C++ map count keys

Maps are used to replicate associative arrays. Maps contain sorted key-value pair, in which each key is unique and cannot be changed, and it can be inserted or deleted but cannot be altered. Value associated with keys can be altered. We can search, remove and insert in a map within O n time complexity. For example: A map of students where roll number is the key and name is the value can be represented graphically as :.

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Notice that keys are arranged in ascending order, its because maps always arrange its keys in sorted order. In case the keys are of string type, they are sorted lexicographically. One thing which is to remembered is that key of a map and corresponding values are always inserted as a pair, you cannot insert only key or just a value in a map.

Both at and [ ] are used for accessing the elements in the map. The only difference between them is that at throws an exception if the accessed key is not present in the map, on the other hand operator [ ] inserts the key in the map if the key is not present already in the map. Since keys are unique in a map, it first checks that whether the given key is already present in the map or not, if it is present the entry is not inserted in the map and the iterator to the existing key is returned otherwise new entry is inserted in the map.

Made with by Abhishek Ahlawat. Ruby Servlet JSP. Operating System. Computer Architecture. Jenkins Maven. Apache Cordova Drools. Available on:. What is Studytonight? All rights reserved.Bit of a weird problem, but is there any way of counting how many keys are stored within a multimap?

Each of these keys have say values for each, so there should be instances of A, of B and so on. I know how to count how many pairs there are for the keys, but is there a way of counting how many unique keys there are in the multimap in this case you should get 4? Just a shot in the dark I was thinking something like that, but the other problem is the multimap could have anything from a few hundred values to a few hundred thousand or more, and finding the unique keys would need to be run maybe every few seconds :S.

Would the values change at all? Yeah it's pretty confusing so it is, but thanks anyways! Might just be a case of re-thinking what I'm doing sadly lol :. That's not just a 'little optimzation'; it's a lot more efficient and also more elegant than my feeble effort.

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c++ map count keys

Hi everyone again : Bit of a weird problem, but is there any way of counting how many keys are stored within a multimap? Many thanks in advance :.

Answered by Eagletalon 34 in a post from 9 Years Ago. Jump to Post. Answered by vijayan 1, in a post from 9 Years Ago. Little optimization if there are millions of keys and million of values per keys. Facebook Like. Twitter Tweet. Be a part of the DaniWeb community. Sign Up — It's Free! This topic is old! No one has contributed to this discussion in over 9 years. Are you sure you have something valuable to add to revive the existing conversation?

Consider starting a new topic instead. Otherwise, please be thoughtful, detailed and courteous, and adhere to our posting rules.Map Control. TMap is similar to TSet in that its structure is based on hashing keys. The difference between these two is that TMap keys are unique, while TMultiMap supports storing multiple, identical keys.

When adding a new key-value pair to a TMap with a key that matches an existing pair, the new pair will replace the old one. In a TMultiMapthe container will store both the new pair and the old. In a TMapkey-value pairs are treated as the element type of the map as if each pair were an individual object. In this document, element means a key-value pair, while individual components are referred to as the element's key or the element's value.

Like TArrayTMap is a homogeneous container, meaning that all of its elements are strictly the same type. TMap is also a value type, and supports the usual copy, assignment, and destructor operations, as well as strong ownership of its elements, which will be destroyed when the map is destroyed.

The key and value must also be value types. Hashing is covered in more detail later. TMap can also take an optional allocator to control the memory allocation behavior. Set allocators, class TSetAllocatordefine how many hash buckets the map should use and which standard UE4 allocators should be used for hash and element storage.

The final TMap template parameter is KeyFuncswhich tells the map how to retrieve the key from the element type, how to compare two keys for equality and how to hash the key.

If your key type supports these functions, you can use it as a map key without supplying a custom KeyFuncs. Unlike TArraythe relative order of TMap elements in memory is not reliable or stable, and iterating over the elements is likely to return them in a different order from the order in which they were added.

Elements are also unlikely to be laid out contiguously in memory. The backing data structure of a map is a sparse array, which is an array that efficiently supports gaps between its elements. As elements are removed from the map, gaps in the sparse array will appear.

Adding new elements into the array can then fill these gaps. However, even though TMap doesn't shuffle elements to fill gaps, pointers to map elements may still be invalidated, as the entire storage can be reallocated when it is full and new elements are added. FruitMap is now an empty TMap of strings that are identified by integer keys.

