I am trying to sort an unordered_map
using sort()
function but I keep getting a compiler error. Can anyone help?
bool comp(pair<char,int> a, pair<char,int> b) {
return a.second < b.second;
}
void rearrangeKDist(char str[], int d) {
int n = strlen(str);
unordered_map<char, int> table;
for (int i=0; i<n; i++) {
unordered_map<char, int>::iterator it = table.find(str[i]);
if (it == table.end()) {
table.insert(make_pair(str[i], 1));
} else {
it->second = it->second+1;
}
}
for (unordered_map<char, int>::iterator it=table.begin(); it!=table.end(); it++)
cout<<it->first<<" "<<it->second<<endl;
sort(table.begin(), table.end(), comp);
for (unordered_map<char, int>::iterator it=table.begin(); it!=table.end(); it++)
cout<<it->first<<" "<<it->second<<endl;
}
This is impossible from both a compilation and logical standpoint. From a type standpoint, std::sort
requires:
-RandomIt must meet the requirements of ValueSwappable and RandomAccessIterator.
-The type of dereferenced RandomIt must meet the requirements of MoveAssignable and MoveConstructible.
The iterator type on std::unordered_map
is a ForwardIterator, not a RandomAccessIterator, so the first requirement is unsatisfied. The type of the dereferenced iterator is pair<const Key, T>
, which is not MoveAssignable (can't assign to const
), so the second requirement is also unsatisfied.
From a logical standpoint, sorting an unordered container makes no sense. It's unordered. And the complexity guarantees that unordered_map
is able to achieve require a very specific ordering that you shouldn't be, and aren't, allowed to mess with.
If you want to "sort" your unordered_map
, put them in a vector
:
std::vector<std::pair<char, int>> elems(table.begin(), table.end());
std::sort(elems.begin(), elems.end(), comp);
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