using grep in the perl script

Question

I have a perl script, such as:


    #!/usr/bin/perl -w

    use strict;

    my @vars = split(" ", "aaa bbb");
    my @list = split(" ", "aaa");

    foreach(@vars) {
        if ( grep(/$_/, @list)) {
            print "1\t";
        }else{
            print "0\t";
        }
    }

I was hoping it gives "1 0" in return, but it is always "1 1". Does anyone know why ? I am using perl 5.8.8.

Thanks.

Chun

mob mob · Accepted Answer · 2016-03-01T01:50:51

Because you are abusing the $_ variable.

Inside the expression of a grep EXPR, LIST construction, $_ is aliased to an element of the list and the expression is evaluated. The previous value of $_ is not restored until the end of the grep statement.

Since a naked /.../ expression is equivalent to $_ =~ /.../, your grep statement is just

grep $_ =~ /$_/, @list

with $_ aliased to an element of @list, and which is true for most normal values of $_.

The lesson is to not make $_ do too much work for you. Say

foreach $var (@vars) {
    if ( grep(/$var/, @list)) {
        print "1\t";
    }else{
        print "0\t";
    }
}

instead. Anyone who looks at your code six months from now (including your future self) will thank you.

using grep in the perl script

1 Answers