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How do I assign & substitute in one statement?

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Contributed by dfaure on Jun 14, 2004 at 07:09 UTC
Q&A  > strings


Description:

Is it possible to assign a value to a variable and then do a substitution on it in a single statement? For example suppose I want to start with a URL, remove a known host-domain name, then assign the remaining dir/filename to a variable. With two statements you could say:

$dirfilename = $url; $dirfilename = s|http://foo.bar.com/||;

Answer: How do I assign & substitute in one statement?
contributed by dfaure

From Re: Assign & substitute in one statement? by Zaxo

Just write parens in their precedence-adjusting role:

($dirfilename = $url) =~ s|\Qhttp://foo.bar.com/\E||;

From the Perl Cookbook, recipe 6.1.Copying and Substituting Simultaneously:

You can even use this technique on an entire array:

@bindirs = qw( /usr/bin /bin /usr/local/bin ); for (@libdirs = @bindirs) { s/bin/lib/ } print "@libdirs\n"; __OUTPUT__ /usr/lib /lib /usr/local/lib

The parentheses are required when combining an assignment if you wish to change the result in the leftmost variable. Normally, the result of a substitution is its success: either "" for failure, or the number of times the substitution was done. Contrast this with the preceding examples where the parentheses surround the assignment itself. For example:

($a = $b) =~ s/x/y/g; # copy $b and then change $a $a = ($b =~ s/x/y/g); # change $b, count goes in $a
Answer: How do I assign & substitute in one statement?
contributed by Roy Johnson

Predicate-for allows you to declare and modify a lexical in the same statement:

s[http://foo.bar.com/][] for (my $dirfilename = $url);
Using block-for does not put the lexical in the surrounding scope. This is an interesting difference, particularly since -MO=Deparse indicates that the above statement is translated into the block form.
Answer: How do I assign & substitute in one statement?
contributed by dimar

You can also use parenthesis to impose 'list context', and then combine that with 'map' to get what you want in a single line. The drawback is you have to shoehorn your string into a list, which is a bit artificial. The benefit is this may be a little bit easier to read and understand what is going on for beginning perl programmers.

$sBegin = "hello world"; ($sEnd) = map{s/hello/goodbye/g;$_;}($sBegin); print $sEnd;

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