Structures in C — structs

Additional Notes

The following commentary comes from Marge Coahran with minor editing.

Some Background

A struct in C is the same as a record in many other (pre- object-oriented) programming languages. A struct is a collection of data. Functions that operate on a struct are not embedded within the struct; rather, we write such functions separately and pass structs to them as arguments.

For example, a program for keeping track of students might use the following collection of variables:

    struct student {
        int number;
        double testGrades[2];
        double grade;
    };

The struct is named student while its members are number, testGrades, and grade. The name of a struct is also called a tag.

A later declaration of

    struct student hannah;

will create a structure variable named hannah. The individual member variables can be referred to using the syntax variableName.memberName, as in the following example:

    hannah.number = 991234567;
    hannah.testGrades[0] = 10.;
    hannah.testGrades[1] = 11.;
    hannah.grade  = (hannah.testGrades[0] / 15. + hannah.testGrades[1] / 12.) / 2.;

The declaration

    struct student csc201[30];

might be used to create an array of student records.

Alternatively, we can define a new data type to describe our student information, using the instruction:

    typedef struct {
        int number;
        double testGrades[2];
        double grade;
    } student_t;

and then declare our variables using this new type, with instructions like:

    student_t hannah;
    student_t csc201[300];

You might find it helpful to think of the typedef instruction as giving a "blueprint" for the creation of a student_t struct variable, while the declarations cause the "construction" of variables having type student_t by setting aside memory.

Representing Time with a Struct

The following struct may be used to represent a time value in hours, minutes and seconds format (e.g., 12:34:56.123):

        typedef struct {
            int hours;
            int mins;
            double secs;
        } timeinfo_t;

The timeinfo_t identifier is the struct "tag". A new type called timeinfo_t is created. We did not call it time, because there is already a C library function called time.

Structure types may be used as return types or argument types in functions. A function that converts time values given in seconds (e.g., 12345.67) to time values given in hh:mm:ss.sss format might have the prototype:

    timeinfo_t convertTime( double realTime )

and would look like:

    timeinfo_t convertTime( double realTime )
    {
        timeinfo_t result;
        .
        .
        .
        return result;
    }