Sections 0101, 0102, 0103 and Honors, Fall 1995

Thursday October 26, 1995

Assigned Reading: 8.2 - 8.5

Handouts (available on-line): none

Topics Covered:

• Discussed some implementation considerations for Project 3. See comments on Project 3.

• We started developing a library for generating random numbers. Our first program simply called the function rand() to print out many random numbers. One problem with this program is that the output from the program is the same set of random numbers.

• We consulted the manual to solve this problem. First we issued the UNIX command:

  	man -k random
       

  to get a list of functions related to the word "random". Then, we looked at the manual pages for the function rand() by typing:

  	man rand
       

• Using the information from the manual pages, we wrote a second program that sets the random seed by asking the user for a random number. Now, the program gives different output if the user types in a different random seed. However, if the user types in the same seed, the program still generates identical output.

• To fix this second problem, we want to use the system clock to set the random seed. So, we consulted the manual pages for the time function.

• The third version of our program uses the time function to set the random seed. This program prints out a different list of random numbers every time. In order to use the time function properly we needed to include the header file time.h at the beginning of the program. (We knew this from reading the manual.)

• We then wrote the fourth version of the program which prints a list of random numbers between 1 and 6, so we can simulate the roll of a six-sided die.

• Next, we broke up our program into several functions and used defined constants instead of magic numbers. The output shows the result of calling the GetRandomNumber() function twice to simulate rolling two six-sided dice.

• 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.

• We modified the craps program from Project 1 to use the random number library instead of taking input from the user. This required very little change. Instead of calling GetInteger, we use GetRandomNumber. The resulting program is compiled with the random number 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 re-compiled the craps program with the new version of the random number library without making any changes. The compiled program runs smoothly. See sample run.