Difference between revisions of "Dictionaries"
| Line 1: | Line 1: | ||
| − | ==Definition== | + | ==Definition== <br> |
| − | A dictionary is a general-purpose data structure for storing a group of objects. A dictionary has a set of keys and each key has a single associated value. When presented with a key, the dictionary will return the associated value. Dictionaries are special lists, whereas every value in the list has a key which is also a variable. A good example for a dictionary is a phone book. | + | A dictionary is a general-purpose data structure for storing a group of objects. A dictionary has a set of keys and each key has a single associated value. When presented with a key, the dictionary will return the associated value. Dictionaries are special lists, whereas every value in the list has a key which is also a variable. A good example for a dictionary is a phone book.<br> |
<b> Declaring & Adding to the Dictionary </b> | <b> Declaring & Adding to the Dictionary </b> | ||
Dictionary<string, long> phonebook = new Dictionary<string, long>(); | Dictionary<string, long> phonebook = new Dictionary<string, long>(); | ||
| Line 8: | Line 8: | ||
Notice that when defining a dictionary, we need to provide a generic definition with two types - the type of the key and the type of the value. In this case, the key is a string whereas the value is an integer. | Notice that when defining a dictionary, we need to provide a generic definition with two types - the type of the key and the type of the value. In this case, the key is a string whereas the value is an integer. | ||
There are also two ways of adding a single value to the dictionary, either using the brackets operator or using the Add method. | There are also two ways of adding a single value to the dictionary, either using the brackets operator or using the Add method. | ||
| − | <b> Checking for a key </b> | + | <br> <b> Checking for a key </b> |
To check whether a dictionary has a certain key in it, we can use the ContainsKey method: | To check whether a dictionary has a certain key in it, we can use the ContainsKey method: | ||
Dictionary<string, long> phonebook = new Dictionary<string, long>(); | Dictionary<string, long> phonebook = new Dictionary<string, long>(); | ||
| Line 18: | Line 18: | ||
Console.WriteLine("Alex's number is " + phonebook["Alex"]); | Console.WriteLine("Alex's number is " + phonebook["Alex"]); | ||
} | } | ||
| + | <br> | ||
<b> Removing a key/value pair </b> | <b> Removing a key/value pair </b> | ||
To remove an item from a dictionary, we can use the Remove method. Removing an item from a dictionary by its key is fast and very efficient. When removing an item from a List using its value, the process is slow and inefficient, unlike the dictionary Remove function. | To remove an item from a dictionary, we can use the Remove method. Removing an item from a dictionary by its key is fast and very efficient. When removing an item from a List using its value, the process is slow and inefficient, unlike the dictionary Remove function. | ||
Revision as of 10:48, 15 May 2017
==Definition==
A dictionary is a general-purpose data structure for storing a group of objects. A dictionary has a set of keys and each key has a single associated value. When presented with a key, the dictionary will return the associated value. Dictionaries are special lists, whereas every value in the list has a key which is also a variable. A good example for a dictionary is a phone book.
Declaring & Adding to the Dictionary
Dictionary<string, long> phonebook = new Dictionary<string, long>();
phonebook.Add("Alex", 4154346543);
phonebook["Jessica"] = 4159484588;
Notice that when defining a dictionary, we need to provide a generic definition with two types - the type of the key and the type of the value. In this case, the key is a string whereas the value is an integer.
There are also two ways of adding a single value to the dictionary, either using the brackets operator or using the Add method.
Checking for a key
To check whether a dictionary has a certain key in it, we can use the ContainsKey method:
Dictionary<string, long> phonebook = new Dictionary<string, long>();
phonebook.Add("Alex", 415434543);
phonebook["Jessica"] = 415984588;
if (phonebook.ContainsKey("Alex")) {
Console.WriteLine("Alex's number is " + phonebook["Alex"]);
}
Removing a key/value pair
To remove an item from a dictionary, we can use the Remove method. Removing an item from a dictionary by its key is fast and very efficient. When removing an item from a List using its value, the process is slow and inefficient, unlike the dictionary Remove function.
Dictionary<string, long> phonebook = new Dictionary<string, long>();
phonebook.Add("Alex", 415434543);
phonebook["Jessica"] = 415984588;
phonebook.Remove("Jessica"); Console.WriteLine(phonebook.Count);
