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copied!<p>Let's try an example.</p> <pre><code>numbers = [ 3, 1, 4, 1, 5, 9 ] sum = numbers.inject(0) do |prefix_sum, number| prefix_sum + number end </code></pre> <p><code>#inject</code> takes one argument and a block. The block should take two values, and return a new value.</p> <p>In the above example, the argument to <code>#inject</code> is <code>0</code>, and the block is <code>do |prefix_sum, number| prefix_sum + number end</code>. The values that will be passed to the block are named in between the two <code>|</code> markers: <code>prefix_sum</code> and <code>number</code>.</p> <p>Each value of the enumerable <code>#inject</code> was called on is passed as the second value to the block in turn. In this example <code>number</code> will be <code>3</code>, then <code>1</code>, then <code>4</code>, then <code>1</code>, then <code>5</code>, then finally <code>9</code>. So in this example, the block will be invoked six times; once for each position in <code>numbers</code>.</p> <p>The first value passed to a block (here named <code>prefix_sum</code>) is usually called an <em>accumulator</em>. Its initial value, the value used the first time the block is called by <code>#inject</code>, is set by the argument passed to <code>#inject</code> (in this example, <code>0</code>). The return value of the block determines the value of the accumulator (<code>prefix_sum</code>) for the next invocation of the block.</p> <p>When there are no more elements to process, the value of the accumulator is returned (and here stored in <code>sum</code>).</p> <p>So lets walk through it:</p> <ul> <li><code>#inject</code> receives <code>0</code> and our block.</li> <li><code>#inject</code> invokes our block, binding <code>prefix_sum</code> to <code>0</code> (the initial accumulator value) and <code>number</code> to <code>3</code> (the first array value).</li> <li>our block calculates <code>0+3</code> as <code>3</code> and returns it.</li> <li><code>#inject</code> invokes our block, binding <code>prefix_sum</code> to <code>3</code> (the returned value) and <code>number</code> to <code>1</code> (the second array value)</li> <li>our block calculates <code>3+1</code> as <code>4</code> and returns it.</li> <li><code>#inject</code> invokes our block, binding <code>prefix_sum</code> to <code>4</code> (the returned value) and <code>number</code> to <code>4</code> (the third array value)</li> <li>our block calculates <code>4+4</code> as <code>8</code> and returns it.</li> <li><code>#inject</code> invokes our block, binding <code>prefix_sum</code> to <code>8</code> (the returned value) and <code>number</code> to <code>1</code> (the fourth array value)</li> <li>our block calculates <code>8+1</code> as <code>9</code> and returns it.</li> <li><code>#inject</code> invokes our block, binding <code>prefix_sum</code> to <code>9</code> (the returned value) and <code>number</code> to <code>5</code> (the fifth array value)</li> <li>our block calculates <code>9+5</code> as <code>14</code> and returns it.</li> <li><code>#inject</code> invokes our block, binding <code>prefix_sum</code> to <code>14</code> (the returned value) and <code>number</code> to <code>9</code> (the sixth array value)</li> <li>our block calculates <code>14+9</code> as <code>23</code> and returns it.</li> <li>since there are no more array elements, <code>#inject</code> returns <code>23</code>, and we bind <code>sum</code> to be that value.</li> </ul> <p>You can look at inject as parenthesizing an operation on a list of items, in this example, caluculating:</p> <pre><code>((((((0 + 3) + 1) + 4) + 1) + 5) + 9) </code></pre> <p>This lets you take any operation which normally only operates on a pair of arguments, and apply it to a list.</p>

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