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    <p>There are a number of problems with this code. First, <code>a/divisor == int(a/divisor)</code> is always <code>True</code>, because division in Python is integral (you will always get an integer result). Use <code>a % divisor == 0</code> instead.</p> <p>Another problem is that <code>prime = True</code> is outside the loop, meaning that as soon as one value is declared not prime, no more values can be prime! (<code>prime</code> will never get set back to <code>True</code> anywhere.)</p> <p>A third issue, more of style, is that it is preferred to use a <code>for ... in</code> loop in python, eg</p> <pre><code>for a in xrange(1, 1000): for divisor in xrange(2, a): ... </code></pre> <p>Edit: Regarding your modified code: your <code>if</code> statement at the very end is not indented, and therefore not part of the <code>for</code> loop. It is only being executed once, after the loop ends, meaning <code>prime</code> will be the last value set (where <code>a</code> is <code>999</code>, not a prime). You want to indent the entire <code>if</code> statement to be in your inner <code>for</code> loop. You could condense this, though:</p> <pre><code>for a in xrange(1, 1000): for divisor in xrange(2, a): if a % divisor == 0: break else: print a numberlist.append(a) </code></pre> <p>Note that this uses the <code>for ... else</code> clause. Putting <code>else</code> at the end of a <code>for</code> loop will cause that <code>else</code> block to run <em>only if</em> the loop is not broken by <code>break</code>.</p> <p>As a further comment, starting the whole loop at <code>1</code> will have <code>1</code> in the list, but <a href="http://en.wikipedia.org/wiki/Prime_number" rel="nofollow"><code>1</code> is not a prime</a>.</p>
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