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POParameters passed to multiple argument lambda expression
 

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    <p>I just watched <a href="http://channel9.msdn.com/Series/C9-Lectures-Stephan-T-Lavavej-Standard-Template-Library-STL-/C9-Lectures-Introduction-to-STL-with-Stephan-T-Lavavej" rel="nofollow">this lecture</a> about STLs by STL.</p> <p>Around 57 minutes into the lecture, we have this code:</p> <pre><code>#include &lt;iostream&gt; #include &lt;vector&gt; #include &lt;string&gt; #include &lt;algorithm&gt; int main() { std::vector&lt;std::string&gt; v; v.push_back("cat"); v.push_back("antelope"); v.push_back("puppy"); v.push_back("bear"); std::sort(v.begin(), v.end(), [](const std::string&amp; left, const std::string&amp; right) { return left.size() &lt; right.size(); } ); for (std::vector&lt;std::string&gt;::iterator i = v.begin(); i != v.end(); ++i) { std::cout &lt;&lt; *i &lt;&lt; " "; } std::cout &lt;&lt; std::endl; return 0; } </code></pre> <p>As expected, this prints the strings in the vector in increasing order of their lengths. My question is about the lambda expression which is the 3rd argument to the <code>sort</code> function. Internally, what gets passed to the input parameters 'left' &amp; 'right'?</p> <p>I added the line:</p> <pre><code>std::cout &lt;&lt; "left: " &lt;&lt; left &lt;&lt; ", right: " &lt;&lt; right &lt;&lt; std::endl; </code></pre> <p>inside the body of the lambda, and the output I got was:</p> <p><em>left: antelope, right: cat<br> left: antelope, right: cat<br> left: puppy, right: cat<br> left: puppy, right: antelope<br> left: puppy, right: cat<br> left: bear, right: cat<br> left: bear, right: antelope<br> left: bear, right: puppy<br> left: bear, right: cat<br> cat bear puppy antelope</em> </p> <p>So it looks like the 'left' and 'right' arguments are somehow related to the internal sorting algorithm in play. Can anyone shed more light on what exactly is going on?</p> <p>My understanding is that if the lambda were to be a unary function, then it's input argument would have been whatever the iterator is currently pointing to. Is this correct?<br> But with a binary function, the input arguments puzzle me.</p>
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    1. COAm I missing something, or are you asking why the comparator function that compares *two* things requires *two* parameters? While the sorting algorithm runs it frequently compares items to determine if their "order" is correct, stressing the **s** in the word "items".
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    2. COI do understand that a comparator function needs two things. My question is 'which' 2 things are being passed into the comparator, and if possible, 'how'. In other words, as a programmer, what can I expect to get as input arguments so that I can decide what check to perform inside the lambda.
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    3. CO@Gautam Ah. You can expect anything in your sequence. When and how-often something appears is entirely dependent on the algorithm being used. And I advise you thoroughly understand the concept of a strict-weak-ordering before writing your own, especially if writing one that orders based on multiple field values in a complex object.
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