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Re: Modification of a read-only value attempted

by BrowserUk (Pope)
on Nov 27, 2012 at 13:22 UTC ( #1005854=note: print w/ replies, xml ) Need Help??


in reply to Modification of a read-only value attempted

You made $c point to a constant: $c=\99;; hence when you try to modify that constant by indirecting through $c: $$c=1000;; Perl won't allow you to do so.

Nor should it.


With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

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Re^2: Modification of a read-only value attempted
by panku (Initiate) on Nov 27, 2012 at 13:34 UTC

    Thanks for the Reply. Please clarify one more doubt. 1.) $c=99; Here $c is var which is holding 99 as value. We can change $c here ? 2.) $c=\99; Here also $c is reference which is pointing to 99. So why we can't change $c=1000? Thanks in advance

      panku:

      You can change $c=1000. What you can't do is change 99 to 1000. When you use $c=1000, it tells perl that $c is no longer pointing to a reference to the constant 99, but is now containing the value 1000. When you use $$c=1000, you're telling perl to change the thing $c points to to 1000. But $c points to the constant value 99. Perl is telling you that it refuses to break the time-space continuum by making 99 be 1000.

      ...roboticus

      When your only tool is a hammer, all problems look like your thumb.

      So why we can't change $c=1000?

      As roboticus points out, you can make that change. That is, you can overwrite the value in $c -- currently the address of the constant 99 -- with any other value.

      What you cannot do is overwrite the value held at the address held in $c, because it is a constant.


      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.

      RIP Neil Armstrong

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