Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>Here is some kind of funny solution using lazy evaluation. First, construct the generator object <code>enumerate_object</code>:</p> <pre><code>template&lt;typename Iterable&gt; class enumerate_object { private: Iterable _iter; std::size_t _size; decltype(std::begin(_iter)) _begin; const decltype(std::end(_iter)) _end; public: enumerate_object(Iterable iter): _iter(iter), _size(0), _begin(std::begin(iter)), _end(std::end(iter)) {} const enumerate_object&amp; begin() const { return *this; } const enumerate_object&amp; end() const { return *this; } bool operator!=(const enumerate_object&amp;) const { return _begin != _end; } void operator++() { ++_begin; ++_size; } auto operator*() const -&gt; std::pair&lt;std::size_t, decltype(*_begin)&gt; { return { _size, *_begin }; } }; </code></pre> <p>Then, create a wrapper function enumerate that will deduce the template arguments and return the generator:</p> <pre><code>template&lt;typename Iterable&gt; auto enumerate(Iterable&amp;&amp; iter) -&gt; enumerate_object&lt;Iterable&gt; { return { std::forward&lt;Iterable&gt;(iter) }; } </code></pre> <p>You can now use your function that way:</p> <pre><code>int main() { std::vector&lt;double&gt; vec = { 1., 2., 3., 4., 5. }; for (auto&amp;&amp; a: enumerate(vec)) { size_t index = std::get&lt;0&gt;(a); double&amp; value = std::get&lt;1&gt;(a); value += index; } } </code></pre> <p>The implementation above is a mere toy: it should work with both <code>const</code> and non-<code>const</code> lvalue-references as well as rvalue-references, but has a real cost for the latter though, considering that it copies the whole iterable object several times. This problem could surely be solved with additional tweaks.</p> <p>Since C++17, decomposition declarations even allow you to have the cool Python-like syntax to name the index and the value directly in the <code>for</code> initializer:</p> <pre><code>int main() { std::vector&lt;double&gt; vec = { 1., 2., 3., 4., 5. }; for (auto&amp;&amp; [index, value] a: enumerate(vec)) { value += index; } } </code></pre> <p>I don't have a C++17-compliant compiler at hand to check it, but I hope that the <code>auto&amp;&amp;</code> in the decomposition is able to infer <code>index</code> as <code>std::size_t</code> and <code>value</code> as <code>double&amp;</code>.</p>
    singulars
    1. This table or related slice is empty.
    1. POPython-like loop enumeration in C++
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USMorwenn
    1. USMorwenn
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COYour code will blow up horribly if you pass a temporary to `enumerate`.
      singulars
      1. PO
      1. USXeo
    2. COWhich compiler are you using? It does not compile with g++ 4.6 or 4.7.
      singulars
      1. PO
      1. USbetabandido
    3. CO@Xeo Isn't it funny? You're probably right and I don't really see how to make a safer version. Anyway, using that function is not even as handy as the plain old solution.
      singulars
      1. PO
      1. USMorwenn
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload