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    <p>Ruby has some built-in methods that make this pretty easy:</p> <pre><code>def has_zero_sum_pair?(a) a.permutation(2).any?{|pair| pair.inject(:+) == 0} end </code></pre> <p><code>a.permutation(2)</code> gives you every pair in <code>a</code>. <code>any?</code> returns <code>true</code> if the block ever returns <code>true</code>. <code>inject(:+)</code> is an easy way to get the sum of an array.</p> <pre><code>has_zero_sum_pair?([1, 2, 3]) # =&gt; false has_zero_sum_pair?([1, 2, 3, -2]) # =&gt; true has_zero_sum_pair?([1, 2, 3, 0]) # =&gt; false has_zero_sum_pair?([0, 1, 2, 3, 0]) # =&gt; true </code></pre> <p><strong>Update:</strong> If I didn't know about <a href="http://ruby-doc.org/core-2.0/Array.html#method-i-permutation" rel="nofollow">Array#permutation</a> and had to accomplish this in the first way that came to mind, or if I was concerned about performance, I'd probably do something like this:</p> <pre><code>def has_zero_sum_pair2?(a) (0..a.length-2).each do |i| (i+1..a.length-1).each do |j| return true if a[i] + a[j] == 0 end end false end </code></pre> <p>I find that uglier and more error-prone, and it took longer to write. But it's about four times faster with small arrays and 10 times faster for larger arrays. It's typical in programming to have an easy way that works well enough in most cases but isn't ideal in performance. Sometimes better performance can be attained without costing clarity by choosing a better algorithm or data structure. Often a trade-off has to be made.</p> <p><strong>Update 2:</strong> If I really cared about performance, I'd use @FMc's technique. That's a great example of using a better data structure and algorithm for huge performance gains.</p>
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    1. USDarshan Rivka Whittle
    1. USDarshan Rivka Whittle
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    1. COI'd suggest showing the version without `inject` alongside the one using it, to make it clearer to a beginner just what it's doing :)
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    2. COAs a warning: `permutation` can get really slow when the array gets big.
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      1. USthe Tin Man
    3. COYea is there a more "beginner" way of doing this without using "permutation"? i just started learning programming last week and I barely even know the inject method. thanks
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