Re: [Seqan-dev] CharString as a key in an unordered_map

Oliver Stolpe <oliver.stolpe@fu-berlin.de> · Fri, 14 Sep 2012 19:55:29 +0200

Hi,

I think the operator mustn't mutate the hash functor, i.e. change

 size_t operator() (const CharString &key) {

into:

 size_t operator() (const CharString &key) const {

Regards,
David

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Am 14.09.2012 um 17:31 schrieb Oliver Stolpe <oliver.stolpe@fu-berlin.de<mailto:oliver.stolpe@fu-berlin.de>>:

Hello,

I tried to build something like in the example from Wikipedia:

struct hashCharString {
 size_t operator() (const CharString &key) {
   Iterator<CharString>::Type CharIt;
   size_t h_value = 7;

   for (CharIt = begin(key); CharIt != end(key); goNext(CharIt)) {
     h_value ^= hash<char>()(value(CharIt));
   }

   return h_value;
 }
};

The call is:

unordered_map<CharString, unsigned int, hashCharString> kmer;

But it fails:

/usr/include/c++/4.6/bits/hashtable_policy.h:753:25: Fehler: keine Übereinstimmung für Aufruf von »(const hashCharString) (const seqan::String<char, seqan::Alloc<> >&)«

I'm a bit lost here.

Oliver

Am 14.09.2012 07:29, schrieb Holtgrewe, Manuel:
Hi Oliver,

does one of these solutions work?

http://en.wikipedia.org/wiki/Unordered_associative_containers_(C%2B%2B)#Custom_hash_functions

*m
________________________________________
From: Oliver Stolpe [oliver.stolpe@fu-berlin.de]
Sent: Thursday, September 13, 2012 11:21 AM
To: seqan-dev@lists.fu-berlin.de
Subject: [Seqan-dev] CharString as a key in an unordered_map

Hello list,

I try to use a SeqAn CharString as a key in an unordered_map (C++11). It
fails when he tries to link. The linker message is posted below. I think
the CharString type is not hashable, therefore I probably need to
provide my own hash function (see below). Am I heading in the right
direction? Before I start doing this I wanted to know why a string is
hashable by the default function but not a CharString? Is there a trick
to easily get around this?

Thanks,
Oliver

This is my map structure:

unordered_map<CharString, unsigned int>

I assume I have to do something like this to provide a hash function
(found at stackoverflow):

template<>
struct hash<typename CharString> {
   size_t operator() (CharString const &key) {
      // calculate hash value
   }
};

Linker message:

Linking CXX executable fastq_reader
CMakeFiles/fastq_reader.dir/fastq_reader.cpp.o: In function
`std::_Hashtable<seqan::String<char, seqan::Alloc<void> >,
std::pair<seqan::String<char, seqan::Alloc<void> > const, unsigned int>,
std::allocator<std::pair<seqan::String<char, seqan::Alloc<void> > const,
unsigned int> >, std::_Select1st<std::pair<seqan::String<char,
seqan::Alloc<void> > const, unsigned int> >,
std::equal_to<seqan::String<char, seqan::Alloc<void> > >,
std::hash<seqan::String<char, seqan::Alloc<void> > >,
std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash,
std::__detail::_Prime_rehash_policy, false, false,
true>::find(seqan::String<char, seqan::Alloc<void> > const&)':
fastq_reader.cpp:(.text._ZNSt10_HashtableIN5seqan6StringIcNS0_5AllocIvEEEESt4pairIKS4_jESaIS7_ESt10_Select1stIS7_ESt8equal_toIS4_ESt4hashIS4_ENSt8__detail18_Mod_range_hashingENSF_20_Default_ranged_hashENSF_20_Prime_rehash_policyELb0ELb0ELb1EE4findERS6_[std::_Hashtable<seqan::String<char,
seqan::Alloc<void> >, std::pair<seqan::String<char, seqan::Alloc<void> >
const, unsigned int>, std::allocator<std::pair<seqan::String<char,
seqan::Alloc<void> > const, unsigned int> >,
std::_Select1st<std::pair<seqan::String<char, seqan::Alloc<void> >
const, unsigned int> >, std::equal_to<seqan::String<char,
seqan::Alloc<void> > >, std::hash<seqan::String<char, seqan::Alloc<void>
 > >, std::__detail::_Mod_range_hashing,
std::__detail::_Default_ranged_hash,
std::__detail::_Prime_rehash_policy, false, false,
true>::find(seqan::String<char, seqan::Alloc<void> > const&)]+0x8a):
undefined reference to `std::hash<seqan::String<char, seqan::Alloc<void>
 > >::operator()(seqan::String<char, seqan::Alloc<void> >) const'
collect2: ld gab 1 als Ende-Status zurück

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