Sieve of Eratosthenes

hi there!

i have to program the sieve of eratosthenes in php as a homework.
after i had created an html file where the maximum is set i wrote a
php script which doesn't work properly - actually it doesn't work at
all.

and here it is:

1 <?php
2 $m = $_POST['max'];
3
4 for ($i=2; $i<=$m; $i=$i+1)
5 {$prime='true';
6 for ($n=2; $n<=sqrt($m); n=n+1)
7 {
8 if (i/n=0)
9 {$prime='false';}
10 if ($prime='true')
11 {echo $i;}
12 }
13 }
14
15 ?>

i constantly get the following error:

Parse error: parse error, unexpected '=', expecting ')' in
C:\Programme\winampp\xampp\htdocs\primzahl.php on line 6

but i cannot find a mistake in line 6. can you? i hope so. but maybe
the mistake lies somewhere else too. i don't know.

i hope someone can help. thanx.

I noticed that Message-ID:
<c1769b91.0412290643.296fbccd@posting.google.com > from MSiegel contained
the following:

>i have to program the sieve of eratosthenes in php as a homework.
>after i had created an html file where the maximum is set i wrote a
>php script which doesn't work properly - actually it doesn't work at
>all.

But the sieve of Eratosthenes doesn't work the way you coded it. Your
assignment was probably intended to get you working with arrays. A
quick google found the following pseudocode

PSEUDOPROGRAM:

Set up an array of integers z[N]:
for i from 0 to N, set z[i] = i.

for i from 2 to N, mark all multiples
of i by setting them to zero
(or in php, you could unset the value in the array)

for i from 2 to N,
print all unmarked multiples.



--
Geoff Berrow (put thecat out to email)
It's only Usenet, no one dies.
My opinions, not the committee's, mine.
Simple RFDs http://www.ckdog.co.uk/rfdmaker/

Since this is homework, no one will just give you the answer, but
here's a few clues:

Line 6: Look at the way you are declaring the variable 'n'

Line 10: Something is wrong with your comparison statement
'if($prime='true')'

I noticed that Message-ID: <gdh5t01g3bhokb2qjf4bhse1feh4vnptsi@4ax.com>
from Geoff Berrow contained the following:

>>i have to program the sieve of eratosthenes in php as a homework.
>>after i had created an html file where the maximum is set i wrote a
>>php script which doesn't work properly - actually it doesn't work at
>>all.
>
>But the sieve of Eratosthenes doesn't work the way you coded it.
http://ccins.camosun.bc.ca/~jbritton/jberatosthenes.htm
--
Geoff Berrow (put thecat out to email)
It's only Usenet, no one dies.
My opinions, not the committee's, mine.
Simple RFDs http://www.ckdog.co.uk/rfdmaker/

On Wed, 29 Dec 2004 06:43:58 -0800, MSiegel wrote:

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> news.glorb.com!postnews.google.com!not-for-mail
> From: msiegel@gmx.de (MSiegel)
> Newsgroups: comp.lang.php
> Subject: Sieve of Eratosthenes
> Date: 29 Dec 2004 06:43:58 -0800
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>
>
> hi there!
>
> i have to program the sieve of eratosthenes in php as a homework.
> after i had created an html file where the maximum is set i wrote a
> php script which doesn't work properly - actually it doesn't work at
> all.
>
> and here it is:
>
> 1 <?php
> 2 $m = $_POST['max'];
> 3
> 4 for ($i=2; $i<=$m; $i=$i+1)
> 5 {$prime='true';
> 6 for ($n=2; $n<=sqrt($m); n=n+1)
> 7 {
> 8 if (i/n=0)
> 9 {$prime='false';}
> 10 if ($prime='true')
> 11 {echo $i;}
> 12 }
> 13 }
> 14
> 15 ?>
>
> i constantly get the following error:
>
> Parse error: parse error, unexpected '=', expecting ')' in
> C:\Programme\winampp\xampp\htdocs\primzahl.php on line 6
>
> but i cannot find a mistake in line 6. can you? i hope so. but maybe
> the mistake lies somewhere else too. i don't know.
>
> i hope someone can help. thanx.

