How can I sort this vector by comparing the pair.first which is an std::string? (without providing a static compare function, nor use boost).
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5 Answers
std::vector<std::pair<std::string, bool> > v;
std::sort(v.begin(), v.end());
std::pair overloads operator< to sort first by the first element then by the second element. Thus, if you just sort the vector using the default sort ordering (operator<), you'll get your desired ordering.
5 Comments
Lara Bailey
This is a C++0x only answer. ;) Edit: Now fixed (
>> token closing two nested template <> pairs is C++0x only).
James McNellis
@Charles: Ha! Yeah, I probably do that in a lot of answers. I'm too used to using a compiler that supports
>>.
Oliver Charlesworth
+1: I didn't know that
std::pair::operator<() was overloaded. I do now!
edA-qa mort-ora-y
Just to be picky, this requires that the second item in the pair has
< defined. If the second type is not sortable then this will fail.James McNellis
@edA: Yes. The OP's pair has a
bool as its second element type, which is comparable.I really like James' answer, but there's one other option you might want to consider - just funnel everything into a std::map:
std::map<std::string, bool> myMap(v.begin(), v.end());
Or, if you have duplicate strings, a std::multimap:
std::multimap<std::string, bool> myMultiMap(v.begin(), v.end());
This does have the added advantage that if you then need to add or remove new key/value pairs, you can do so in O(lg n), as opposed to O(n) for the sorted vector.
If you really must use a vector, then go with James' answer. However, if you have a vector of pairs, there's a good chance that you really want a std::map.
answered Jan 5, 2011 at 23:36
templatetypedef
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Answer to "duplicate question" of this: link: Sort a vector of pairs by first element then by second element of the pair in C++?
bool cmp(const pair<int,int>&x,const pair<int,int>y){
if(x.first==y.first){
return(x.second<y.second);
}
return(x.first<y.first);
}
array of pairs before:
5 2
4 2
8 2
8 3
8 1
array of pairs after:
4 2
5 2
8 1
8 2
8 3
Comments
You can use a custom comparator to order on the pairs' .first only.
sort(begin, end,
compose2(less<string>(),
select1st<pair<string, bool> >(),
select1st<pair<string, bool> >()));
2 Comments
James McNellis
Note that
select1st is not part of the C++ Standard Library.
ephemient
Mmm. Luckily, it's trivial to write:
template<class T> struct select1st : public unary_function<T, typename T::first_type> { const typename T::first_type& operator()(const T& x) const {return x.first;} };You don't need to provide any compare function since by default since the sort() function will sort vector in ascending order of values. As each element is a pair, so one pair will be smaller than other one if the first value of one pair is smaller than that for other pair.
vector<pair<string,int>>v;
v = {{"xyz",1},{"pq",2}}; // Sample input
sort(v.begin(),v.end()); // Requires #include<algorithm> header
