UMBC CMSC201, Computer Science I, Fall 1994 Sections 0101, 0102 and Honors

Tuesday December 13, 1994

Assigned Reading: 17.1

Handouts (available on-line): none

Topics Covered:

• We discussed the meaning of the keywords static and extern in front of a variable declaration. If a local variable in a function is declared with the storage class static, then its value is preserved from between calls to that function, as shown in this example. Static local variables are similar to global variables because the variable is preserved between function calls. However, static local variables can only be used in the function where it is declared. So, a good way to think of static local variables is as a private global variable that can only be used by that function.

  Functions can also be given the static designation. In that case, only the functions implemented in that file can use the static function. So, static functions are private functions that cannot be used by functions implemented in a separate file.

  When a global variable is given the extern storage class, this indicates that the variable is a global variable declared in a separate file. Changes made to this function would be seen by functions in both files.

  If a global variable is declared with the static storage class, then it cannot be used by functions in a separate file using the extern storage class. Again, we can think of static global variables as private global variable that can only be used by functions in that file.

  Here's an example with two files: file1.c and file2.c. The variable file_1_var is a global variable declared in file1.c. The main function in file2.c can use this variable by declaring it as an external variable. There is also a variable called private declared in file1.c with the storage class static. The main function in file2.c cannot access this variable directly. It must use the functions set_private and print_private to use this static global variable. Using static global variables gives us better control over how and when global variables are used. Sample run.

• Next, we talked about structures (or records). Structures 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. Sample run.

• Our third example expands the student record type to include two new fields that point to other student records. Note that in the type definition of this new structure, we need to use the dummy tag. Sample run. Suppose that student2 is a student record and ptr is a pointer to student records. Then after the assignment p = &student2, the name field of student2 can be accessed as either (*p).name or p->name. Note that in the first case the parentheses are necessary because the field operator . has higher precedence than the dereference operator *. Also, what was said in class about the -> operator was correct.

• Finally, we only had time to look at one example of a recursive function. A recursive function is a function that calls itself. Each time the function calls itself, it gets a new set of parameters and local variables. The parameters and local variables from a previous call to the recursive function will still be preserved until the function returns from the previous call. Our example of a recursive function simply counts down to 0 and returns. Sample run. In this example, if countdown called itself with countdown(k) instead of countdown(k-1), the function calls would never terminate.

• The students filled out the Student Course Evaluation Questionnaire (SCEQ).

• Quiz 3 was handed back.