Data Types. Chapter 8

Transcription

1 Data Types Chapter 8

2 Chapter Topics Enumeration Data Types Declaration Assignment Operations Looping with Enumeration Types Anonymous Data Types The typedef statement Namespaces The string type 2

3 Enumeration Data Types A data type is A set of values together with A set of operations on those values. In order to define a new simple data type, called enumeration type, we need: A name for the data type. A set of values for the data type. A set of operations on the values. 3

4 Enumeration Data Types C++ allows the user to define a new simple data type by specifying: Its name and the values But not the operations. The values that we specify for the data type must be legal identifiers The syntax for declaring an enumeration type is: enum typename{value1, value2,...}; 4

5 Declaration of Enumerated Types Consider the colors of the rainbow as an enumerated type: enum rainbowcolors = { red, orange, yellow, green, blue, indigo, violate } The identifiers between { } are called enumerators The order of the declaration is significant red < orange < yellow 5

6 Declaration of Enumerated Types 6 Why are the following illegal declarations? enum grades{'a', 'B', 'C', 'D', 'F'}; They do not have legal identifiers in the list enum places{1st, 2nd, 3rd, 4th}; What could you do to make them legal? enum grades{a, B, C, D, F}; enum places{first, second, third, fourth};

7 Declaration of Enumerated Types As with the declaration of any object Specify the type name Followed by the objects of that type Given: enum daysofweek { Sun, Mon, Tue,Wed, Thu, Fri, Sat } Then we declare: daysofweek Today, payday, dayoff; 7

8 Assignment with Enumerated Types Once an enumerated variable has been declared It may be assigned an enumerated value Assignment statement works as expected payday = Fri; // note no quotes // Fri is a value, a constant Enumerated variables may receive only 8 values of that enumerated type

9 Operations on Enumerated Type Objects Incrementing variables of an enumerated type Do NOT use workaday += 1; NOR today++; Instead, use explicit type conversion today = daysofweek (today + 1); 9

10 Operations on Enumerated Type Objects Comparison I/O normal, OK in order of the enumeration definition generally not possible to do directly can be sort of done, indirectly Used primarily for program control, branching, looping Possible to have functions return an enumerated type 10

11 Looping with Enumeration Types Use an enumerated type variable as the loop control variable of a for loop for (day = Mon; day < Sat; day = static_cast<daysofweek>(day + 1)) {... This works because the values are } represented internally as integers 11

12 Functions with Enumerated Types Enumeration type can be passed as parameters to functions either by value or by reference. A function can return a value of the enumeration type. daysofweek nextday (daysofweek d) { return (daysofweek) ((d + 1)%7); } 12

13 Anonymous Data Types Named Type user defined type declaration includes typedef As with daysofweek or Boolean Anonymous Type does not have an associated type enum (MILD, MEDIUM, HOT) salsa_sizzle; variable declared without typedef 13

14 Anonymous Data Types Disadvantages Cannot pass an anonymous type as a parameter to a function. A function cannot return a value of an anonymous type. Problems when: enum {English, French, Spanish, German, Russian} languages; enum {English, French, Spanish, German, Russian} foreignlanguages; languages = foreignlanguages; //illegal Same values used but variables treated as non compatible types 14

15 The typedef statement Syntax: typedef existing_type_name new_type_name; Example: typedef int Boolean; Does not really create a new type is a valuable tool for writing self-documenting programs 15

16 Namespaces Recall use of using namespace std; Namespace is another word for scope In C++ it is a mechanism programmer creates a "named scope" namespace std { int abs ( int );... } 16

17 Namespaces Identifiers within a namespace can be used outside the body of the declaration only if use scope resolution operator x = std::abs(y); a using declaration using std::abs; z = abs(q); a using directive using namespace std; p = abs(t); Note the distinction between declaration and directive 17

18 Namespaces We usually place the using directive in global scope All blocks {... } then have identifiers available from the std namespace 18

19 The string type We have used arrays of char to hold "string" information char name[30]; cin >> name; There are some problems with doing this There is no assignment statement Must use strcpy (name, "Clyde"); Cannot do comparisons with < or == or > Must use if (strcmp (s1, s2) == 0) For all these must use #include <string.h> 19

20 The string type C++ has a string type which bypasses all the problems we've encountered Gain these capabilities by #include <string> // note no.h Now we can use statements as shown: string name = "Clyde"; if (title1 < title2) str1 = str1 + "Day"; // assignment and concatenation 20

21 The string type 21 Some functions are available string name, title; name = "Alexander"; cout << name.length()<<endl; cout << name.find('x') <<endl; cout << name.substr(1,3) <<endl; title = "Big Cheese"; title.swap(name); cout << name<<endl; Guess what will be the output of these lines of code