1. 2008/09/24: Lecture 6b CMSC 104, Section 0101 John Y. Park
More Algorithms
2008/09/24: Lecture 6b CMSC 104, Section 0101
John Y. Park
[New lecture, some reused slides]

2. More Algorithms Topics Algorithms: Elements in Depth
In-Class Project: Euclid’s Algorithm
In-Class Project: Student Voluteerizer
Reading
None

3. Pseudocode: Control Structures
Any problem can be solved using only three logical control structures:
Sequence
Selection
Repetition

4. Sequence
A series of steps or statements that are executed in the order they are written.
Example:
Display “Enter two numbers: “
Read <number1>
Read <number2>
<sum> = <number1> + <number2>
Display “sum = “, <sum>

5. Sequence Some languages… Have line numbers
Allow a set of sequenced steps to be grouped as a “statement block”
Explicitly bracketed (e.g., with “begin…end” or {…}
Allow “goto”s
Evil-evil-evil!!!!!

6. Sequence Goto e.g.: <a> = 0 goto Label3: Label1: DISPLAY “Hello”
DISPLAY “bye”
Label2: <a> = <a> + 1
DISPLAY <a>
goto Label1:
DISPLAY “Never get here…”
Label3: DISPLAY “Starting up”
goto Label2:

7. Sequence You should try to avoid the “goto” like the plague!
In this class, you will never need to use a “goto”
DON’T USE GOTOs!!!
Why did I even mention it?
It is a vehicle for explaining what other control structures are doing, in a more logical manner

8. Selection
Defines one or more courses of action depending on the evaluation of a condition.
Synonyms: conditional, branching, decision
Examples: If (condition is true) If (condition is true) do this do this
End_if Else do that End_if

9. Selection More complex examples:
DISPLAY “Enter number to invert” READ <my_num> If (<my_num> < 0) DISPLAY “Don’t like negative numbers” if (<my_num> < -999) DISPLAY “… but I guess you really do!” End_if Else_if (<my_num> == 0) <result> = 0 Else <result> = 1 / <my_num> End_if

10. Repetition
Allows one or more statements to be repeated as long as a given condition is true.
Synonyms: looping, iteration
Example:
While (condition is true)
do this End_while

11. Repetition More complex example (with mistakes)
DISPLAY “Enter number to compute factorial for” READ <my_num> While (<my_num> > 0) <factorial> = <factorial> * <my_num> <my_num> = <my_num> - 1 End_while DISPLAY “The factorial of”, <my_num>, “ is “, <factorial>

12. Repetition More complex example (with mistakes)
DISPLAY “Enter number to compute factorial for” READ <my_num> <factorial> = 0 While (<my_num> > 0) <factorial> = <factorial> * <my_num> <my_num> = <my_num> - 1 End_while DISPLAY “The factorial of”, <my_num>, “ is “, <factorial>

13. Repetition More complex example (with mistakes)
DISPLAY “Enter number to compute factorial for” READ <my_num> <factorial> = 0 <saved_my_num> = <my_num> While (<my_num> > 0) <factorial> = <factorial> * <my_num> <my_num> = <my_num> - 1 End_while <my_num> = <saved_my_num> DISPLAY “The factorial of”, <my_num>, “ is “, <factorial>

14. Pseudocode Style
Any user prompts should appear exactly as you wish the programmer to code them.
The destination of any output data should be stated, such as in “Display”, which implies the screen.
Make the data items clear (e.g., surround them by < and > ) and give them descriptive names.
Use formulas wherever possible for clarity and brevity.
Use keywords (such as Read and While) and use them consistently. Accent them in some manner.

15. Pseudocode (con’t) [Review]
Use indentation for clarity of logic.
Avoid using code. Pseudocode should not be programming language-specific.
Always keep in mind that you may not be the person translating your pseudocode into programming language code. It must, therefore, be unambiguous.
You may make up your own pseudocode guidelines, but you MUST be consistent.

16. Euclid’s Algorithm
Problem: Find the largest positive integer that divides evenly into two given positive integers (i.e., the greatest common divisor).
Algorithm:
Assign M and N the values of the larger and smaller of the two positive integers, respectively.
Divide M by N and call the remainder R.
If R is not 0, then assign M the value of N, assign N the value of R, and return to Step 2. Otherwise, the greatest common divisor is the value currently assigned to N.

17. Finding the GCD of 24 and 9 So, 3 is the GCD of 24 and 9. M N R 24 9 6
So, 3 is the GCD of 24 and 9.

18. Euclid’s Algorithm Tips about problem # 1 :
The user should specify the two numbers to factor. She may enter them in any order (i.e., don't assume first is greater).
You will need a new arithmetic operation for computing remainders: the '%' operator. E.g.:
"24 % 7" equals "3"
“-5 % 2” equals “-1”, and "18 % -8" equals "+2"
"7 % 0" equals "the end of the world as we know it."
The user may input one or both values as negative numbers. You must either:
explicitly test for and reject negative numbers
make sure your algorithm computes the correct answer with them

19. Student Voluteerizer
Problem: Write a generic algorithm for helping call on student “volunteers” in a “fair” manner <obviously underspecified…>