Stuart White wrote:
>
> I'm working through Beginning Perl, and I'm stuck on a
> particular exercise. The exercise asks me to take a
> number from <STDIN> and print all the prime numbers
> between 2 and that number. (ch 4, pg 145, #3)
>
> I can take the number from the user and save it in a
> scalar.
> My solution to listing the numbers between 2 and it,
> is to use the range operator on it and 2, and then
> store that into an array.
> Of course, then all the numbers are squished together.
> (How else might I say that?)
I don't know, what do you mean by "squished together"?
> So my solution is to
> "stringify" the array of numbers by putting the array
> inside double quotes and assigning it to itself. That
> seems to work.
How does that "seem to work"?
> However, when I test it by calling print "$list[3]" or
> print "@list" I get an error.
> "Use of unitialized value in concatenation (.) or
> string at primeNumbers.pl line 18, STDIN line 1."
> This makes me think that my array is still acting like
> a string, which I don't want.
> I thought Perl was supposed to convert numbers to
> strings and back when needed, right?
> Am I correct in my thinking?
>
> After, assuming that I can get the array into number
> form again, I'll work that out later.
>
> I'll show my code, but, if it's ok with you all,
> please don't write code for me, but rather, write an
> explanation of why my code is wrong or my thinking
> wrong. If you think I'm heading in the wrong
> direction, please let me know. Thanks.
>
> #!/usr/bin/perl
> # primeNumbers.pl
> use warnings;
> use strict;
> my $input;
> my @list;
> my @primelist;
> print "I want you to enter a list of numbers bigger
> than 2.\n";
> print "To mark the end of your list, enter 0.\n";
> print "Enter a number that is bigger than 2:\t";
> $input = <STDIN>;
> chomp($input);
> print "This is your number: $input \n";
> @list = (2 .. $input);
> print "list first: @list\n";
> @list = "@list";
That is the same as:
$list[ 0 ] = join $", @list;
Given that $" defaults to ' ' and assuming @list contains ( 2, 3, 4, 5,
6 ) you will have the string '2 3 4 5 6' in $list[0] and the rest of the
array will be empty.
> print "list second: @list\n";
> print "this is this the fourth element: $list[3]\n";
That will give you the warning "Use of unitialized value in
concatenation" because $list[3] is now undefined.
> #foreach (@list)
> #{
> # if ($_ % 2 == 0)
> # {
> # unshift(@primelist, $_);
> # print "this is the inloop primelist: @primelist\n";
> # }
> #}
> #print "this is the outloop primelist: @primelist\n";
John
--
use Perl;
program
fulfillment
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