{"id":29261,"date":"2023-01-06T14:29:34","date_gmt":"2023-01-06T08:59:34","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-perl-programming\/"},"modified":"2023-01-06T14:29:34","modified_gmt":"2023-01-06T08:59:34","slug":"solved-perl-programming","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-perl-programming\/","title":{"rendered":"[Solved] Perl Programming"},"content":{"rendered":"

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You’re talking about regular expressions and how to use them in Perl. Your question seems to be whether the answers you picked to homework are correct.<\/p>\n

The code you’ve added should do what you want, but it has syntax errors.<\/p>\n

if ( ^x*$ ) {\n    print \"This is\";\n}\n<\/code><\/pre>\n
Your pattern<\/em> is correct, but you don’t know how to use a regular expression in Perl. You’re missing the actual operator to tell Perl that you want a regular expression.<\/p>\n
The short form is this, where I’ve highlighted the important part with #<\/code><\/p>\n
if ( \/^x*$\/ ) {\n     #    #\n<\/code><\/pre>\n
The slashes \/\/<\/code> tell Perl that it should match a pattern. The long form of it is:<\/p>\n
if ( $_ =~ m\/^x*$\/ ) {\n     ## ## ##    #\n<\/code><\/pre>\n
$_<\/code> is the variable that you are matching against a pattern. The =~<\/code> is the matching operator<\/em>. The m\/\/<\/code> constructs a pattern to match with. If you use \/\/<\/code> you can leave out the m<\/code>, but it’s clearer to put it in.<\/p>\n
The $_<\/code> is called topic<\/em>. It’s like a default variable that stuff goes into in Perl if you don’t specify another variable.<\/p>\n
while ( <$fh> ) {\n    print $_ if $_ =~ m\/foo\/; # print all lines that contain foo\n}\n<\/code><\/pre>\n
This code can be written as $_<\/code>, because a lot of commands in Perl assume that you mean $_<\/code> when you don’t explicitly name a variable.<\/p>\n
while ( <$fh> ) {    # puts each line in $_\n    print if m\/foo\/; # prints $_ if $_ contains foo\n}\n<\/code><\/pre>\n
You code looks like you wanted to do that, but in fact you have a $row<\/code> in your loop. That’s good, because it is more explicit. That means it’s easier to read. So what you need to do for your match is:<\/p>\n
while ( my $row = <$fh> ) {\n    if ( $row =~ m\/^x*$\/ ) {\n        print \"This is\";\n    }\n}\n<\/code><\/pre>\n
Now you will iterate<\/em> each line of the file behind the $fh<\/code> filehandle, and check if it matches the pattern ^x*$<\/code>. If it does, you print _”This is”. That doesn’t sound very useful.<\/p>\n
Consider this example, where I am using the __DATA__<\/code> section instead of a file.<\/p>\n
use strict;\nuse warnings;\n\nwhile ( my $row = <DATA> ) {\n    if ( $row =~ m\/^x*$\/ ) {\n        print \"This is\";\n    }\n}\n\n__DATA__\nfoo\nxxx\n\nx\nxxxxx\nbar\n<\/code><\/pre>\n
This will print:<\/p>\n
This isThis isThis isThis is\n<\/code><\/pre>\n
It really does not seem to be very useful. It would make more sense to include the line that matched.<\/p>\n
if ( $row =~ m\/^x*$\/ ) {\n    print \"match: $row\";\n}\n<\/code><\/pre>\n
Now we get this:<\/p>\n
match: xxx\nmatch: \nmatch: x\nmatch: xxxxx\n<\/code><\/pre>\n
That’s almost what we expected. It matches a single x<\/code>, and a bunch of x<\/code>s. It did not match foo<\/code> or bar<\/code>. But it does match an empty line.<\/p>\n
That’s because you picked the wrong pattern. <\/p>\n
The *<\/code> multiplier means  match as many as possible, as least none<\/strong><\/em>.
\nThe +<\/code> multiplier means match as many as possible, at least one<\/strong><\/em>.<\/p>\n
So your pattern should be the one with +<\/code>, or it will match if there is nothing, because start of the line<\/em>, no x<\/code>, end of the line<\/em> matches an empty line.<\/p>\n
While you’re at it, you could also rename your variable. Unless you’re dealing with CSV, which has rows of data, you have lines<\/em>, not rows<\/em>. So $line<\/code> would be a better name for your variable. Giving variables good, descriptive names is very important because it makes it easier to understand your program.<\/p>\n
use strict;\nuse warnings;\n\nmy $filename=\"data.txt\";\n\nopen( my $fh, '<:encoding(UTF-8)', $filename ) \n    or die \"Could not open file '$filename' $!\";\n\nwhile ( my $line = <$fh> ) {\n    if ( $line =~ m\/^x+$\/ ) {\n        print \"match: $line\";\n    }\n}\n<\/code><\/pre>\n<\/p><\/div>\n
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solved Perl Programming <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] You’re talking about regular expressions and how to use them in Perl. Your question seems to be whether the answers you picked to homework are correct. The code you’ve added should do what you want, but it has syntax errors. if ( ^x*$ ) { print “This is”; } Your pattern is correct, but … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[442],"class_list":["post-29261","post","type-post","status-publish","format-standard","hentry","category-solved","tag-perl"],"yoast_head":"\n[Solved] Perl Programming - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-perl-programming\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] Perl Programming - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] You’re talking about regular expressions and how to use them in Perl. 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