Sections 0101, 0102, 0103 and Honors, Fall 1995

Thursday September 221, 1995

Assigned Reading: 3.4 - 3.6

Handouts (available on-line): none

Topics Covered:

• Programs which show the "dangling else". Where there are two if's and only one else, the else is matched with the closest if.
  First program and sample run.
  Second program and sample run.

• Designed a program that computes the minimum, average and sum of a list of numbers using the "loop unrolling" method. First we wrote down a list of statements that we want to accomplish. Then, we use the repeating part as the body of a while loop. The resulting program may not be the tightest code, but this is a methodical way to get started. The sample run shows that the program handles empty lists and lists with 1 or 2 elements correctly.

• The trouble with integer variables is that they can only store integer values within a certain range. The C compilers on our UNIX system store integers in 4 bytes. These variables can store numbers between -2,147,483,648 and 2,147,483,647. The program which computes the powers of 2, shows that we can exceed this range easily when working with large numbers (see sample run). When this happens, large positive numbers can become large negative numbers. This can happen without error or warning. It is the responsibility of the programmer to make sure that the integer values do not exceed the available range. Some compilers use 2 byte integers, which is equivalent to short int on our UNIX C compiler. In this case, we can get into trouble much sooner. (See program and sample run).

• "99 Bottles of Beer on the Wall". A program that demonstrates that you should not test floating point numbers for equality. The culprit is that 1/5 is not a terminating fractional number in base 2 (similar to 1/3 in base 10). Program and sample run.

• A note on how to represent fractions in base 2.