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POMultiply operator applied to list(data structure)
 

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    <p>I'm reading <a href="http://openbookproject.net/thinkCSpy/" rel="noreferrer">How to think like a computer scientist</a> which is an introductory text for "Python Programming".</p> <p>I want to clarify the behaviour of multiply operator (<code>*</code>) when applied to lists.</p> <p>Consider the function <strong>make_matrix</strong></p> <pre><code>def make_matrix(rows, columns): """ &gt;&gt;&gt; make_matrix(4, 2) [[0, 0], [0, 0], [0, 0], [0, 0]] &gt;&gt;&gt; m = make_matrix(4, 2) &gt;&gt;&gt; m[1][1] = 7 &gt;&gt;&gt; m [[0, 0], [0, 7], [0, 0], [0, 0]] """ return [[0] * columns] * rows </code></pre> <p>The actual output is </p> <pre><code>[[0, 7], [0, 7], [0, 7], [0, 7]] </code></pre> <p>The correct version of <strong>make_matrix</strong> is :</p> <pre><code>def make_matrix(rows, columns): """ &gt;&gt;&gt; make_matrix(3, 5) [[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]] &gt;&gt;&gt; make_matrix(4, 2) [[0, 0], [0, 0], [0, 0], [0, 0]] &gt;&gt;&gt; m = make_matrix(4, 2) &gt;&gt;&gt; m[1][1] = 7 &gt;&gt;&gt; m [[0, 0], [0, 7], [0, 0], [0, 0]] """ matrix = [] for row in range(rows): matrix += [[0] * columns] return matrix </code></pre> <p>The reason why first version of <strong>make_matrix</strong> fails ( as explained in the book at 9.8 ) is that</p> <p><em>...each row is an alias of the other rows...</em></p> <p>I wonder why </p> <pre><code>[[0] * columns] * rows </code></pre> <p>causes <em>...each row is an alias of the other rows...</em></p> <p>but not </p> <pre><code>[[0] * columns] </code></pre> <p>i.e. why each <code>[0]</code> in a row is not an alias of other row element.</p>
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