No memory has been allocated at this point.

MAP Container in STL

The standard way to populate a map is to call the Add function with a key and a value:. While the elements are listed here in the order of insertion, there is no guarantee as to their actual order in memory. For a new map, they are likely to be in order of insertion, but as more insertions and removals happen, it becomes increasingly unlikely that new elements will appear at the end.

This is not a TMultiMapso keys are guaranteed to be unique. The following is the result of attempting to add a duplicate key:. The map still contains three elements, but the previous "Grapefruit" value with a key of 2 has been replaced with "Pear".

The Add function can accept a key without a value.Join Stack Overflow to learn, share knowledge, and build your career. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information.

If you have a compiler that supports lambda expressions, you don't need any custom predicate functor at all; you can use a much simpler lambda:. You may want to consider using an extra map to keep track of the count of each value.

Do notice that this is not really all that different from previous answers, but it should be reusable with anything that iterates over pairs as well. Just rename Condition to something more meaningful. Learn more. Counting number of same values in map Ask Question. Asked 9 years, 9 months ago.

How to Iterate over a map in C++

Active 9 years, 9 months ago. Viewed 14k times. Muggen: That should be the last thing that comes to mind. You're just introducing a great deal of complexity to accomplish a very simple task for which there are already toos. John Dibling: What do you mean by a great deal of complexity? I am inclined to believe that using another map will require less amount of code. Also using map has better runtime complexity i.

This approach also requires that the elements stored in the container are comparable. James McNellis: I can see your point and I also think your answer is nice and of course educational. Yes, it's waste of memory if you only count it once even though poster didn't specify. But complexity in implementation? Active Oldest Votes. Also I don't see a way the predicate taking two arguments: en. That lamda is sexy, no need for many extra lines of code when only used once.

Definitely the simpler solution when carrying an extra variable around isn't an issue.In case of std::multimapno sorting occurs for the values of the same key. The basic difference between std::map and std::multimap is that the std::map one does not allow duplicate values for the same key where std::multimap does. Maps are implemented as binary search trees. An std::map takes key, value pairs as input. Consider the following example of std::map initialization:.

In an std::mapelements can be inserted as follows:. In the above example, if the key stackoverflow is already present, its value will be updated to 2. If it isn't already present, a new entry will be created.

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In an std::mapelements can be accessed directly by giving the key as an index:. Note that using the operator[] on the map will actually insert a new value with the queried key into the map. This means that you cannot use it on a const std::mapeven if the key is already stored in the map. To prevent this insertion, check if the element exists for example by using find or use at as described below.

In both containers std::map and std::multimapelements can be accessed using iterators:. The container std::map has a member function emptywhich returns true or falsedepending on whether the map is empty or not. The member function size returns the number of element stored in a std::map container:. In order to be able to use a class as the key in a map, all that is required of the key is that it be copiable and assignable.

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The ordering within the map is defined by the third argument to the template and the argument to the constructor, if used.

Just write a comparison operator preferably as a functional object :. In particular, compare X,X must return false for any X. This is a mathematical term to define a relationship between two objects.

Its definition is:. Two objects x and y are equivalent if both f x, y and f y, x are false. Note that an object is always by the irreflexivity invariant equivalent to itself.

As std::map does not allow duplicate keys and comma operator performs right to left, the pair on right would be overwritten with the pair with same key on the left. An element can be inserted into a std::map only if its key is not already present in the map.

Given for example:. A key-value pair is inserted into a std::map through the insert member function. It requires a pair as an argument:. The insert function returns a pair consisting of an iterator and a bool value:.

For convenience, the std::map container provides the subscript operator to access elements in the map and to insert new ones if they don't exist:. While simpler, it prevents the user from checking if the element already exists.

If an element is missing, std::map::operator[] implicitly creates it, initializing it with the default constructor before overwriting it with the supplied value.

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This version of insert returns void:. Time complexity for an insertion operation is O log n because std::map are implemented as trees.

c++ map count keys

If the new element can be inserted just before hintthen the insertion can be done in constant time.Used for the storage and retrieval of data from a collection in which each element is a pair that has both a data value and a sort key. The value of the key is unique and is used to automatically sort the data. The value of an element in a map can be changed directly.

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The key value is a constant and can't be changed. Instead, key values associated with old elements must be deleted, and new key values must be inserted for new elements. Traits The type that provides a function object that can compare two element values as sort keys to determine their relative order in the map.

