Structures and Unions

Transcription

1 Structures and Unions Structures Structures are a collection of related items (variables) under one name. In contrast to arrays, structures can contain variables of different types. For instance, suppose that you want to store the details of a person in a program. A person has a number of attributes, such as name surname and age. The attributes name and surname are both string but the age is of an integer type. Therefore the best way to represent this person is by using a structure Defining and declaring a structure Defining a structure is very equivalent to defining a new data type. For this reason, the definition of a structure is written before the main() header and has the following structure: struct _identifier_ { _variable members that make up the structure_ }; Consider the following example: struct person { char name[20]; char surname[20]; int age; } The above code snippet illustrates a structure that represents a person that is made up of a name, surname and age. This structure can now be declared and used in the main() program by simply by creating a variable of its type as shown in the following code snippet: struct person p; In the above code snippet, p is the variable identifier while person is representing that data type. As this is a structure, the keyword struct is included before the structure name. Note that the more that one variable of the same structure type can be created by using a comma as shown in the following code snippet: struct person p1, p2, p3; A variable of a structure type can be assigned values as soon as it is declared. This can be done as shown in the code snippet below: Matthew Xuereb 2012/2013 1

2 struct person p1 = { Matthew, Xuereb, 26}; Accessing fields in a structure In order to access a particular filed in a structure the dot notation is used. Consider the following example: In the above example, a structure person is defined and a variable of this structure type is declared and assigned values in the main method. Then the contents of this structure are displayed on the screen using the dot notation. Items of a structure can also be assigned individually using the dot notation. Numerical or character items can be either assigned directly using the assignment (=) operator or else assigned values that are entered by the user from the keyboard. On the other hand, string values cannot be directly assigned using the assignment operator as in reality in C a string is not a primitive data type but is represented as an array of characters. For this reason, the string function strcpy is used as shown in the code snippet below: strcpy(p.name, Matthew ); strcpy(p.surname, Xuereb ); p.age = 26; In the following example, first the user is asked to enter his details via the keyboard. These details are stored in a structure and then they are displayed on the screen: Matthew Xuereb 2012/2013 2

3 Structure within a structure Sometimes it might be useful to have a structure type in another structure. For instance, in the above person structure, we can add another structure that is used to hold the person s home address. Consider the following example: Matthew Xuereb 2012/2013 3

4 The typedef keyword The typedef keyword is used to create a new data type names. It is usually used in conjunction with structures so that such structures can be used without the need to declare them using the struct keyword. Consider the following example: Matthew Xuereb 2012/2013 4

5 In the above example the struct person is also defined as a data type using the typedef keyword. This means that in order to declare a variable of type person there is no need to write the struct keyword at the beginning of the declaration statement. Unions A union is a special variable that at different times may hold items of types and sizes. Unions provide a way to manipulate different kinds of data in a single area of storage, without embedding any machine dependent information in the program. A union is declared in the same format as a structure using the keyword union instead of struct. Consider the example below: Matthew Xuereb 2012/2013 5

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