Is there an way of inserting/deleting an element from the vectors other than the following..
The formal method of using 'push_back'
Using 'find()' in this way... find(v.begin(), v.end(), int)
I have read some where that inserting in the middle can be achieved by inclusive insertion/deletion.
So, is it really possible?
You can use std::vector::insert; however, note that this operation is O(.size()). If your code needs to perform insertions in the middle frequently, you may want to switch to a linked-list structure.
Is there an way of inserting/deleting an element from the vectors other than the following
Yes, you can use std::vector::insert() to insert element at a specified position.
Because vectors use an array as their underlying storage, inserting elements in positions other than the vector end causes the container to move all the elements that were after position to their new positions. This is generally an inefficient operation compared to the one performed for the same operation by other kinds of sequence containers (such as std::list).
std::vector is standard container, you could apply standard STL algorithms on it.
vector::insert seems to be what you want.
Related
Is there any function to resize a vector in both directions?Can we manipulate the pointer or the element from where it starts adding new empty elements?
Here is the entire interface of std::vector. It makes it very obvious there's no direct way to do what you ask, although you can reserve and then insert(begin,...) if you really want to (it has linear complexity, so is usually avoided).
The usual advice would be to use std::deque instead, since it's specifically designed for this operation.
Yes, you can insert elements to the front of the vector using insert:
vec.insert(vec.begin(), numer_of_elements_to_insert, {});
Note, however, that front-inserting into a vector is very inefficient, because it will require moving all the current elements in the vector past the newly inserted ones. If you need a double-ended container, look into std::deque.
I have a doubt that I would like to clarify in my head. I am aware of the different behavior for std::vector between erase and std::remove where the first physically removes an element from the vector, reducing size, and the other just moves an element leaving the capacity the same.
Is this just for efficiency reasons? By using erase, all elements in a std::vector will be shifted by 1, causing a large amount of copies; std::remove does just a 'logical' delete and leaves the vector unchanged by moving things around. If the objects are heavy, that difference might matter, right?
Is this just for efficiency reason? By using erase all elements in a std::vector will be shifted by 1 causing a large amount of copies; std::remove does just a 'logical' delete and leaves the vector unchanged by moving things around. If the objects are heavy that difference mihgt matter, right?
The reason for using this idiom is exactly that. There is a benefit in performance, but not in the case of a single erasure. Where it does matter is if you need to remove multiple elements from the vector. In this case, the std::remove will copy each not removed element only once to its final location, while the vector::erase approach would move all of the elements from the position to the end multiple times. Consider:
std::vector<int> v{ 1, 2, 3, 4, 5 };
// remove all elements < 5
If you went over the vector removing elements one by one, you would remove the 1, causing copies of the remainder elements that get shifted (4). Then you would remove 2 and shift all remainding elements by one (3)... if you see the pattern this is a O(N^2) algorithm.
In the case of std::remove the algorithm maintains a read and write heads, and iterates over the container. For the first 4 elements the read head will be moved and the element tested, but no element is copied. Only for the fifth element the object would be copied from the last to the first position, and the algorithm will complete with a single copy and returning an iterator to the second position. This is a O(N) algorithm. The later std::vector::erase with the range will cause destruction of all the remainder elements and resizing the container.
As others have mentioned, in the standard library algorithms are applied to iterators, and lack knowledge of the sequence being iterated. This design is more flexible than other approaches on which algorithms are aware of the containers in that a single implementation of the algorithm can be used with any sequence that complies with the iterator requirements. Consider for example, std::remove_copy_if, it can be used even without containers, by using iterators that generate/accept sequences:
std::remove_copy_if(std::istream_iterator<int>(std::cin),
std::istream_iterator<int>(),
std::ostream_iterator<int>(std::cout, " "),
[](int x) { return !(x%2); } // is even
);
That single line of code will filter out all even numbers from standard input and dump that to standard output, without requiring the loading of all numbers into memory in a container. This is the advantage of the split, the disadvantage is that the algorithms cannot modify the container itself, only the values referred to by the iterators.
