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Sort list using stl sort function
The C++ standard library gives strict linear sequence container, linear sequence container, associative container.
std::sort() is available for all kinds of containers. But why only it provides sort for list. std::list::sort()?
std::sort only works on random access containers. And the only non-random access container in the standard library that it makes sense to sort is std::list.
std::sort certainly doesn't work on associative containers as you seem to think. What sense would that make? Associative containers are accessed by the value of their key, not by position.
As noted by Mike, C++11 also has std::forward_list, which, by no accident, also has it's own sort function.
std::sort only works for random access iterators, but a std::list only provides biderectional iterators. Since it therefore cannot be used with std::sort, it needs its own implementation, which is also likely to be more optimized for a doubly linked list.
Likewise you cannot use std::map or std::set iterators with std::sort. But for these you don't need it anyway, as they are always sorted.
As a side note, there are also std::map::find and the like. These are indeed not required, as you can use all iterators with std::find. But the member function versions provide optimized algorithms for the individual containers, which are more efficient than std::find's linear complexity.
Related
Many STL containers have both ordered and unordered version, e.g. map and unondered_map, set and unordered_set. And on the other hand, some STL algorithms have to be fed in an ordered container to give the correct output. For instance, std::set_difference will not work correctly on unordered_set. So my question is, why isn't there a named requirement that specifies a container is always sorted, such that inputs to algorithms like set_difference are constraint to those sorted containers?
In a way, the STL concept contradicts the original idea of
object-oriented programming: The STL separates data and algorithms
rather than combining them. However, the reason for doing so is very
important. In principle, you can combine every kind of container with
every kind of algorithm, so the result is a very flexible but still
rather small framework.
-- from Nicolai M. Josuttis <<The C++ Standard Library A Tutorial and Reference Second Edition>>
There are three classes of STL containers :
sequence containers(array, vector, deque, forward_list, list)
associative containers(set, map, multiset, and multimap)
unordered associative containers(unordered_set, unordered_map, unordered_multiset, and unordered_multimap)
You can get sorted sequence containers by calling std::sort for array, vector, deque, sort() member function for forward_list and list.
Hi i have used some stl algorithms on vectors like find_if,count_if, sort, push_back etc. Now i want to make a generic code for all container objects(vectors,lists,maps,sets). Do i need to modify my stl functions for them or are they all same?
For starters push_back is not an algorithm. It is a method of some sequential containers like std::vector or std::list. However std::forward_list does not have such a method.
Standard algoreithm std::sort requires that the underlaying container had random access iterators. Some sequential containers like std::list and std::forward_list have their own methods sort. The standard algorithm std::sort can not be applied to these containers.
On the other hand such associative containers like std;:set or std::map are already ordered containers.
As for standard algorithms std::find_if or std::count_if then they can be applied to all standard containers because they are based on input iterators.
I am wondering in which uses cases, we can be interested by having an ordered associative container.
In other terms, why use std::map and no std::unorderd_map
You use an ordered map when you need to be able to iterate over the keys in an ordered fashion.
As Mat pointed out, it depends on whether you need to access your elements in sorted order. Also, map has guaranteed, worst-case logarithmic access time (in the number of elements), whereas unordered map has average constant access time.
The main reason for using map instead of unordered_map is that map is in standard and for sure you have it. Some companies have policy not to use definitions from tr1.
But you ask when order is important - it is needed for operations like lower_bound, upper_bound, set_union, set_intersection etc.
If the comparison between unordered containers is needed, the complexity
might be an issue.
It seems to be difficult to implement operator==/operator!= between
unordered containers efficiently.
N3290 §23.2.5/11 says
the complexity of operator== ... is
proportional to ... N^2 in the worst
case, where N is a.size().
while other containers have linear complexity.
Unordered_map use more memory than map. If there is a constraint in memory then its recommended to use map. So the operations become slower when using unordered_map
EDIT
Quoting from the wiki article
It is similar to the map class in the C++ standard library but has different
constraints. As its name implies, unlike the map class, the elements of an
unordered_map are not ordered. This is due to the use of hashing to store objects.
unordered_map can still be iterated through like a regular map.
There is a sort() method for lists in STL. Which is absurd, because I would be more inclined to sort an array/vector.
Why isn't sort() provided for vector? Is there some underlying philosophy behind the creation of the vector container or its usage, that sort is not provided for it?
As has already been said, the standard library provides a nonmember function template that can sort any range given a pair of random access iterators.
It would be entirely redundant to have a member function to sort a vector. The following would have the same meaning:
std::sort(v.begin(), v.end());
v.sort();
One of the first principles of the STL is that algorithms are not coupled to containers. How data is stored and how data is manipulated should be as loosely coupled as possible.
Iterators are used as the interface between containers (which store data) and algorithms (which operate on the data). In this way, you can write an algorithm once and it can operate on containers of various types, and if you write a new container, the existing generic algorithms can be used to manipulate its contents.
The reason that std::list provides its own sort function as a member function is that it is not a random accessible container; it only provides bidirectional iterators (since it is intended to represent a doubly linked list, this makes sense). The generic std::sort function requires random access iterators, so you cannot use it with a std::list. std::list provides its own sort function in order that it can be sorted.
In general, there are two cases in which a container should implement an algorithm:
If the generic algorithm cannot operate on the container, but there is a different, container-specific algorithm that can provide the same functionality, as is the case with std::list::sort.
