Structures

Motivation

• We have plenty of simple types for storing single items like numbers, characters. But is this really enough for storing more complex things, like patient records, address books, tables, etc.?
• It would be easier if we had mechanisms for building up more complex storage items that could be accessed with single variable names
• Compound Storage -- there are some built-in ways to encapsulate multiple pieces of data under one name
  • Array -- we already know about this one. Indexed collections, and all items are the same type
  • Structure -- keyword struct gives us another way to encapsulate multiple data items into one unit. In this case, items do not have to be the same type
• Structures are good for building records -- like database records, or records in a file.

What is a Structure?

• A structure definition is like a blueprint for the structure. It takes up no storage space itself -- it just specifies what variables of this structure type will look like
• An actual structure variable is like a box with multiple data fields inside of it. Consider the idea of a student database. One student record contains multiple items of information (name, address, SSN, GPA, etc)
• Properties of a structure:
  • internal elements may be of various data types
  • order of elements is arbitrary (no indexing, like with arrays)
  • Fixed size, based on the combined sizes of the internal elements

Creating Structure definitions and variables

Structure Definitions

• The basic format of a structure definition is:

    struct structureName
    {
        // data elements in the structure
    };
  

  struct is a keyword
. The data elements inside are declared as normal variables
 
• Examples:

    /* A structure representing the parts of a fraction (a rational number) */
    struct fraction
    {
       int num;		// the numerator of the fraction 
       int denom;		// the denominator of the fraction
    };
  
    /* A structure representing a record in a student database */
    struct student
    {
       char fName[20];		// first name
       char lName[20];		// last name
       int socSecNumber;		// social security number
       double gpa;		// grade point average
    };
  

• Note that the two examples above are both just blueprints specifying what will be in corresponding structure variables if and when we create them. By themselves, these definitions above are not variables and do not take up storage

Structure variables

• To create an actual structure variable, we use the keyword struct and the structure name. Format:

    struct structureName variableName;
  

  Variations on this format include the usual forms for creating arrays and pointers, and the comma-separated list for multiple variables
   
• Examples (using the above structure definitions):

    struct fraction f1;		// f1 is now a 'fraction'
    struct fraction fList[10];    // an array of 'fraction' structures
    struct fraction * fptr;	// a pointer to a 'fraction' structure
  
    struct student stu1;		// a student structure variable
    struct student mathclass[10]; // an array of 10 students
    struct student s1, s2, s3;	// three student variables
  

Legal variations in declaration syntax

• The definition of a structure and the creation of variables can be combined into a single declaration, as well. Just list the variables after the structure definition block (the blueprint), and before the semi-colon:

    struct structureName
    {
        // data elements in the structure
    } variable1, variable2, ... , variableN;
  

• Example:

    struct fraction
    {
       int num;		// the numerator of the fraction 
       int denom;		// the denominator of the fraction
  
    } f1, fList[10], *fptr;	// variable, array, and pointer created
  

• In fact, if you only want structure variables, but don't plan to re-use the structure type (i.e. the blueprint), you don't even need a structure name:

    struct		// note:  no structure NAME given
    {
       int num;
       int denom;
    } f1, f2, f3;		// three variables representing fractions
  

• Of course, the advantage of giving a structure definition a name is that it is reusable. It can be used to create structure variables at any point later on in a program, separate from the definition block.
• You can even declare structures as variables inside of other structure defintions (of different types):

    struct date		// a structure to represent a date
    {
       int month;
       int day;
       int year;
    };
  
    struct employee	// a structure to represent an employee of a company
    {
       char firstName[20];
       char lastName[20];
       struct date hireDate;
       struct date birthDate;
    };
  

• And, typedef can be used to create an actual type name, which can be used to declare variables without repeating the keyword struct:

    typedef struct date
    {
       int month;
       int day;
       int year;
    } Date;		// "Date" is now a type name
  
    /* ------------------ */
  
    Date d1, d2, d3;	// d1, d2, and d3 are now Date variables
  			//  keyword struct was not necessary to declare
  

Using structures

• Once a structure variable is created, how do we use it? How do we access its internal variables (often known as its members)?
• To access the contents of a structure, we use the dot-operator. Format:

    structVariableName.dataVariableName
  

• Example, using the fraction structure:

    struct fraction f1, f2;
  
    f1.num = 4;			// set f1's numerator to 4
    f1.denom = 5;			// set f1's denominator to 5
    f2.num = 3;			// set f2's numerator to 3
    f2.denom = 10;		// set f2's denominator to 10
  
    printf("f1 = %d/%d\n", f1.num, f1.denom);	// prints 4/5
    printf("f2 = %d/%d\n", f2.num, f2.denom);	// prints 3/10
  

