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Sum - adding numbers

Here's a parser that sums a comma-separated list of numbers.

Ok we've glossed over some details in our previous examples. First, our includes:

```#include <boost/spirit/home/x3.hpp>
#include <iostream>
#include <string>
```

Then some using directives:

```namespace x3 = boost::spirit::x3;
namespace ascii = boost::spirit::x3::ascii;

using x3::double_;
using ascii::space;
using x3::_attr;
```

Now the actual parser:

```template <typename Iterator>
bool adder(Iterator first, Iterator last, double& n)
{
auto assign = [&](auto& ctx){ n = _attr(ctx); };
auto add = [&](auto& ctx){ n += _attr(ctx); };

bool r = x3::phrase_parse(first, last,

//  Begin grammar
(
double_[assign] >> *(',' >> double_[add])
)
,
//  End grammar

space);

if (first != last) // fail if we did not get a full match
return false;
return r;
}
```

The full cpp file for this example can be found here: sum.cpp

This is almost like our original numbers list example. We're incrementally building on top of our examples. This time though, like in the complex number example, we'll be adding the smarts. There's an accumulator (`double& n`) that adds the numbers parsed. On a successful parse, this number is the sum of all the parsed numbers.

The first `double_` parser attaches this action:

```[&](auto& ctx){ n = _attr(ctx); }
```

This assigns the parsed result (actually, the attribute of `double_`) to `n`. The second `double_` parser attaches this action:

```[&](auto& ctx){ n += _attr(ctx); }
```

So, subsequent numbers add into `n`.

That wasn't too bad, was it :-) ?

 Copyright © 2001-2018 Joel de Guzman, Hartmut Kaiser Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)