sashac88 has asked for the wisdom of the Perl Monks concerning the following question:

Hello again, good monks.This is my case: 
foreach (readline FILE1) {
        if (/$hashnew{$com}/){
              print WFILE "$_\n";
        }
}

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$hashnew{$com} has value - the word "context". How do I make the script to treat $hashnew{$com} as it's value,i.e. in this case to treat it as context. Thanks a lot in advance.

Edited by Chady -- code tags, fixed spaces in the code part.

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Re: File parsing
by dave_the_m (Monsignor) on Jun 30, 2004 at 11:31 UTC
    As an aside, you might want to replace 
    foreach (readline FILE) {

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    with 
    while (<FILE>) {

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    The former reads the whole file into memory before processing it, which may become a problem with large files.

    Dave.

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Re: File parsing
by tachyon (Chancellor) on Jun 30, 2004 at 11:29 UTC

    As borisz says it should work. Add a sanity check to ensure it really contains what you expect like this (note the quotes around the value let you see hidden spaces/newlines which may be the issue) 

    warn "Value of \$hashnew{\$com}: '$hashnew{$com}'\n";

while(<FILE1>) { 
    print WFILE $_ if m/$hashnew{$com}/;
}

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    A while loop is a more usual way to iterate over a files content and more efficient that your foreach. Newlines remain attached so you don't need to add one when you print the file out (unless you are using -l switch)

    On unix you could get the same functionality of this snippet with something like grep context file1 > wfile

    cheers

    tachyon

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Re: File parsing
by borisz (Canon) on Jun 30, 2004 at 11:20 UTC
    do nothing, it works already that way.
    Boris
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Re: File parsing
by calin (Deacon) on Jun 30, 2004 at 14:17 UTC

    In addition:

    If you want to treat the value of $hashnew{$com} as a string literal (avoid regexp semantics), use the \Q...\E construct: 

    if (/\Q$hashnew{$com}\E/) {
# ...

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    This is highly recommended if $hashnew{$com} comes from untrusted sources (because otherwise it can open a security hole). It also helps avoid weird behaviour or sudden death (in case $hashnew{$com} happens to contains regexp control characters).

    If you want to avoid the regexp overhead, you can use index: 

    if (index($_, $hashnew{$com}) + 1) {
# ...

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    All of the above applies only if you want $hashnew{$com} to behave as a simple substring, not as a sub-regexp. (your post is vague, so I'm guessing)

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