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- The following example creates a dictionary containing the atomic masses of some chemical elements. The element names are the keys and the masses are the values. The dictionary is a set of key:value pairs: In : atomic_mass = {'H': 1., 'C': 12, 'S': 32} Then we can add another key:value entry using: In : atomic_mass ['O'] = 16.
csc.ucdavis.edu › ~chaos › coursesPart C: Dictionaries, Arrays, Functions, and Modules - UC Davis
Top results related to what are the function of dictionary keys in c example list of elements
Top Answer
Answered Jul 19, 2017 · 3 votes
- Same as regular typdefs, incredibly useful for 2 and 4. Instead of typing the whole thing, you can now use myFuncDef
- A function declaration for a function that recieves a function pointer. A function that takes a function pointer of 1. would thus look int add2to3(myFuncDef functionPtr);
- -
- A function that takes an int and (probably based on that) returns you a function pointer. Using the typedef it would have looked like myFuncDef functionFactory(int n);
Without typedefs these chains of pointers to pointers could get incredibly long as can be seen in this question: C syntax for functions returning function pointers
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Top Answer
Answered Mar 25, 2018 · 2 votes
N1570 6.11.6:
The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.
This same wording appears in the 1990, 1999, and 2011 editions of the ISO C standard. There has been no change. The word obsolescent says that the feature may be removed in a future edition of the Standard, but so far the committee has not done so. (Function pointer declarations are just one of several contexts where function declarators can appear.)
The Introduction section of the C standard explains what obsolescent means:
Certain features are obsolescent, which means that they may be considered for withdrawal in future revisions of this International Standard. They are retained because of their widespread use, but their use in new implementations (for implementation features) or new programs (for language [6.11] or library features [7.31]) is discouraged.
A call to a function declared with an old-style declarator is still required to pass the correct number and type(s) of arguments (after promotion) as defined by the function's actual definition. A call with incorrect arguments has undefined behavior, which means that the compiler is not required to diagnose the error; the burden is entirely on the programmer.
This is why prototypes were introduced, so that the compiler could check correctness of arguments.
On an i386 system with GCC, “extra” arguments passed in a call to an empty-parentheses-type’d function pointer are ignored, because of how stack frames work ...
Yes, that's well within the bounds of undefined behavior. The worst symptom of undefined behavior is having the program work exactly as you expect it to. It means that you have a bug that hasn't exhibited itself yet, and it will be difficult to track it down.
You should not depend on that unless you have a very good reason to do so.
If you change
int (*fp)() = foo;-
to
int (*fp)(int) = foo;-
the compiler will diagnose the incorrect call.
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Top Answer
Answered Jan 28, 2024 · 0 votes
Calling a constructor and calling a function pointer of a constructor in C++ are very similar in the sense that they are both quite impossible:
- You cannot call a constructor in C++.
- You cannot obtain a pointer to a constructor in C++.
According to the C++ spec about constructors:
Constructors have no names and cannot be called directly.
So, in C++, you do not call constructors. The language does that for you when you use the new keyword, when you declare an object on the stack, etc. The expression new MyClass() might look like a constructor invocation, and it does in fact contain a constructor invocation, and so it might be mistaken for a constructor invocation, but that's not what it is; it is a "new expression", which includes, among other things, an invocation of a constructor. But the other things are crucial: you cannot just skip them and directly invoke the constructor.
Function pointers of constructors do not exist because normally you cannot take the address of a constructor. If you examine the names exported by the DLL, and somehow figure out which name corresponds to the constructor, and use that name to obtain the entry point of that constructor from the DLL, you are cheating: you are doing something which is not normally supported by the language. As a result, you should not attempt to invoke that entry point. A constructor is only supposed to be invoked by the language.
Also according to the C++ spec:
A constructor is a special non-static member function of a class.
(Emphasis mine.)
A constructor assumes that memory has been allocated, and proceeds to initialize that memory. The language takes care of the memory allocation, but invoking the constructor directly does not. Since a constructor is non-static, it expects an uninitialized this to be passed to it, but you are not allocating any memory and you are not passing anything to the constructor. The result is exactly as expected: a crash.
And besides, think about it: the constructor could not be internally allocating the memory, because how would it work then if the object was allocated on the stack? And how would it work if the constructor was being invoked by the constructor of a derived class which presumably occupies more memory?
You can get your DLL to work by using a factory method. A factory method is a public static method which executes new ClassA() and returns the result, or declares ClassA() and returns it by value. Then, obtain the address of the factory method from the DLL, and invoke it.
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Top Answer
Answered Jun 26, 2017 · 5 votes
Answering questions in reverse order:
C does not have array variables....but this is really just working with pointers with an alternative syntax.
This is incorrect, and you need to toss that bookmark in the trash. It's a common misconception that arrays and pointers are the same thing, but they are not. An array expression will be converted to a pointer expression under most circumstances, and array subscripting is accomplished through pointer arithmetic, but an array object is an actual array, not a pointer.
