Introduction to Computers and Programming (PowerPoint) by dfhdhdhdhjr

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									Introduction to Computers and
        Programming

              Class 24
        Structures (structs)
      Professor Avi Rosenfeld
                Structures
• Structures are collections of related
  variables under one name
• Derived data type – i.e. constructed using
  objects of other types
• Unlike arrays, structures can store variables
  of many different types (heterogeneous and
  not homogeneous)
           Parts of the structure
• Keyword struct introduces the structure
• Structure tag names the structure definition
   – Used with keyword struct when you declare variables
     of the structure type
• Members are the variables declared within the
  structure definition
   – Must have unique names within the structure definition
   – In databases, called “fields”
             structure definitions
              keyword struct: tells
              the compiler you are
              defining a struct
struct student {         structure tag: used to
     char nameFirst[10]; name the structure
     char nameLast[10]; definition
     int firstExam;           members of the structure
     int secondExam;
     int final;
};
      don't forget the semi-colon
    structure definition (cont’d)
• The structure definition makes a new type
• It does not allocate space in memory to
  store variables
• The structure is another example of a
  complex (or derived) data type
                 members
• A struct’s members can be simple data
  types (int, char, etc) or complex data types
  (arrays, other structs)
• No two members of a struct can share the
  same name
• A struct cannot contain another instance of
  itself (you need to use a pointer for that)
      declaring struct variables
• Once the struct is defined, we can declare
  variables of our new type.

  struct student currentStudent, class[50];

keyword   struct   variable to      array to hold
struct    tag      hold one         50 student
                   student record   records
           struct comparison
• You cannot compare structs using == or !=
  – trying to do so should cause a syntax error
  – same problems as arrays
• Instead you must write code which will
  compare the structs
           Initializing structs
• The syntax for initializing structs is similar
  to the syntax for arrays.
struct student currentStudent =
  {“Bart”, “Simpson”, 55, 64};
• Declares the variable currentStudent and
  initializes nameFirst, nameLast, firstExam,
  secondExam with the values provided. final
  would be initialized to zero because no
  value was specified.
  Accessing members of a struct
• We use the struct member operator (dot
  operator) to access members of a struct
  (there is another method which uses
  pointers)
• currentStudent.firstExam would reference
  the value stored in the variable
  currentStudent, member firstExam
• You can use this notation to reference a
  struct’s values any place you can use other
  variables (including function parameters)
    Passing structs to functions
• You can pass entire structs to function.
• Unlike arrays, a struct is passed to a
  function using call by value (in other words,
  if you change the value of the struct in a
  function, that change will not be reflected in
  the calling function)
 Makin’ your own variable types
• C provides a mechanism through which you
  can simplify struct usage and enable you to
  use it as you would another type
• Use the keyword typedef
                 typedef
• The keyword typedef provides a mechanism
  for creating synonyms (or aliases) for
  previously defined data types.
• The statement:
  typedef struct student TStudent;
 defines a new type “TStudent” as a
 synonym for type struct student.
• Once the typedef synonym is defined, you
  can declare your variables like this:
  TStudent class[ 50 ] ;
            typedef (cont’d)
• It is good practice to capitalize the first
  letter of typedef synonym.
• It is better practice to preface it with a
  capital “T” to indicate it’s one of your new
  types e.g.
  – TRobot, TStudent, etc.
Simple Struct Program
An Array of Structs
Structs with functions

								
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