• Example, using the student structure:

    struct student sList[10];	// array of 10 students 
  
    // set first student's data: (John Smith, SSN: 123456789, GPA: 3.75)
    strcpy(sList[0].fName, "John");	
    strcpy(sList[0].lName, "Smith");
    sList[0].socSecNumber = 123456789;
    sList[0].gpa = 3.75;
  
    // assume there's more code here that initializes other students
  
    // This loop prints all 10 students -- their names and their GPA
    int i;
    for (i = 0; i < 10; i++)
       printf("%s %s: %.2f\n", sList[i].fName, sList[i].lName, sList[i].gpa);
  

• struct1.c -- Simple example of accessing the internal elements of a structure

A shortcut for initializing structs

• While we can certainly initialize each variable in a structure separately, we can use an initializer list on the declaration line, too
  • This is similar to what we saw with arrays
  • This is only usable on the declaration line (like with arrays)
  • The initializer set should contain the struct contents in the same order that they appear in the struct definition
  
  
• Example (using the fraction structure):

    struct fraction f1 = {3, 5};		// initializes num = 3, denom = 5
  
    // This would be the same as doing the following:
    f1.num = 3;
    f1.denom = 5;
  

• Example (using the student structure):

    struct student s1 = {"John", "Smith", 123456789, 3.75};
    struct student s2 = {"Alice", "Jones", 123123123, 2.66};
  

Accessing internal data using a pointer to a structure

• If we have a pointer to a structure, then things are a little trickier:

    struct fraction f1;		// a fraction structure
    struct fraction *fPtr;	// pointer to a fraction
  
    fPtr = &f1;			// fPtr now points to f1
  
    f1.num = 3;			// this is legal, of course
  
    fPtr.num = 10;		// but how about this? NO!  ILLEGAL
  				// cannot put a pointer on the left side
  				// of the dot-operator
  

  Remember that to get to the target of a pointer, we dereference it. The target of fPtr is *fPtr. So how about this?

    *fPtr.num = 10;		// closer, but still NO (not quite)
  

  The problem with this is that the dot-operator has higher precedence, so this would be interpreted as:

    *(fPtr.num) = 10;	// cannot put a pointer on the left of the dot
  

  But if we use parintheses to force the dereference to happen first, then it works:

    (*fPtr).num = 10;		// YES!
  

• Alternative operator for pointers: While the above example works, it's a little cumbersome to have to use the parintheses and the dereference operator all the time. So there is a special operator for use with pointers to structures. It is the arrow operator:

    pointerToStruct -> dataVariable
  

  Example:

    struct fraction * fPtr;	// pointer to a fraction
  
    // assume this has been pointed at a valid target
  
    fPtr->num = 10;		// set the fraction's numerator to 10
    fPtr->denom = 11;		// denominator set to 11
  
    printf("%d/%d\n", fPtr->num, fPtr->denom);	// prints: 10/11
  
    // Note that fPtr->num is just a shortcut for (*fPtr).num
  

Accessing members of nested structures

• Earlier, we saw an example of a structure variable used within another structure definition

    typedef struct date		// making it a typedef
    {
       int month;
       int day;
       int year;
    } Date;			// so that "Date" is the type name
  
    struct employee
    {
       char firstName[20];
       char lastName[20];
       Date hireDate;		// using the typedef "Date" to declare
       Date birthDate;
    };
  

• Here's an example of initializing all the data elements for one employee variable:

    struct employee emp;			// emp is an employee variable
  
    // Set the name to "Alice Jones"
    strcpy(emp.firstName, "Alice");
    strcpy(emp.lastName, "Jones");
  
    // set the hire date to March 14, 2001
    emp.hireDate.month = 3;
    emp.hireDate.day = 14;
    emp.hireDate.year = 2001;
  
    // sets the birth date to Sept 15, 1972
    emp.birthDate.month = 9;
    emp.birthDate.day = 15;
    emp.birthDate.year = 1972;
  

• Here's an example of an employee initialization using our shortcut initializer form:

    struct employee emp2 =
  	{ "John", "Smith", {6, 10, 2003}, {2, 19, 1981} };
  
    // John Smith, whose birthday is Feb 19, 1981, was hired on June 10, 2003
  

• struct2.c -- See examples of nested structure access here