How to use unordered_map in which gregorian_date will be used as a key ?
unordered_map<boost::gregorian::date,int>date_map;
boost::gregorian::date sample_date{2018,01,01};
date_map[sample_date]=1;
Can any one please help me.
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Anamul Hasan
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3what's the error? – Joseph D. May 14 '18 at 05:12
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error: no match for ‘operator[]’ (operand types are ‘std::unordered_map
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sorry brother that was my mistake. check the updated code. gives same error. – Anamul Hasan May 14 '18 at 05:26
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i think, you can use only date iterators – Swift - Friday Pie May 14 '18 at 05:32
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3Be careful using the leading `0` for int literals in dates, it will cause errors for `08` and `09` since they aren't valid octal. – o11c May 14 '18 at 05:53
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i tried with your instruction but sill gives error. – Anamul Hasan May 14 '18 at 05:54
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1When I try it, the error is that there's no `std::hash
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now how can i overcome this???? – Anamul Hasan May 14 '18 at 06:12
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1Possible duplicate of [unordered\_map with gregorian dates](https://stackoverflow.com/questions/24084445/unordered-map-with-gregorian-dates) – SHR May 14 '18 at 06:26
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@AnamulHasan I'd suggest you don't need quite so many question marks. If it's a genuine question, spend the energy typing the excess interpunction to search for "unordered_map std::hash implementation" instead? – sehe May 14 '18 at 08:38
1 Answers
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Your question is closely related to unordered_map with gregorian dates, except you are using a std::unordered_map instead of the boost::unordered_map. You need to solve the same problem: If you want to use any data type as a key in an unordered_map, you need to provide a specialization of std::hash for that type (in your case ::boost::gregorian::date). Building on the answer given for the question I linked, you can use this specialization:
#include <boost/date_time/gregorian/gregorian.hpp>
#include <unordered_map>
namespace std {
// Note: This is pretty much the only time you are allowed to
// declare anything inside namespace std!
template <>
struct hash<boost::gregorian::date>
{
size_t operator () (const boost::gregorian::date& date) const
{
return std::hash<decltype(date.julian_day())>()(date.julian_day());
}
};
}
int main()
{
std::unordered_map<boost::gregorian::date, int> date_map;
boost::gregorian::date sample_date{2018, 1, 1};
date_map[sample_date] = 1;
}
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@Anamul If this answers your question, you can accept the answer. See https://stackoverflow.com/help/someone-answers. If your problem is still not solved, maybe you can clarify your original question. – mindriot May 17 '18 at 07:19