If pointer also keeps the variable's address in it and at the same time we can get the variable's address using & sign, can't we do the same task pointer does by deriving the address of any variable? I mean if I don't declare it as pointer and use it as int ptr = &i; in 2nd code snippet and use it as normal variable, what would be the differences?
That code doesn't illustrate why pointers exist, or why they are useful.
C actually requires us to use pointers in the following cases:
- To write to a function's parameters;
- To track dynamically allocated memory;
Pointers also make dynamic data structures like trees and lists easy to implement, but they aren't required for it (unless you're using dynamic memory allocation in those structures).
Writing to a function's parameters
C passes all function arguments by value; the formal parameter in the function definition is a separate object in memory from the actual parameter in the function call, so any change to the formal parameter is not reflected in the actual parameter. For example, assume the following swap function:
void swap( int a, int b ) { int t = a; a = b; b = t; }-
This function exchanges the values in a and b. However, when we call the function as
int x = 4, y = 5;swap( x, y );-
the values of x and y won't be updated, because they are different objects than a and b. If we want to update x and y, we have to pass pointers to them:
swap( &x, &y );-
and update the function definition as follows:
void swap( int *a, int *b ) { int t = *a; *a = *b; *b = t; }-
Instead of swapping the contents of a and b, we swap the contents of the objects that a and b point to. This crops up all the time - think about the scanf function, and how you have to use the & operator on scalar arguments.
Tracking dynamically allocated memory
The dynamic memory allocation functions malloc, calloc, and realloc all return pointers to dynamic memory buffers; there's no variable associated with that memory as such.
char *buffer = malloc( sizeof *buffer * some_length );-
A pointer is the only way to track that memory.
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Top Answer
Answered Nov 10, 2009 · 21 votes
From here and here: (after i googled your title)
- Alternate key - An alternate key is any candidate key which is not selected to be the primary key
- Candidate key - A candidate key is a field or combination of fields that can act as a primary key field for that table to uniquely identify each record in that table.
- Compound key - compound key (also called a composite key or concatenated key) is a key that consists of 2 or more attributes.
- Primary key - a primary key is a value that can be used to identify a unique row in a table. Attributes are associated with it. Examples of primary keys are Social Security numbers (associated to a specific person) or ISBNs (associated to a specific book). In the relational model of data, a primary key is a candidate key chosen as the main method of uniquely identifying a tuple in a relation.
- Superkey - A superkey is defined in the relational model as a set of attributes of a relation variable (relvar) for which it holds that in all relations assigned to that variable there are no two distinct tuples (rows) that have the same values for the attributes in this set. Equivalently a superkey can also be defined as a set of attributes of a relvar upon which all attributes of the relvar are functionally dependent.
- Foreign key - a foreign key (FK) is a field or group of fields in a database record that points to a key field or group of fields forming a key of another database record in some (usually different) table. Usually a foreign key in one table refers to the primary key (PK) of another table. This way references can be made to link information together and it is an essential part of database normalization
Other Answers
Answered Sep 21, 2014 · 89 votes
(I) Super Key – An attribute or a combination of attribute that is used to identify the records uniquely is known as Super Key. A table can have many Super Keys.
E.g. of Super Key
- ID
- ID, Name
- ID, Address
- ID, Department_ID
- ID, Salary
- Name, Address
- Name, Address, Department_ID
So on as any combination which can identify the records uniquely will be a Super Key.
(II) Candidate Key – It can be defined as minimal Super Key or irreducible Super Key. In other words an attribute or a combination of attribute that identifies the record uniquely but none of its proper subsets can identify the records uniquely.
E.g. of Candidate Key
- ID
- Name, Address
For above table we have only two Candidate Keys (i.e. Irreducible Super Key) used to identify the records from the table uniquely. ID Key can identify the record uniquely and similarly combination of Name and Address can identify the record uniquely, but neither Name nor Address can be used to identify the records uniquely as it might be possible that we have two employees with similar name or two employees from the same house.
(III) Primary Key – A Candidate Key that is used by the database designer for unique identification of each row in a table is known as Primary Key. A Primary Key can consist of one or more attributes of a table.
E.g. of Primary Key - Database designer can use one of the Candidate Key as a Primary Key. In this case we have “ID” and “Name, Address” as Candidate Key, we will consider “ID” Key as a Primary Key as the other key is the combination of more than one attribute.
(IV) Foreign Key – A foreign key is an attribute or combination of attribute in one base table that points to the candidate key (generally it is the primary key) of another table. The purpose of the foreign key is to ensure referential integrity of the data i.e. only values that are supposed to appear in the database are permitted.
E.g. of Foreign Key – Let consider we have another table i.e. Department Table with Attributes “Department_ID”, “Department_Name”, “Manager_ID”, ”Location_ID” with Department_ID as an Primary Key. Now the Department_ID attribute of Employee Table (dependent or child table) can be defined as the Foreign Key as it can reference to the Department_ID attribute of the Departments table (the referenced or parent table), a Foreign Key value must match an existing value in the parent table or be NULL.
(V) Composite Key – If we use multiple attributes to create a Primary Key then that Primary Key is called Composite Key (also called a Compound Key or Concatenated Key).
