Sections 0101, 0102, 0103 and Honors, Fall 1995

Thursday December 7, 1995

Assigned Reading: 17.1

Handouts (available on-line): Quiz 3

Topics Covered:

• Structures (also called records) allow us to combine several pieces of data with differing types under one variable name. In our first example, we declare a new type called student_record using the typedef facility. Variables of the type student_record can hold two strings and a double in the fields called name, major and gpa. Sample run. If the variable rec is of type student record, then the fields accessed using rec.name, rec.major and rec.gpa. These fields can then be used just like any other string or double variable. ANSI C also allows us to pass structures as parameters, use structures as return values from functions, and assign one structure to another (older versions of C do not).

• Our second example uses an array of structures to hold information about a series of students. Sample run.

• Our third example sorts the array of structures according to the gpa field. Sample run. To do this, we modified the sample run.) We did not need to change this procedure very much. We just changed the type of the array A and the type of the variable temp. Also, in the FindSmallest function we compare smallest_value against A[i]->gpa instead of A[i]. Otherwise the new SelectionSort function is identical to the old one.

• Finally, in our fourth example we noted that our previous example did a lot of copying to sort the array. For large records, this is very inefficient. It would be much faster to sort an array of pointers to records than an array of records, because when we swap two entries, we just exchange the pointers instead of the entire record. (See sample run.) Here, it is convenient to define a new type of pointers called rec_ptr which can point to a student_record. Note that in this version, the function read_record returns a pointer to student_record and not a student_record. This makes sense because students is an array of pointers, not an array of student_records. Thus, read_record must call malloc to allocate a block of memory to store the record it just read.

• Quiz 3