Read file and split and trim each line with cpp20

Question

I have created a code that fits my specific need - to split the string (read from a file) at comma stripping any whitespaces. also, I want to convert these substrings to double and store them in std::vector. I use ranges library in c++20 and implement like bellow:

#include <iostream>
#include <fstream>
#include <ranges>
#include <algorithm>
#include <vector>

auto join_character_in_each_subranges = [](auto &&rng) { return std::string_view(&*rng.begin(), std::ranges::distance(rng)); };
auto trimming = std::ranges::views::filter([](auto character){ return !std::isspace(character);});

typedef std::vector<double> LineList;
typedef std::vector<LineList> List;

int main () {
    std::ifstream myfile; 
    std::string myline;
    List list;

    myfile.open("data.txt");
    
    while (std::getline(myfile, myline))
    {
        LineList line_list;
        for (auto words : myline 
                | std::ranges::views::split(',') 
                | std::ranges::views::transform(join_character_in_each_subranges))
        {
            auto words_trimming = words | trimming;
            std::string clean_numbers;
            std::ranges::for_each(words_trimming, [&](auto character){ clean_numbers += character;});

            line_list.push_back(atof(clean_numbers.c_str()));
        }

        list.push_back(line_list);
    }
}

First, iterate on myline sentences and splits the view into subranges on the delimiter

 myline | std::ranges::views::split(',')

append each character inside subranges and view into the std::string with transform function

std::transform applies the given function to a range and stores the result in another range.

 std::ranges::views::transform(join_character_in_each_subranges)

also, remove any prefix and suffix from view ranges

auto words_trimming = words | trimming;

and convert view ranges to std::string with

std::ranges::for_each(words_trimming, [&](auto character){ clean_number += character;});

finally, convert each clean_number to double and push_back into the list.

line_list.push_back(atof(clean_words.c_str()));

but when I change this code to

for (auto words : myline 
                | std::ranges::views::split(',') 
                | std::ranges::views::transform(join_character_in_each_subranges)
                | trimming)

I've got a lot of errors.