If the container can provide a specific implementation of the algorithm that is more efficient than the generic algorithm, as is the case with std::map::find, which allows an element to be found in the map in logarithmic time (the generic std::find algorithm performs a linear search because it cannot assume the range is sorted).
There are already interesting elements of answer, but there is actually more to be said about the question: while the answer to "why doesn't std::vector has a sort member function?" is indeed "because the standard library provides member functions only when they offer more than generic algorithms", the real interesting question is "why does std::list have a sort member function?", and a few things haven't been explained yet: yes, std::sort only works with random-access iterators and std::list only provides bidirectional iterators, but even if std::sort worked with bidirectional iterators, std::list::sort would still offer more. And here is why:
First of all, std::list::sort is stable, while std::sort isn't. Of course there is still std::stable_sort, but it doesn't work with bidirectional iterators either.
std::list::sort generally implements a mergesort, but it knows that it is sorting a list and can relink nodes instead of copying things. A list-aware mergesort can sort the list in O(n log n) time with only O(log n) additional memory, while your typical mergesort (such as std::stable_sort) uses O(n) additional memory or has a O(n log² n) complexity.
std::list::sort doesn't invalidate the iterators. If an iterator was pointing to a specific object in the list, it will still be pointing to the same object after the sort, even if its position in the list isn't the same than before the sort.
Last but not least, std::list::sort doesn't move or swap the objects around since it only relinks nodes. That means that it might be more performant when you need to sort objects that are expensive to move/swap around, but also that it can sort a list of objects that aren't even moveable, which is totally impossible for std::sort!
Basically, even if std::sort and std::stable_sort worked with bidirectional or forward iterators (and it would totally be possible, we know sorting algorithms that work with them), they still couldn't offer everything std::list::sort has to offer, and they couldn't relink nodes either since the standard library algorithms aren't allowed to modify the container, only the pointed values (relinking nodes counts as modifying the container). On the other hand, a dedicated std::vector::sort method wouldn't offer anything interesting, so the standard library doesn't provide one.
Note that everything that has been said about std::list::sort is also true for std::forward_list::sort.
A vector-specific sort would provide no advantage over std::sort from <algorithm>. However, std::list provides its own sort because it can use the special knowledge of how list is implemented to sort items by manipulating the links instead of copying objects.
You can easily sort a vector with:
sort(v.begin(), v.end());
UPDATE: (answer to the comment): Well, they have certainly provided it by default. The difference is that it's not a member function for vector. std::sort is a generic algorithm that's supposed to work for anything that provides iterators. However, it really expects a random access iterator to sort efficiently. std::list, being a linked list, cannot provide random access to its elements efficiently. That's why it provides its own specialized sort algorithm.
std::sort() in <algorithm> does sorting on containers with random access iterators like std::vector.
There is also std::stable_sort().
edit - why does std::list have its own sort() function versus std::vector?
std::list is different from both std::vector and std::deque (both random access iterable) in how it's implemented, so it contains its own sort algorithm that is specialized for its implementation.
Answering the question of "Why?"
A sorting algorithm for std::vector is the same as sorting a native array and is the same (probably) as sorting a custom vector class.
The STL was designed to separate containers and algorithms, and to have an efficient mechanism for applying an algorithm to data that has the right characteristics.
This lets you write a container that might have specific characteristics, and to get the algorithms free. Only where there is some special characteristic of the data that means the standard algorithm is unsuitable is a custom implementation supplied, as in the case of std::list.
what's the alternative?
Should I write by myself?
There is the std::find() algorithm, which performs a linear search over an iterator range, e.g.,
std::vector<int> v;
// Finds the first element in the vector that has the value 42:
// If there is no such value, it == v.end()
std::vector<int>::const_iterator it = std::find(v.begin(), v.end(), 42);
If your vector is sorted, you can use std::binary_search() to test whether a value is present in the vector, and std::equal_range() to get begin and end iterators to the range of elements in the vector that have that value.
The reason there is no vector::find is because there is no algorithmic advantage over std::find (std::find is O(N) and in general, you can't do better for vectors).
But the reason you have map::find is because it can be more efficient (map::find is O(log N) so you would always want to use that over std::find for maps).
Who told you that? There's is "find" algorithm for vector in C++. Ironically Coincidentally, it is called std::find. Or maybe std::binary_search. Or something else, depending on the properties of the data stored in your vector.
Containers get their own specific versions of generic algorithms (implemented as container methods) only when the effective implementation of the algorithm is somehow tied to the internal details of the container. std::list<>::sort would be one example.
In all other cases, the algorithms are implemented by standalone functions.
Having a 'find' functionality in the container class violates 'SRP' (Single Responsibility Principle). A container's core functionality is to provide interfaces for storage, retrieval of elements in the container. 'Finding', 'Sorting', 'Iterating' etc are not core functionality of any container and hence not part of it's direct interface.
However as 'Herb' states in Namespace Principle, 'find' is a part of the interface by being defined in the same namespace as 'vector' namely 'std'.
Use std::find(vec.begin(), vec.end(), value).
And don't forget to include <algorithm>
what's the alternative?
The standard offers std::find, for sequential search over arbitrary sequences of like-elements (or something like that).
This can be applied to all containers supporting iterators, but for internally sorted containers (like std::map) the search can be optimized. In that case, the container offers it's own find member function.
why there is no find for vector in C++?
There was no point in creating a std::vector<???>::find as the implementation would be identical to std::find(vector.begin(), vector.end(), value_to_find);.
Should I write by myself?
No. Unless you have specific limitations or requirements, you should use the STL implementation whenever possible.