Structures

1. Structures putting data together

2. Structure Definitions • aggregate construct allows to manipulate several data items as a single whole • structures are an aggregate construct; • what other aggregate constructs have we studied? • example: a particular date has month, day and year • structure definition: struct Date{ int month; int day; int year; }; • note the semicolon at the end of the structure definition • elements of the structure are called members or member variables • structure definition is not executable – good candidate for a header file • members of the same structure must have different names, different structures can contain members with the same name • that is: the scope of member variable name is the structure definition

3. Preprocessor Directives (Review) • each definition (e.g. global constant def.) can be encountered only once during compilation • when definition is placed in a header file, it may be included multiple times • header file must structured so it is safe in case of multiple inclusion; term – multiple inclusion protection • mechanism - preprocessor directives #define name value note that substitution is textual • problem: #define press 50+5 int myvar = press * 20; changes order of operations, do not use #define instead of global constants #ifdef name - true if name defined, #ifndefname - true if not #endif - completes #if • header file myheader.h containing definitions usually has the following structure: #ifndef MYHEADER_H #define MYHEADER_H // text of the header file goes here #endif

4. Structure Variables • defined structure Date is a type • part of the language types such as int, char are built-in or basic • a structure variables can be declared as built-in type variable : Date today, birthday; • each structure variable of Date contains three member variables • programmer can refer to member variable by specifying structure variable name “dot” member variable name • for example today contains: today.month today.day today.year • member variables are of type specified in the structure definition • a member variable can be used as any other (scalar) variabletoday.year=2001; today.day = birthday.day+1; cin >> today.month;

5. Assigning Value to Structure Variable • initialization • structure variable can be initialized at declaration: Date birthday={10, 31, 2035}; • order of initialization corresponds to the order member of member variables in structure definition • more values than variable members – error • less values than variable members – the rest initialized to zero • structure variable can be assigned the value of another structure variable of the same structure type Date deadline; deadline=today; • structure variables cannot be directly compared if (deadline == today) // error how would you compare two structure variables?

6. Passing Structures as Parameters,Returning Structures structures can be passed by value and by reference, a function can return a structure: Date setDate(int m, int d, int y){ Date tmp; tmp.month=m; tmp.day=d; tmp.year=y; return tmp; } what does this code do? Date projectDue; projectDue=setDate(10,31,2035);

7. Complex Structures • a member may be of basic type or of type structure • substructure – member variable of type structure // example structure struct Example{ int a; string b; }; // example complex structure struct CompExample{ int c; Example d; // substructure }; • if there is a declaration compExample me;how do you access substructure member variables?

8. Structures and Arrays • a member may be an array struct ExampleWArray{ // complex structure definition int a; int b[5]; // member array }; ExampleWArray se; // declaring structure variable se.a = 123; se.b[3]=456; // accessing members • an array of structures may be declared struct Example{ //structure int a; string b; }; Example as[4]; // declaring array of structures as[3].a=123; as[3].b=”Hello”; // accessing members how do you access characters of string b? • more complicated constructs (arrays of structures with substructures, etc.) are possible