.</p> <p>To really solve the problem, all I actually need is to add one line of code to force Perl to ignore the BOM:</p> <pre><code>#!perl -w use Data::Dumper; use strict; use autodie; open my $in,'&lt;',"./hash_test.txt"; open my $out,'&gt;',"./hash_result.txt"; seek $in,3,0; # force Perl to ignore the BOM! my %hash = map {split/\t/,$_,2} &lt;$in&gt;; print $out Dumper(\%hash); print $out $hash{abacus}; print $out $hash{abalone}; print $out $hash{abandon}; </code></pre> <p>Now, the output is exactly what I expected:</p> <pre> $VAR1 = { 'abalone' => 'æbәlәuni ', 'abandon' => 'әbændәn', 'abacus' => 'æbәkәs ' }; æbәkәs æbәlәuni әbændәn </pre> <p>Please note the script is saved as UTF-8 encoding and the code does not have to include any utf-8 labels because the input file and the output file are both pre-saved as UTF-8 encoding.</p> <p>Finally thanks again to all of you. And thank you, @Sinan, for the insightful guidance. Without your help, I would stay in the dark for God know how long.</p> <p><strong>Note</strong> To clarify a little more, if I use:</p> <pre><code>open my $in,'&lt;:utf8',"./hash_test.txt"; open my $out,'&gt;:utf8',"./hash_result.txt"; my %hash = map {split/\t/,$_,2} &lt;$in&gt;; print $out Dumper(\%hash); print $out $hash{abacus}; print $out $hash{abalone}; print $out $hash{abandon}; </code></pre> <p>The output is this:</p> <pre> $VAR1 = { 'abalone' => "\x{e6}b\x{4d9}l\x{4d9}uni ", 'abandon' => "\x{4d9}b\x{e6}nd\x{4d9}n", "\x{feff}abacus" => "\x{e6}b\x{4d9}k\x{4d9}s " }; æbәlәuni әbændәn </pre> <p>And the warning message:</p> <pre> Use of uninitialized value in print at C:\hash_test.pl line 13, line 3. </pre>
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    1. COYour code works fine to me, although I would `chomp` inside the map {}
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    2. CO@Sinan, it's weird. I'm sure the entries are seprated by tabs and that's why I use \t. But when I use \s like apbianco suggested, things starts getting okay. Well, it's really bizzare!
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    3. CO@Kemper, so you tested the code on your machine and everything's okay? I manually typed tabs to separate the entries but the code fails for the first line entry.
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