In other news, line 8 will evaluate "true" only when $i is zero (never).
Maybe you meant some other operator? I'm looking sort of in the direction
of the middle-sized number keys... ;)

MSiegel wrote:

> hi there!
>
> i have to program the sieve of eratosthenes in php as a homework.
> after i had created an html file where the maximum is set i wrote a
> php script which doesn't work properly - actually it doesn't work at
> all.
>
> and here it is:
>
> 1 <?php
> 2 $m = $_POST['max'];
> 3
> 4 for ($i=2; $i<=$m; $i=$i+1)
> 5 {$prime='true';
> 6 for ($n=2; $n<=sqrt($m); n=n+1)
> 7 {
> 8 if (i/n=0)
> 9 {$prime='false';}
> 10 if ($prime='true')
> 11 {echo $i;}
> 12 }
> 13 }
> 14
> 15 ?>
>
> i constantly get the following error:

You are constantly forgetting the $ infront of a variable name, which is
required in PHP, unless it is a constant, and in that case, you can't
assign to it.
You might also want to have a look at the ++ operator.

--
Tommy

"Steve" <schigh@comcast.net> wrote:
> Since this is homework, no one will just give you the answer, but
> here's a few clues:
>
> Line 6: Look at the way you are declaring the variable 'n'

And by "declaring" he doesn't mean the initial declaration ("$n = 2"), but
the iterative step declaration ("n=n+2").


> Line 10: Something is wrong with your comparison statement
> 'if($prime='true')'
>

Here's the list of comparison operators:
http://www.php.net/manual/en/languag...comparison.php

-Noah

Tommy Gildseth wrote:

> MSiegel wrote:
>
>> i have to program the sieve of eratosthenes in php as a homework.

Well, since you're being so honest about it....other people have pointed out
some of the *errors* in the script, but your programming style is abysmal.

Have a read at http://pear.php.net/manual/en/standards.php for starters.

Use readable variable names.

>> 4 for ($i=2; $i<=$m; $i=$i+1)
>> 5 {$prime='true';
>> 6 for ($n=2; $n<=sqrt($m); n=n+1)


$m does not change throughout the life of the program but you are
calculating it's square root $m^1.5 times! It should be calculated once,
outside of both loops.

Once you know it's not prime, why bother to continue the loop?

>> 7 {
>> 8 if (i/n=0)
>> 9 {$prime='false';}
>> 10 if ($prime='true')
>> 11 {echo $i;}
>> 12 }
>> 13 }
>> 14
>> 15 ?>

There are also semantic errors in there too.

HTH

C.

"MSiegel" <msiegel@gmx.de> wrote in message
news:c1769b91.0412290643.296fbccd@posting.google.c om...
> hi there!
>
> i have to program the sieve of eratosthenes in php as a homework.
> after i had created an html file where the maximum is set i wrote a
> php script which doesn't work properly - actually it doesn't work at
> all.
>
> and here it is:
>
> 1 <?php
> 2 $m = $_POST['max'];
> 3
> 4 for ($i=2; $i<=$m; $i=$i+1)
> 5 {$prime='true';
> 6 for ($n=2; $n<=sqrt($m); n=n+1)

Along with everyone else's comments about your programming, try examining
this little program:

<?php

$m = 100;

for($i = 2; $i < 100; $i++)
{
echo "$i) " . sqrt($m) . "<br>\n";
}

?>

You'll notice that the square root of $m does not change during the
execution of the program, so calculating it every time you do a loop is:

a) unessecary
b) time consuming

> 7 {
> 8 if (i/n=0)

The sieve of of eratosthenes is a question about multiplication and
elimination, not division. If you have to divide something then you're doing
it wrong.

> 9 {$prime='false';}
> 10 if ($prime='true')
> 11 {echo $i;}
> 12 }
> 13 }
> 14
> 15 ?>
>
> i constantly get the following error:
>
> Parse error: parse error, unexpected '=', expecting ')' in
> C:\Programme\winampp\xampp\htdocs\primzahl.php on line 6
>
> but i cannot find a mistake in line 6. can you? i hope so. but maybe
> the mistake lies somewhere else too. i don't know.
>
> i hope someone can help. thanx.

Pay particular attention to Geoff Berrow's advice and link.

CJ Llewellyn wrote:
<snip>
> The sieve of of eratosthenes is a question about multiplication and
> elimination, not division. If you have to divide something then you're doing
> it wrong.
<snip>

After looking at the explanation of the algorithm here:

http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

I'm wondering, How one goes about determining if a number is a multiple
of another without dividing?

NM