std::remove is an algorithm from the STL which is quite container agnostic. It requires some concept, true, but it has been designed to also work with C arrays, which are static in sizes.
std::remove simply returns a new end() iterator to point to one past the last non-removed element (the number of items from the returned value to end() will match the number of items to be removed, but there is no guarantee their values are the same as those you were removing - they are in a valid but unspecified state). This is done so that it can work for multiple container types (basically any container type that a ForwardIterator can iterate through).
std::vector::erase actually sets the new end() iterator after adjusting the size. This is because the vector's method actually knows how to handle adjusting it's iterators (the same can be done with std::list::erase, std::deque::erase, etc.).
remove organizes a given container to remove unwanted objects. The container's erase function actually handles the "removing" the way that container needs it to be done. That is why they are separate.
I think it has to do with needing direct access to the vector itself to be able to resize it. std::remove only has access to the iterators, so it has no way of telling the vector "Hey, you now have fewer elements".
See yves Baumes answer as to why std::remove is designed this way.
Yes, that's the gist of it. Note that erase is also supported by the other standard containers where its performance characteristics are different (e.g. list::erase is O(1)), while std::remove is container-agnostic and works with any type of forward iterator (so it works for e.g. bare arrays as well).
Kind of. Algorithms such as remove work on iterators (which are an abstraction to represent an element in a collection) which do not necessarily know which type of collection they are operating on - and therefore cannot call members on the collection to do the actual removal.
This is good because it allows algorithms to work generically on any container and also on ranges that are subsets of the entire collection.
Also, as you say, for performance - it may not be necessary to actually remove (and destroy) the elements if all you need is access to the logical end position to pass on to another algorithm.
Standard library algorithms operate on sequences. A sequence is defined by a pair of iterators; the first points at the first element in the sequence, and the second points one-past-the-end of the sequence. That's all; algorithms don't care where the sequence comes from.
Standard library containers hold data values, and provide a pair of iterators that specify a sequence for use by algorithms. They also provide member functions that may be able to do the same operations as an algorithm more efficiently by taking advantage of the internal data structure of the container.
To get the pointer to the data in a vector we can use
vector<double> Vec;
double* Array_Pointer = &(Vec[0]);
Function(Array_Pointer);
Is that possible to get the pointer to the data in a set? Can I use that as array pointer like above?
If not possible, what is the best way to make a vector out of set? I mean without loop over all elements.
No, this is not necessarily possible. The C++ ISO standard explicitly guarantees contiguous storage of elements in a std::vector, so you can safely take the address of the first element and then use that pointer as if you were pointing at a raw array. Other containers in the standard library do not have this guarantee.
The reason for this is to efficiently support most operations on a std::set, the implementation needs to use complex data structures like balanced binary search trees to store and organize the data. These structures are inherently nonlinear and require nodes to be allocated and linked together. Efficiently getting this to work with the elements in a flat array would be difficult, if not impossible, in the time constraints laid out by the standard (amortized O(log n) for most operations.)
EDIT: In response to your question - there is no way to build a std::vector from a std::set without some code somewhere iterating over the set and copying the elements over. You can do this without explicitly using any loops yourself by using the std::vector range constructor:
std::vector<T> vec(mySet.begin(), mySet.end());
Hope this helps!
No. It's not possible to implement set in such a way that you can do this.
If you implement set in such a way that elements are stored in a single array, then when you add more elements, that array will inevitably need to be reallocated at some point. At that time, any references to existing elements will be invalidated.
One of features of set is that it guarantees that references to elements will never be invalidated if you add (or remove) other elements. As stated in [associative.reqmts]:
The insert and emplace members shall not affect the validity of iterators and references to the container, and the erase members shall invalidate only iterators and references to the erased elements.