E.g. of Composite Key, if we have used “Name, Address” as a Primary Key then it will be our Composite Key.
(VI) Alternate Key – Alternate Key can be any of the Candidate Keys except for the Primary Key.
E.g. of Alternate Key is “Name, Address” as it is the only other Candidate Key which is not a Primary Key.
(VII) Secondary Key – The attributes that are not even the Super Key but can be still used for identification of records (not unique) are known as Secondary Key.
E.g. of Secondary Key can be Name, Address, Salary, Department_ID etc. as they can identify the records but they might not be unique.
Other Answers
Answered Nov 10, 2009 · 9 votes
Ólafur forgot the surrogate key:
A surrogate key in a database is a unique identifier for either an entity in the modeled world or an object in the database. The surrogate key is not derived from application data.
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People also ask
What is Dict keys() function in Python?
- Syntax: dict.keys () Parameters: There are no parameters. Returns: A view object is returned that displays all the keys. This view object changes according to the changes in the dictionary. A simple example to show how the keys () function works in the dictionary. Output: Demonstrating the practical application of keys () using the Python loop.
Python Dictionary keys() method - GeeksforGeeks
www.geeksforgeeks.org/python-dictionary-keys-method/How to get a key from a dictionary in Python?
- Returns the value for a key if it exists in the dictionary. The Python dictionary .get () method provides a convenient way of getting the value of a key from a dictionary without checking ahead of time whether the key exists, and without raising an error. d.get ( ) searches dictionary d for and returns the associated value if it is found.
Dictionaries in Python – Real Python
realpython.com/python-dicts/How do I access a dictionary element in Python?
- Dictionary elements are accessed via keys. Take the Quiz: Test your knowledge with our interactive “Python Dictionaries” quiz. You’ll receive a score upon completion to help you track your learning progress: Dictionaries are Python’s implementation of a data structure that is more generally known as an associative array.
Dictionaries in Python – Real Python
realpython.com/python-dicts/Can an object be used as a dictionary key?
- Technically, it is not quite correct to say an object must be immutable to be used as a dictionary key. More precisely, an object must be hashable, which means it can be passed to a hash function.
Dictionaries in Python – Real Python
realpython.com/python-dicts/www.geeksforgeeks.org › implementation-on-map-orImplementation on Map or Dictionary Data Structure in C
www.geeksforgeeks.org › implementation-on-map-orMar 27, 2023 · Functions used to implement Map in C. The getIndex function searches for a key in the keys array and returns its index if found, or -1 if not found. The insert function inserts a key-value pair into the map. If the key already exists, it updates the value.
stackoverflow.com › questions › 13552898Remove elements from Dictionary<Key, Item> - Stack Overflow
stackoverflow.com › questions › 13552898Nov 26, 2012 · var keysToRemove = myDictionary.Keys.Except(keysToInclude).ToList(); foreach (var key in keysToRemove) myDictionary.Remove(key); Note the ToList () call - it's important to materialize the list of keys to remove.
Code sample
var keysToRemove = myDictionary.Keys.Except(keysToInclude).ToList();foreach (var key in keysToRemove)myDictionary.Remove(key);csc.ucdavis.edu › ~chaos › coursesPart C: Dictionaries, Arrays, Functions, and Modules - UC Davis
csc.ucdavis.edu › ~chaos › coursesThe following example creates a dictionary containing the atomic masses of some chemical elements. The element names are the keys and the masses are the values. The dictionary is a set of key:value pairs:
www.geeksforgeeks.org › python-dictionary-keys-methodPython Dictionary keys() method - GeeksforGeeks
www.geeksforgeeks.org › python-dictionary-keys-methodJun 16, 2023 · The keys() method in Python Dictionary, returns a view object that displays a list of all the keys in the dictionary in order of insertion using Python. Syntax: dict.keys()
realpython.com › python-dictsDictionaries in Python – Real Python
realpython.com › python-dictsMark as Completed. Table of Contents. Defining a Dictionary. Accessing Dictionary Values. Dictionary Keys vs. List Indices. Building a Dictionary Incrementally. Restrictions on Dictionary Keys. Restrictions on Dictionary Values. Operators and Built-in Functions. Built-in Dictionary Methods. d.clear () d.get (<key> [, <default>]) d.items ()
www.knowledgeboat.com › learn › sumita-arora-pythonChapter 13: Dictionaries | Solutions of ... - KnowledgeBoat
www.knowledgeboat.com › learn › sumita-arora-python1. By using curly brackets and separating key-value pairs with commas as per the syntax below: <dictionary-name> = {<key>:<value>, <key>:<value>...} For example: d = { 'a': 1, 'b': 2, 'c': 3 } . 2. By using dictionary constructor dict (). There are multiple ways to provide keys and values to dict () constructor: i.
pynative.com › python-dictionariesDictionaries in Python – PYnative
pynative.com › python-dictionariesNov 3, 2022 · Python dictionary represents a mapping between a key and a value. In simple terms, a Python dictionary can store pairs of keys and values. Each key is linked to a specific value. Once stored in a dictionary, you can later obtain the value using just the key.
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