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    <p>Prasoon has already offered up a host of different (and good) ways to do this, none of which need repeating here. I'd like to suggest an alternative approach for speed however.</p> <p>If you're going to be doing this quite a bit, you may want to consider "sub-classing" your vector so that a sum of elements is maintained separately (not <em>actually</em> sub-classing vector which is iffy due to the lack of a virtual destructor - I'm talking more of a class that contains the sum and a vector within it, <code>has-a</code> rather than <code>is-a</code>, and provides the vector-like methods).</p> <p>For an empty vector, the sum is set to zero. On every insertion to the vector, add the element being inserted to the sum. On every deletion, subtract it. Basically, <em>anything</em> that can change the underlying vector is intercepted to ensure the sum is kept consistent.</p> <p>That way, you have a very efficient O(1) method for "calculating" the sum at any point in time (just return the sum currently calculated). Insertion and deletion will take slightly longer as you adjust the total and you should take this performance hit into consideration.</p> <p>Vectors where the sum is needed more often than the vector is changed are the ones likely to benefit from this scheme, since the cost of calculating the sum is amortised over all accesses. Obviously, if you only need the sum every hour and the vector is changing three thousand times a second, it won't be suitable.</p> <p>Something like this would suffice:</p> <pre><code>class UberVector: private Vector&lt;int&gt; vec; private int sum; public UberVector(): vec = new Vector&lt;int&gt;(); sum = 0; public getSum(): return sum; public add (int val): rc = vec.add (val) if rc == OK: sum = sum + val return rc public delindex (int idx): val = 0 if idx &gt;= 0 and idx &lt; vec.size: val = vec[idx] rc = vec.delindex (idx) if rc == OK: sum = sum - val return rc </code></pre> <p>Obviously, that's pseudo-code and you may want to have a little more functionality, but it shows the basic concept.</p>
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    1. POHow to sum up elements of a C++ vector?
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    1. COInteresting approach, +1 :)
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      1. USPrasoon Saurav
    2. COinteresting, but be careful as `std::vector` isn't meant for subclassing.
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      1. USEvan Teran
    3. COSorry, I should have been clearer - you could create your own class with the same methods as vector which maintained a `has-a` vector within it, rather than being a proper subclass (`is-a`).
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