Suppose I have a float-integer map m:
m[1.23] = 3
m[1.25] = 34
m[2.65] = 54
m[3.12] = 51
Imagine that I know that there's a mapping between 2.65 and 54, but I don't know about any other mappings.
Is there any way to visit the adjacent mappings without iterating from the beginning or searching using the find function?
In other words: can I directly access the adjacent values by just knowing about a single mapping...such as m[2.65]=54?
UPDATE Perhaps a more important "point" than my answer, brought up by #MattMcNabb:
Floating point keys in std:map
Can I directly access the adjacent values by just knowing about a single mapping (m[2.65]=54)
Yes. std::map is an ordered collection; which is to say that if an operator< exists (more generally, std::less) for the key type you can expect it to have sorted access. In fact--you won't be able to make a map for a key type if it doesn't have this comparison operator available (unless you pass in a predicate function to perform this comparison in the template invocation)
Note there is also a std::unordered_map which is often preferable for cases where you don't need this property of being able to navigate quickly between "adjacent" map entries. However you will need to have std::hash defined in that case. You can still iterate it, but adjacency of items in the iteration won't have anything to do with the sort order of the keys.
UPDATE also due to #MattMcNabb
Is there any way to visit the adjacent mappings without iterating from the beginning or searching using the find function?
You allude to array notation, and the general answer here would be "not really". Which is to say there is no way of saying:
if (not m[2.65][-2]) {
std::cout << "no element 2 steps prior to m[2.65]";
} else {
std::cout << "the element 2 before m[2.65] is " << *m[2.65][-2];
}
While no such notational means exist, the beauty (and perhaps the horror) of C++ is that you could write an augmentation of map that did that. Though people would come after you with torches and pitchforks. Or maybe they'd give you cult status and put your book on the best seller list. It's a fine line--but before you even try, count the letters and sequential consonants in your last name and make sure it's a large number.
What you need to access the ordering is an iterator. And find will get you one; and all the flexibility that it affords.
If you only use the array notation to read or write from a std::map, it's essentially a less-capable convenience layer built above iterators. So unless you build your own class derived from map, you're going to be stuck with the limits of that layer. The notation provides no way to get information about adjacent values...nor does it let you test for whether a key is in the map or not. (With find you can do this by comparing the result of a lookup to end(m) if m is your map.)
Technically speaking, find gives you the same effect as you could get by walking through the iterators front-to-back or back-to-front and comparing, as they are sorted. But that would be slower if you're seeking arbitrary elements. All the containers have a kind of algorithmic complexity guarantee that you can read up on.
When dereferencing an iterator, you will receive a pair whose first element is the key and second element is the value. The value will be mutable, but the key is constant. So you cannot find an element, then navigate to an adjacent element, and alter its key directly...just its value.
Related
I understand that the underlying data structure for map in C++ is a self-balancing Binary Search Tree. Since in these data structures, finding a lower bound and an upper bound to a key has lots of use, you'd think that map lower_bound, and upper_bound functions will give you that capability. It's a bummer that these functions don't deliver that.
Does anyone know why lower_bound behaves the way it does? (it gives you the key that is NOT BEFORE the given key).
I've been using C++ since before SGI even introduced the STL, and for some reason still manage to mess up using these methods, including even embarrassing myself when presenting them to a class. I think my problems are that:
The names already have an intuitive but different meaning in mathematics. Given the mathematical meanings, it seems weird that in a big set or map, upper_bound and lower_bound are actually the same or adjacent elements.
The names "upper_bound" and "lower_bound" sound like there is a some kind of symmetry between the two, when there is absolutely not. I'd have a much easier time if the names were something like least_ge (least greater than or equal to) for lower_bound and least_gt (least greater than) for upper_bound.
If someone has a mnemonic or logic to make these easy to internalize, please share it. Otherwise, it just feels like they wrote two useful functions but used two random mathematical terms to name those functions, with no way to derive the semantics from the names. At that point, why not use up made up names like egptr and pbase? I mean at least I don't have any pre-existing intuitions to overcome about the names of streambuf methods...
At any rate here are what I believe are the basic rules you have to remember:
lower_bound(X) returns the lowest element v such that v >= X
upper_bound(X) returns the lowest element v such that v > X
To traverse the half-open interval [L,H), start with lower_bound(L) and stop at (don't process) lower_bound(H). This is usually what you want, because it's most common to traverse half-open intervals in C++--e.g., [buf, buf+nbytes), or [0,array_size), or [begin(), end()).
To traverse the closed interval [L,H], start at lower_bound(L) and stop at upper_bound(H).
To traverse the open interval (L,H), start at upper_bound(L) and stop at lower_bound(H).
