Path: utzoo!attcan!uunet!decwrl!ucbvax!iwarp.intel.com!news
From: merlyn@iwarp.intel.com (Randal Schwartz)
Newsgroups: comp.lang.perl
Subject: Re: Changing the first character of a string.
Message-ID: <1990Jun30.211102.14938@iwarp.intel.com>
Date: 30 Jun 90 21:11:02 GMT
References: <sean.646706730@s.ms.uky.edu>
Sender: news@iwarp.intel.com
Reply-To: merlyn@iwarp.intel.com (Randal Schwartz)
Organization: Stonehenge; netaccess via Intel, Beaverton, Oregon, USA
Lines: 57
In-Reply-To: sean@ms.uky.edu (Sean Casey)

In article <sean.646706730@s.ms.uky.edu>, sean@ms (Sean Casey) writes:
| 
| How can I change an arbitrary character of a string? I've got a fixed length
| string, $flags, that is used in a printf. I need to be able to do something
| like:
| 
| 	$flags = "   ";
| 
| 	if ($mb_flags[$num] && $FL_UNREAD) {
| 		set first character of $flags to 'U';
| 	}
| 	if ($mb_flags[$num] && $FL_SENT) {
| 		set second character of $flags to 'S';
| 	}
| 	and so on

substr() can be used as an lvalue.  From the manpage:

     substr(EXPR,OFFSET,LEN)
             Extracts a substring out of  EXPR  and  returns  it.
             First  character  is at offset 0, or whatever you've
             set $[ to.  If OFFSET is negative, starts  that  far
             from  the  end  of  the  string.   You  can  use the
                                                =================
             substr() function as an lvalue, in which  case  EXPR
             ====================================================
             must  be an lvalue.  If you assign something shorter
             ===================
             than LEN, the string will shrink, and if you  assign
             something  longer  than LEN, the string will grow to
             accommodate it.  To keep the string the same  length
             you  may  need  to  pad  or  chop  your  value using
             sprintf().

In English (or something resembling English :-), this means that you
can select portions of strings with substr, and then assign into them.
So, a sample of what you wanted to do is:

	$flags = " " x 3; # hard to count spaces... :-)
	substr($flags,0,1) = "U";
	print "<$flags>\n";

| That's why we need a perl book. I've written 40 or so odd perl scripts, and
| something this simple evades me.

I've written probably ten times that many *production* scripts (and
many more one-ups), and I still find myself reading the manpage (and
occasionally the code :-) from cover to cover to get what I need to
know.  I hope the book clears up some of the folklore and allows you
to have access to our collective experience.

for("hacker","Perl","another","Just"){substr($x,0,0)="$_ ";}substr($x,-1,1)=",";print$x
-- 
/=Randal L. Schwartz, Stonehenge Consulting Services (503)777-0095 ==========\
| on contract to Intel's iWarp project, Beaverton, Oregon, USA, Sol III      |
| merlyn@iwarp.intel.com ...!any-MX-mailer-like-uunet!iwarp.intel.com!merlyn |
\=Cute Quote: "Welcome to Portland, Oregon, home of the California Raisins!"=/