So it's impossible to implement set in such a way that all of the elements of the set are stored in a single array.
Note that this has nothing to do with the efficiency requirements such as O(log n) insert/delete/lookup (if you squint really hard and allow for amortized O(log n) insertion time, at least), or maintaining sorted order, or anything like that. If it was just these, they could easily be handled with a data structure on top of the underlying elements, and the elements themselves could be stored in an array. It also doesn't even have anything to do with guarantees about iterator invalidation, since iterators are abstract.
No, the only thing holding you back is the reference invalidation requirement.
I came across this answer to the question of removing elements by value in C++:
C++ Erase vector element by value rather than by position?
Basically:
vec.erase(std::remove(vec.begin(), vec.end(), valueToRemove), vec.end());
The answer makes sense, but isn't this bad style? The logic is consists of a double negative... is there a cleaner way to do this?
Deleting an element from a collection consists of two steps:
Moving down all subsequent elements to fill in the holes created by matches
Marking the new end
With the C++ standard library, these are two separate functions, remove and erase, respectively.
One could certainly imagine an erase_if type of function which would be easier to use, but evidently the current code is considered good enough. Of course you can write your own remove_if.
This isn't bad and in fact an efficient way of removing elements from a vector based on a condition in linear time. Watch this video from 35th minute. STL explanation for the Erase and Remove Idiom
Remember that there are different types of containers: Contiguous vs node-based, and sequential vs associative.
Node-based containers allow efficient erase/insert. Sequential containers organize elements by insertion order (i.e. position), while associative containers arrange them by (key) value.
All current associative containers (map/set/unordered) are node-based, and with them you can erase elements directly, and you should use the element-wise member erase function directly. Lists are node-based sequence containers, so you can erase individual elements efficiently, but finding an element by value takes linear time, which is why lists offer a member remove function. Only sequence containers (vector and deque) have no easy way to erase elements by value, and that's where the free remove algorithm comes in, which first rearranges the sequence to then allow the container's member erase to perform an efficient erasure at the end of the container.
Unlike the many generic aspects of the standard library which work without any knowledge of the underlying container, the copy/erase idiom is one of those things which require a bit of detail knowledge about the differences between the containers.
Can some one please tell me what is the difference between vector vs deque. I know the implementation of vector in C++ but not deque. Also interfaces of map and set seem similar to me. What is the difference between the two and when to use one.
std::vector: A dynamic-array class. The internal memory allocation makes sure that it always creates an array. Useful when the size of the data is known and is known to not change too often. It is also good when you want to have random-access to elements.
std::deque: A double-ended queue that can act as a stack or queue. Good for when you are not sure about the number of elements and when accessing data-element are always in a serial manner. They are fast when elements are added/removed from front and end but not when they're added/removed to/from the middle.
std::list: A double-linked list that can be used to create a 'list' of data. The advantage of a list is that elements can be inserted or deleted from any part of the list without affecting an iterator that is pointing to a list member (and is still a member of the list after deletion). Useful when you know that elements will be deleted very often from any part of the list.
std::map: A dictionary that maps a 'key' to a 'value'. Useful for applications like 'arrays' whose index are not an integer. Basically can be used to create a map-list of name to an element, like a map that stores name-to-widget relationship.
std::set: A list of 'unique' data values. For e.g. if you insert 1, 2, 2, 1, 3, the list will only have the elements 1, 2, 3. Note that the elements in this list are always ordered. Internally, they're usually implemented as binary search trees (like map).
See here for full details:
What are the complexity guarantees of the standard containers?
vector Vs deque
A deque is the same as a vector but with the following addition:
It is a "front insertion sequence"
This means that deque is the same as a vector but provides the following additional gurantees:
push_front() O(1)
pop_front() O(1)
set Vs map
A map is a "Pair Associative Container" while set is a "Simple Associative Container"
This means they are exactly the same. The difference is that map holds pairs of items (Key/Value) rather than just a value.
std::vector
Your default sequential containers should be a std::vector. Generally, std::vector will provide you with the right balance of performance and speed. The std::vector container is similar to a C-style array that can grow or shrink during runtime. The underlying buffer is stored contiguously and is guaranteed to be compatible with C-style arrays.