In a non-empty container, the mirror image of lower_bound(X) is std::prev(upper_bound(X)) and the mirror image of upper_bound(X) is std::prev(lower_bound(X)). Of course, if an element is equal to begin(), then you can't step it backwards with std::prev, so you need extra logic to deal with the fact that this point cannot be represented with an iterator value.
In a multiset/multimap, the first v is lower_bound(v) if that element is indeed v. The last v is std::prev(upper_bound(v)) if the container is not empty and that element is v, but remember to check that the container is not empty before attempting prev on end().
From the point view of usual math convention, the upper_bound is the "least true upper-bound" (never equal) and the lower_bound is the "least upper-bound" (could equal). The fact that lower_bound is actually an "upper-bound" in usual math convention may cause confusion among users.
A way to rationalize the name of lower_bound/upper_bound is considering them in the context of another method called equal_range. The lower_bound is really the "lower_bound" of the equal_range, similarly the upper_bound.
This is not only in the map. It is in STL.
lower_bound for your x find such y that x <= y. And the upper_bound x < y.
I am trying to understand ordered and unordered map
I understand that ordered map will sort your list and unordered map will not sort it. base from this site
But why is this code get sorted anyway
std::unordered_map<std::string, int> map;
map.insert(std::make_pair("hello", 1));
map.insert(std::make_pair("world", 2));
map.insert(std::make_pair("call", 3));
map.insert(std::make_pair("ZZ", 4));
for(auto it = map.begin(); it != map.end();it++)
{
std::cout << it->first << std::endl;
}
result:
world
hello
call
ZZ
I dont get it. its supposed to be hello then world then call then ZZ. How did the world came first if its unordered container is not suppose to sort it.
std::hash<std::string> produces hashes for your input keys. Then, a transformation is applied to these hashes to get a valid index to a bucket in your map (The most basic one one can think of is a simple modulo). These operations take constant time and give you the location of your items. In order to get the good item from a transformed hash, you need it to be at the right place. That is why you observe an apparently random ordering of your inputs.
Now there is NO assumption you can make on the order of the computed indices. Suppose you would add some more strings in your map. Maybe a rehash would be needed for some strings and your input strings could appear in yet another order.
This has nothing to do with the order of insertion which has no reason to be kept, the purpose of the data structure is to place the items at the appropriate location so that their retreival can be made in amortized constant time (here by placing them at the indices given by the transformed hashes).
Probably, the best answer to this question is to say: "study a little about data structures.
Unordered map is supposed to store the data into a special data structure specially designed to be efficient but without any restrictions on storage order, and is therefore "unorderer".
Note that if you get the order of insertion is a type of restriction about the order.
Check out this data structure: hash table.
What they mean by "unordered", is that the elements are returned in a random order (based on element hash).
I need a container to store a value (int) according to two attributes, source (int) and destination (int) i.e. when a source sends something to a destination, I need to store it as an element in a container. The source is identified by a unique int ID (an integer from 0-M), where M is in the tens to hundreds, and so is the destination (0-N). The container will be updated by iterations of another function.
I have been using a vector(vector(int)) which means goes in the order of source(destination(value)). A subsequent process needs to check this container, to see if an element exists in for a particular source, and a particular destination - it will need to differentiate between an empty 'space' and a filled one. The container has the possibility of being very sparse.
The value to be stored CAN be 0 so I haven't had success trying to find out if the space is empty, since I can't seem to do something like container[M][N].empty().
I have no experience with maps, but I have seen another post that suggests a map might be useful, and an std::map<int, int> seems to be similar to a vector<vector<int>>.
To summarise:
Is there a way to check if a specific vector of vector 'space' is empty (since I can't compare it to 0)
Is a std::map<int, int> better for this purpose, and how do I use one?
I need a container to store a value (int) according to two attributes,
source (int) and destination (int)
std::map<std::pair<int, int>, int>
A subsequent process needs to check this container, to see if an
element exists in for a particular source, and a particular
destination - it will need to differentiate between an empty 'space'
and a filled one.
std::map::find
http://www.cplusplus.com/reference/map/map/find/
The container has the possibility of being very sparse.
Use a std::map. The "correct" choice of a container is based on how you need to find things and how you need to insert/delete things. If you want to find things fast, use a map.
First of all, assuming you want an equivalent structure of
vector<vector<int>>
you would want
std::map<int,std::vector<int>>
because for each key in a map, there is one unique value only.
If your sources are indexed very closely sequentially as 0...N, will be doing a lot of look-ups, and few deletions, you should use a vector of vectors.
