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

Thursday November 3, 1994

Assigned Reading: 9.2 - 9.3

Handouts (available on-line): none

Topics Covered:

• We are running a bit behind the posted syllabus. We will continue developing our random number library. We want to allow our client to set the range of numbers that GetRandomNumber() would produce. Instead of passing, extra parameters to GetRandomNumber() we will use a new feature of C called global variables. We traced through a program that illustrates the lexical scoping of global and local variables in C.
• Then we wrote a new version of our program which has a SetRandomRange function. The output of the program is still the same as the previous version.
• We broke up our program into three pieces: the header file, the implementation of the functions, and the main program. These programs can be compiled separately, and we can put the implemented functions in a library. See sample run.
• It turns out that the rand() function that we used is not very good, because the lower digits of the generated numbers are predictable. Since we used the modulo operator to reduce the range of the random number, this means the random numbers we generated may be quite predictable. Again, we consulted the manual pages of a different random number generator. According to the manual the random() function produces numbers where every digit appears to be random.
• We incorporated the random() and srandom() functions into a new implementation of our random number library. This new library can be linked to our main program without changing or recompiling the main program because the interface between these two parts remains the same. (See sample run.)
• We moved on to explain what the preprocessor directive

         #ifndef
       

  really means. We looked at a program where the constant MARKER was defined; and a program where the constant was not defined. The compiled programs run differently, but it is important to note that in each case the C preprocessor stripped a printf statement from the file before passing the file to the C compiler.
• The purpose of the C preprocessor is to take care of the preprocessor directives, such as #include, #define and #ifndef, before the "real" C compiler looks at the file. The preprocessor usually strips the comments from the source file and replaces every occurrence of the constants with their definitions. We looked at a sample program and the output from the preprocessor.
• We also looked at what happens when a program includes a header file. The output from the preprocessor has a copy of the header file in the middle of the main program.
• In our second example, the main program includes both the header.h file and a second header file. The second header file also includes header.h, so the output from the preprocessor contains two copies of header.h. This is bad because the global variable global_variable is initialized twice, which would result in a syntax error.
• The solution to this problem is to realize that we can use the #ifndef directive to make sure that the same header file we not be given to the C compiler twice. The C preprocessor will remove one of the copies. For example, in this program the constant MARKER is defined, so the first printf statement is not part of the output from the preprocessor. In the second program, the constant MARKER is not defined, so the second printf statement is not part of the output from the preprocessor.
• Thus, we can write the new header files: new_header.h and new_header2.h. When new_header.h is read the first time, the constant _new_header_h is not defined, so all of the file is included. Also, the constant _new_header_h is defined when the file new_header.h is read the first time. If new_header.h is included a second time, the constant _new_header_h would be defined, so the body of the file would be stripped by the #ifndef directive. As a result, when we give our new main program to the C preprocessor, the output from the preprocessor only has one copy of the body of new_header.h.