in reply to showing the error ,Can't modify concatenation (.) or string in scalar assignment

G'day gayu_justin,

Firstly, this code is highly confusing and a potential source of errors:

my $check_ = "chck";
my $chck_ = "check";

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Regarding the code generating the error, instead of:

foreach $usr( @
#print $usr . "\n";
#print $check_.$usr;
my $check_.$usr = $cgi->param("$chck_.$usr");
print $chck_.$usr;
}

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I suspect you want something closer to this (untested):

for my $user_id (@userchecklist_id) {
    my $checkbox_name = "check_$user_id";
    my $checkbox_value = $cgi->param($checkbox_name);
    print "Checkbox name:  $checkbox_name\n";
    print "Checkbox value: $checkbox_value\n";
}

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By using meaningful variable names and taking a little care when laying out your code, you'll improve readability and maintainability. Take a look at perlstyle for some tips on the latter.

As a final point, note how you've reused $usr: 

my $usr = $user->id;
...
foreach $usr( @userchecklist_id) {

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This, again, is a potential source of errors. Consider what might happen if you wanted to use the $user->id version of $usr after the loop. By using my when you start the loop (as I did) you get a different variable (scoped to the loop) and avoid possible future problems. [In case you didn't know, for and foreach are synonymous so you can save yourself some typing (see perlsyn - Foreach Loops).]

-- Ken

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Re^2: showing the error ,Can't modify concatenation (.) or string in scalar assignment
by AnomalousMonk (Archbishop) on Jul 18, 2013 at 10:43 UTC

    The following is more for the benefit gayu_justin than for kcott:

    Consider what might happen if you wanted to use the $user->id version of $usr after the loop. By using my when you start the loop (as I did) you get a different variable (scoped to the loop) and avoid possible future problems.

    This is an oddity, IMHO, of the way for-loop variables behave. The variable is fully localized even though it is lexical. So this code doesn't work as one (I myself, on more than one occasion) would naively expect: 

    >perl -wMstrict -le
"my $max_x = 99;
 ;;
 for $max_x (0 .. 5) {
   print $max_x;
   last if $max_x == 2;
   }
 ;;
 print qq{max_x: $max_x};
"
0
1
2
max_x: 99

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    Nor does the loop variable do something really odd like becoming a package variable: 

    >perl -wMstrict -le
"my $x = 'foo';
 print qq{x before loop: '$x'};
 ;;
 for $x (0 .. 2) {
   func();
   our $y;
   print qq{$x, $y};
   }
 ;;
 print qq{x after loop: '$x'};
 sub func {
   our $x;
   our $y;
   $x = 99;
   $y = 42;
   }
"
x before loop: 'foo'
0, 42
1, 42
2, 42
x after loop: 'foo'

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    So what's the point of ever using a lexical loop variable in the  for my $x (...) { ... } form? If  $x was a package variable to start with, the loop variable  $x would then be fully lexically scoped. Remove the  my from the following example to see the difference. 

    >perl -wMstrict -le
"our $max_x = 99;
 ;;
 for my $max_x (0 .. 4) {
   func();
   print $max_x;
   last if $max_x == 2;
   }
 ;;
 print qq{max_x after loop: $max_x};
 ;;
 sub func {
   $max_x = 42;
   }
"
0
1
2
max_x after loop: 42

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      ++ You're absolutely right. I've been writing for my $var ... for so long that I'd forgotten about that rather unintuitive behaviour.

      By the way, you're not the only one to have been bitten by that. :-)

      -- Ken

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