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    <p>This is how I would approach it:</p> <p>Pretend you have a <code>vector&lt;SomeValue&gt;</code> that is sorted. This vector is accessible using <code>someFunction(index)</code>.</p> <p>Now, <a href="http://en.wikipedia.org/wiki/Binary_search#algorithm" rel="nofollow">look at this</a>. That is pseudocode for the binary search. Continuing with the thought process above, replace <code>A[imid]</code> with <code>someFunction(imid)</code> and make <code>key</code> a <code>SomeValue</code>. Make sure <code>SomeValue</code> has a valid <code>operator &lt;</code> (or a comparator function that you use in place of it).</p> <p>This will of course only work if <code>someFunction(x) &lt; someFunction(x + 1)</code> for all <code>x</code>. You have stated this to be true, so it should be good.</p> <p>I would suggest using the iterative approach because both have the same asymptotic runtime, and the iterative version makes it easier to report a key that was not found, and tends to use less memory.</p> <p><strong>EDIT</strong> I am unaware of a simple way to do it using the <code>std</code> stuff. As you mentioned above, <code>int</code> wouldn't work as an iterator, and all of the functions that you would probably want to use take iterators. <em>Technically</em> though, the iterators in these functions are templated types, so you write your own <code>IntIter</code> class or something like that. Using <code>std::lower_bound</code> would need the <code>operator *()</code>, <code>operator ++()</code>, and <code>operator +(int)</code>. <code>operator *()</code> would probably return <code>someFunction(n)</code>, where <code>n</code> is the int value associated with the <code>IntIter</code>. However, <strong><em>I don't know if this would actually work</em></strong>, and it would probably take more time and coding. You should look at <a href="http://www.cplusplus.com/reference/algorithm/lower_bound/" rel="nofollow"><code>std::lower_bound</code></a> and <a href="http://www.cplusplus.com/reference/iterator/advance/" rel="nofollow"><code>std::advance</code></a> (called in <code>lower_bound</code>) if you want to take this approach.</p>
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    1. PObinary search over function arguments with/without boost
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    1. COI've asked how can I use built-in functions (std::lower_bound and similar) instead of writing my own specialized function. I can write customized binary search function myself, but this isn't exactly "right" way to do it.
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    2. CO@SigTerm added my edit. I'm not sure of a straightforward way to do this with `std` since you don't have a container/iterator. What I originally posted was the solution that seemed simplest to me with what you have
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