See Heterogeneous Lookup in Associative Containers for more information. Allocator The type that represents the stored allocator object that encapsulates details about the map's allocation and deallocation of memory. A container of variable size that efficiently retrieves element values based on associated key values.

Sorted, because its elements are ordered by key values according to a specified comparison function. A class template, because the functionality it provides is generic and independent of element or key type. The data types used for elements and keys are specified as parameters in the class template together with the comparison function and allocator. The iterator provided by the map class is a bidirectional iterator, but the insert and map class member functions have versions that take as template parameters a weaker input iterator, whose functionality requirements are fewer than those guaranteed by the class of bidirectional iterators.

The different iterator concepts are related by refinements in their functionality. Each iterator concept has its own set of requirements, and the algorithms that work with it must be limited by those requirements. An input iterator may be dereferenced to refer to some object and may be incremented to the next iterator in the sequence.

We recommend that you base the choice of container type on the kind of searching and inserting that is required by the application. Associative containers are optimized for the operations of lookup, insertion, and removal.

The member functions that explicitly support these operations do them in a worst-case time that is proportional to the logarithm of the number of elements in the container.

Inserting elements invalidates no iterators, and removing elements invalidates only those iterators that specifically pointed to the removed elements.And touts, because of the fees they charge for picks, have to be even more successful than that to make their clients money in the long run.

Bell demands that Pregame customers trust him on this.

c++ map count keys

Bell loves to say that every pick ever sold on Pregame is archived and available for review. This is ostensibly true, but to access results older than 30 days requires clicking through a calendar, day by day, and entering CAPTCHA codes for each one.

Before long you find yourself in an infinite CAPTCHA loop, unable to continue, blocked from any attempts to tabulate the hard evidence. Two real-world bettors, one of whom is a former financial analyst in his late 20s who is highly regarded by oddsmakers, developed a script to scrape Pregame. You can view and download a spreadsheet containing all the data, including every single Pregame pick dating back to Jan.

It is a large file and may take several minutes to open.

C++ Programming Language Tutorial - Map in C++ STL - GeeksforGeeks

Based on full record of Pregame picks. The data covers 49 touts who sold their plays during this period (not including those whose existences have been completely expunged from the archives, like David Glisan, Mike Hook, and Stan Sharp), and of those 49, only 11 of them showed a profit. Of those gains, most were marginal and would be wiped out by standard fees.

c++ map count keys

Flipping a coin would have been a more cost-effective strategy. Based on full record of Pregame picks plus standard per-pick prices. His touts, he claims, win 55 percent of the time. Such a strike rate would be a license to print money.

Bell is smart enough to understand that shouting about winning records of 75-90 percent, as some barrel-bottom touts do, would only garner ridicule. It tells you something that for all their supposed professionalism, touts still use aliases. At Pregame, the big ones go by names like Fezzik, Goodfella, Spartan, Sleepyj, VegasButcher, and King Creole. In July 2014, Bell patted himself on the back when he announced on Pregame a new standard for his touts: They had to publicly provide their real name and a signed declaration declaring any past legal troubles.

As touts bounce from site to site, sometimes under different names, there really is no way to determine their true track records. Touts plug away despite columns of red, all the while advertising useless cherry-picked short-term streaks or outright falsified ones.

Like most touts, Coach sold picks in a few places. Every Super Bowl, places like The PBS NewsHour turn to the bespectacled Covers as the prototypical long-time professional bettora sharp.

Teddy Covers is as sharp as a marble. A documented six-year loser at SportsMemo, he became one of the faces of Pregame after joining in August 2014. How rare was it. He had sold another one nine days earlier. Fezzik, whose real name is Steve Fenic, joined Pregame with much hullabaloo in early 2013.

Pregame co-founder Johnny Detroit (another alias, of course) said at the time that Fezzik, a former actuary and two-time NFL handicapping contest winner, had never sold picks before. For Pregame, this was a boon: Fezzik came in with a lot of hype, and no documented record for clients to consider. At a site called Las Vegas Advisors, or LVA Sports, his record showed a 147-unit loss from 2009 to 2012.

The answer is just that: selling. I combed through Pregame materials that Bell probably wishes had disappearedcontracts obtained via public legal filings, archived web pages, affiliate presentationsand nowhere is the importance of winning mentioned. Commissions are the normthe more you sell, the more you earn.