If your sources have arbitrary IDs that do not closely follow a sequential order or if you are going to do a lot of insertions/deletions, you should use a map<int,vector<int>> - usually implemented by a binary tree.
To check the size of a vector, you use
myvec.size()
To check whether a key exists in a map, you use
mymap.count(ID) //this will return 0 or 1 (we cannot have more than 1 value to a key)
I have used maps for a while and even though I'm nowhere close to an expert, they've been very convenient for me to use for storing and modifying connections between data.
P.S. If there's only up to one destination matching a source, you can proceed with
map<int,int>
Just use the count() method to see whether a key exists before reading it
If you want to keep using a vector but want to add a check for whether the item contains a valid value, look at boost::optional. The type would now be std::vector<std::vector<boost::optional<int>>>.
You can also use a map, but the key into the map needs to be both IDs not just one.
std::map<std::pair<int,int>,int>
Edit: std::pair implements a comparison operator operator< that should be sufficient for use in a map, see http://en.cppreference.com/w/cpp/utility/pair/operator_cmp.
I have to store information about contents in a lookup table such that it can be accessed very quickly.I might need to refer some of the elements in look up table recursively to get complete information about contents. What will be better data structure to use:
Map with one of parameter, which will be unique to all the entries in look up table, as key and rest of the information as value
Use static array for each unique entries and access them when needed according to key(which will be same as the one used in MAP).
I want my software to be robust as if we have any crash it will be catastrophic for my product.
It depends on the range of keys that you have.
Usually, when you say lookup table, you mean a smallish table which you can index directly ( O(1) ). As a dumb example, for a substitution cipher, you could have a char cipher[256] and simply index with the ASCII code of a character to get the substitution character. If the keys are complex objects or simply too many, you're probably stuck with a map.
You might also consider a hashtable (see unordered_map).
Reply:
If the key itself can be any 32-bit number, it wouldn't make sense to store a very sparse 4-billion element array.
If however your keys are themselves between say 0..10000, then you can have a 10000-element array containing pointers to your objects (or the objects themselves), with only 2000-5000 of your elements containing non-null pointers (or meaningful data, respectively). Access will be O(1).
If you can have large keys, then I'd probably go with the unordered_map. With a map of 5000 elements, you'd get O(log n) to mean around ~12 accesses, a hash table should be pretty much one or two accesses tops.
I'm not familiar with perfect hashes, so I can't advise about their implementation. If you do choose that, I'd be grateful for a link or two with ideas to keep in mind.
The lookup times in a std::map should be O=ln(n), with a linear search in a static array in the worst case O=n.
I'd strongly opt for a std::map even if it has a larger memory footprint (which should not matter, in the most cases).
Also you can make "maps of maps" or even deeper structures:
typedef std::map<MyKeyType, std::map<MyKeyType, MyValueType> > MyDoubleMapType;
I have created a vector which contains several map<>.
vector<map<key,value>*> v;
v.push_back(&map1);
// ...
v.push_back(&map2);
// ...
v.push_back(&map3);
At any point of time, if a value has to be retrieved, I iterate through the vector and find the key in every map element (i.e. v[0], v[1] etc.) until it's found. Is this the best way ? I am open for any suggestion. This is just an idea I have given, I am yet to implement this way (please show if any mistake).
Edit: It's not important, in which map the element is found. In multiple modules different maps are prepared. And they are added one by one as the code progresses. Whenever any key is searched, the result should be searched in all maps combined till that time.
Without more information on the purpose and use, it might be a little difficult to answer. For example, is it necessary to have multiple map objects? If not, then you could store all of the items in a single map and eliminate the vector altogether. This would be more efficient to do the lookups. If there are duplicate entries in the maps, then the key for each value could include the differentiating information that currently defines into which map the values are put.
If you need to know which submap the key was found in, try:
unordered_set<key, pair<mapid, value>>
This has much better complexity for searching.
If the keys do not overlap, i.e., are unique througout all maps, then I'd advice a set or unordered_set with a custom comparision functor, as this will help with the lookup. Or even extend the first map with the new maps, if profiling shows that is fast enough / faster.
If the keys are not unique, go with a multiset or unordered_multiset, again with a custom comparision functor.
You could also sort your vector manually and search it with a binary_search. In any case, I advice using a tree to store all maps.
It depends on how your maps are "independently created", but if it's an option, I'd make just one global map (or multimap) object and pass that to all your creators. If you have lots of small maps all over the place, you can just call insert on the global one to merge your maps into it.
That way you have only a single object in which to perform lookup, which is reasonably efficient (O(log n) for multimap, expected O(1) for unordered_multimap).
This also saves you from having to pass raw pointers to containers around and having to clean up!