Consider using a std::vector if:
You need your data to be stored contiguously in memory
Especially useful for C-style API compatibility
You do not know the size at compile time
You need efficient random access to your elements (O(1))
You will be adding and removing elements from the end
You want to iterate over the elements in any order
Avoid using a std::vector if:
You will frequently add or remove elements to the front or middle of the sequence
The size of your buffer is constant and known in advance (prefer std::array)
Be aware of the specialization of std::vector: Since C++98, std::vector has been specialized such that each element only occupies one bit. When accessing individual boolean elements, the operators return a copy of a bool that is constructed with the value of that bit.
std::array
The std::array container is the most like a built-in array, but offering extra features such as bounds checking and automatic memory management. Unlike std::vector, the size of std::array is fixed and cannot change during runtime.
Consider using a std::array if:
You need your data to be stored contiguously in memory
Especially useful for C-style API compatibility
The size of your array is known in advance
You need efficient random access to your elements (O(1))
You want to iterate over the elements in any order
Avoid using a std::array if:
You need to insert or remove elements
You don’t know the size of your array at compile time
You need to be able to resize your array dynamically
std::deque
The std::deque container gets its name from a shortening of “double ended queue”. The std::deque container is most efficient when appending items to the front or back of a queue. Unlike std::vector, std::deque does not provide a mechanism to reserve a buffer. The underlying buffer is also not guaranteed to be compatible with C-style array APIs.
Consider using std::deque if:
You need to insert new elements at both the front and back of a sequence (e.g. in a scheduler)
You need efficient random access to your elements (O(1))
You want the internal buffer to automatically shrink when elements are removed
You want to iterate over the elements in any order
Avoid using std::deque if:
You need to maintain compatibility with C-style APIs
You need to reserve memory ahead of time
You need to frequently insert or remove elements from the middle of the sequence
Calling insert in the middle of a std::deque invalidates all iterators and references to its elements
std::list
The std::list and std::forward_list containers implement linked list data structures. Where std::list provides a doubly-linked list, the std::forward_list only contains a pointer to the next object. Unlike the other sequential containers, the list types do not provide efficient random access to elements. Each element must be traversed in order.
Consider using std::list if:
You need to store many items but the number is unknown
You need to insert or remove new elements from any position in the sequence
You do not need efficient access to random elements
You want the ability to move elements or sets of elements within the container or between different containers
You want to implement a node-wise memory allocation scheme
Avoid using std::list if:
You need to maintain compatibility with C-style APIs
You need efficient access to random elements
Your system utilizes a cache (prefer std::vector for reduced cache misses)
The size of your data is known in advance and can be managed by a std::vector
A map is what is often refered to as an associative array, usually implemented using a binary tree (for example). A deque is a double-ended queue, a certain incarnation of a linked list.
That is not to say that the actual implementations of the container library uses these concepts - the containr library will just give you some guarantees about how you can access the container and at what (amortized) cost.
I suggest you take a look at a reference that will go into detail about what those guarantees are. Scott Meyers book "Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library" should talk a bit about those, if I remember correctly. Apart from that, the C++ standard is obviously a good choice.
What I really want to say is: containers really are described by their properties, not by the underlying implementation.
set: holds unique values. Put 'a' in twice, the set has one 'a'.
map: maps keys to values, e.g. 'name' => 'fred', 'age' => 40. You can look up 'name' and you'll get 'fred' out.
dequeue, like a vector but you can only add/remove at the ends. No inserts into the middle. http://en.wikipedia.org/wiki/Deque
edit: my dequeue description is lacking, see comments below for corrections