read.cpp:26:17: error: no match for 'operator|' (operand types are 'std::ranges::transform_view<std::ranges::split_view<std::ranges::ref_view<std::__cxx11::basic_string<char> >, std::ranges::single_view<char> >, <lambda(auto:16&&)> >' and 'std::ranges::views::__adaptor::_RangeAdaptorClosure<std::ranges::views::__adaptor::_RangeAdaptor<_Callable>::operator()<{<lambda(auto:17&&)>}>::<lambda(_Range&&)> >')
   23 |         for (auto words : myline
      |                           ~~~~~~
   24 |                 | std::ranges::views::split(',')
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   25 |                 | std::ranges::views::transform(join_character_in_each_subranges)
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                 |
      |                 std::ranges::transform_view<std::ranges::split_view<std::ranges::ref_view<std::__cxx11::basic_string<char> >, std::ranges::single_view<char> >, <lambda(auto:16&&)> >
   26 |                 | trimming)
      |                 ^ ~~~~~~~~
      |                   |
      |                   std::ranges::views::__adaptor::_RangeAdaptorClosure<std::ranges::views::__adaptor::_RangeAdaptor<_Callable>::operator()<{<lambda(auto:17&&)>}>::<lambda(_Range&&)> >
In file included from read.cpp:3:
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/ranges:1183:4: note: candidate: 'template<class _Tp> constexpr auto std::ranges::views::__adaptor::operator|(const std::ranges::views::__adaptor::_RangeAdaptorClosure<_Callable>&, const std::ranges::views::__adaptor::_RangeAdaptorClosure<std::ranges::views::__adaptor::_RangeAdaptor<_Callable>::operator()<{<lambda(auto:17&&)>}>::<lambda(_Range&&)> >&)'
 1183 |    operator|(const _RangeAdaptorClosure<_Tp>& __x,
      |    ^~~~~~~~
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/ranges:1183:4: note:   template argument deduction/substitution failed:
read.cpp:26:19: note:   'std::ranges::transform_view<std::ranges::split_view<std::ranges::ref_view<std::__cxx11::basic_string<char> >, std::ranges::single_view<char> >, <lambda(auto:16&&)> >' is not derived from 'const std::ranges::views::__adaptor::_RangeAdaptorClosure<_Callable>'
   26 |                 | trimming)
      |                   ^~~~~~~~
In file included from read.cpp:3:
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/ranges:1178:4: note: candidate: 'constexpr auto std::ranges::views::__adaptor::operator|(_Range&&, const std::ranges::views::__adaptor::_RangeAdaptorClosure<_Callable>&) [with _Range = std::ranges::transform_view<std::ranges::split_view<std::ranges::ref_view<std::__cxx11::basic_string<char> >, std::ranges::single_view<char> >, <lambda(auto:16&&)> >; _Callable = std::ranges::views::__adaptor::_RangeAdaptor<_Callable>::operator()<{<lambda(auto:17&&)>}>::<lambda(_Range&&)>]'
 1178 |    operator|(_Range&& __r, const _RangeAdaptorClosure& __o)
      |    ^~~~~~~~
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/ranges:1178:4: note: constraints not satisfied
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/ranges: In instantiation of 'constexpr auto std::ranges::views::__adaptor::operator|(_Range&&, const std::ranges::views::__adaptor::_RangeAdaptorClosure<_Callable>&) [with _Range = std::ranges::transform_view<std::ranges::split_view<std::ranges::ref_view<std::__cxx11::basic_string<char> >, std::ranges::single_view<char> >, <lambda(auto:16&&)> >; _Callable = std::ranges::views::__adaptor::_RangeAdaptor<_Callable>::operator()<{<lambda(auto:17&&)>}>::<lambda(_Range&&)>]':
read.cpp:26:19:   required from here
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/ranges:1178:4:   required by the constraints of 'template<class _Callable> template<class _Range>  requires (viewable_range<_Range>) && requires{(declval<_Callable>)()((declval<_Range>)());} constexpr auto std::ranges::views::__adaptor::operator|(_Range&&, const std::ranges::views::__adaptor::_RangeAdaptorClosure<_Callable>&)'
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/ranges:1176:13:   in requirements  [with _Range = std::ranges::transform_view<std::ranges::split_view<std::ranges::ref_view<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::ranges::single_view<char> >, ._anon_104>; _Callable = std::ranges::views::__adaptor::_RangeAdaptor<std::ranges::views::._anon_83>::operator()::._anon_106]
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/ranges:1176:44: note: the required expression 'declval<_Callable>()(declval<_Range>())' is invalid
 1176 |    requires requires { declval<_Callable>()(declval<_Range>()); }
      |                        ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
cc1plus: note: set '-fconcepts-diagnostics-depth=' to at least 2 for more detail
In file included from /usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/ios:42,
                 from /usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/ostream:38,
                 from /usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/iostream:39,
                 from read.cpp:1:
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/bits/ios_base.h:87:3: note: candidate: 'constexpr std::_Ios_Fmtflags std::operator|(std::_Ios_Fmtflags, std::_Ios_Fmtflags)'
   87 |   operator|(_Ios_Fmtflags __a, _Ios_Fmtflags __b)
      |   ^~~~~~~~
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/bits/ios_base.h:87:27: note:   no known conversion for argument 1 from 'std::ranges::transform_view<std::ranges::split_view<std::ranges::ref_view<std::__cxx11::basic_string<char> >, std::ranges::single_view<char> >, <lambda(auto:16&&)> >' to 'std::_Ios_Fmtflags'
   87 |   operator|(_Ios_Fmtflags __a, _Ios_Fmtflags __b)
      |             ~~~~~~~~~~~~~~^~~
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/bits/ios_base.h:129:3: note: candidate: 'constexpr std::_Ios_Openmode std::operator|(std::_Ios_Openmode, std::_Ios_Openmode)'
  129 |   operator|(_Ios_Openmode __a, _Ios_Openmode __b)
      |   ^~~~~~~~
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/bits/ios_base.h:129:27: note:   no known conversion for argument 1 from 'std::ranges::transform_view<std::ranges::split_view<std::ranges::ref_view<std::__cxx11::basic_string<char> >, std::ranges::single_view<char> >, <lambda(auto:16&&)> >' to 'std::_Ios_Openmode'
  129 |   operator|(_Ios_Openmode __a, _Ios_Openmode __b)
      |             ~~~~~~~~~~~~~~^~~
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/bits/ios_base.h:169:3: note: candidate: 'constexpr std::_Ios_Iostate std::operator|(std::_Ios_Iostate, std::_Ios_Iostate)'
  169 |   operator|(_Ios_Iostate __a, _Ios_Iostate __b)
      |   ^~~~~~~~
/usr/local/Cellar/gcc@10/10.3.0/include/c++/10.3.0/bits/ios_base.h:169:26: note:   no known conversion for argument 1 from 'std::ranges::transform_view<std::ranges::split_view<std::ranges::ref_view<std::__cxx11::basic_string<char> >, std::ranges::single_view<char> >, <lambda(auto:16&&)> >' to 'std::_Ios_Iostate'
  169 |   operator|(_Ios_Iostate __a, _Ios_Iostate __b)
      |             ~~~~~~~~~~~~~^~~

how fix this problem?

Thanks

Trimming operates on characters, but you are now passing it `string_view`s. You need to apply it via `std::ranges::views::transform()` if it's done outside of the loop. — , Jul 19 '21 at 12:06
What I mean is that before in the code that works, you are appling `trimming` to a single string, but in your second version, you are trying to apply it to a *list* of strings. — , Jul 19 '21 at 12:13

score 0 · Answer 1 · answered Jul 19 '21 at 17:34

0

According to Frank's comment change trimming like this:

auto trimming = std::ranges::views::transform(std::ranges::views::filter([](char character){return !std::isspace(character); }));

answered Jul 19 '21 at 17:34

kingpin

21
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Read file and split and trim each line with cpp20

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