LYNBROOK COMPUTER SCIENCEMember Meeting, November 10, 2008

UPCOMING COMPETITIONS

Today: USACO November Round 11/12 (Wed): TopCoder SRM Round 12/5-12/8 (Fri-Mon): USACO December Round 12/15 (Mon): ACSL Contest #1

Check http://LynbrookCS.com for full ACSL and USACO schedules!

TESTED MATERIAL ON THE ACSL

What does this program do? Number Systems (different base operations) Recursive Functions (f(x) = f(x-1) + 1) Boolean Algebra (not A + B = …) Bit String Flicking (right shift, left shift) LISP Evaluation (ADD, SUB, DIV, MULT) Digital Electronics (AND, OR, XOR, NAND,

NOR, XNOR) Prefix/Infix/Postfix Notation (+ 3 4) Data Structures (heaps, binary trees, stacks,

etc.)

WHAT DOES THIS PROGRAM DO?

Usually written in an easy-to-understand language such as BASIC or Pascal

Trick is to read the code as if it were in english, and then evaluate the operations as they come.

program S(input, output);

var a,b:integer;

begin

a:=0;

b:=0;

for a:=0 to 5 do begin

b:=b+1;

end;

end; Simple evaluation leads us to b = 6

INFIX NOTATION

Operators are written in-between operands Exactly like normal mathematical evaluation

A * ( B + C ) / D First add B to C Then multiply the result by A Then divide by D

PREFIX NOTATION

Operators are written before their operands Trick is to group them with parenthesis, then

convert into infix notation, and evaluate

/ * A + B C D (/ (* A (+ B C) ) D ) Infix notation: A * (B + C) / D

POSTFIX NOTATION

Operators are written after their operands Trick is to group them with parenthesis, then

convert into infix notation, and evaluate

A B C + * D / ( (A (B C +) *) D / ) Infix notation: A * (B + C) / D

WHAT DOES THIS PROGRAM DO? program SR(input,output);

var a,b,c,x:integer;

begin

b:=0;

for a:=1 to 5 do begin

x:=0;

while x <= 5 do begin

c:=5;

repeat

b:=b+c;

c:=c-1;

until c=0;

x:=x+1;

end;

end

end;

What is the final value of b after SR is executed?

SOLUTION

1. Read code as if it were in english 2. Translate code into math

( ( 5+4+3+2+1 ) * 6 ) * 5 3. Evaluate mathematical operation

= (15) * 6 * 5 =15 * 30 = 450

PREFIX/INFIX/POSTFIX NOTATION

Translate the following from prefix to postfix:

- + * 2 x * 3 y z

SOLUTION

1. Group problem with parenthesis (- (+ (* 2 x) (* 3 y)) z)

2. Translate problem into infix notation (((2 * x) + (3 * y)) - z)

3. Work backwards to translate from infix notation to postfix notation (((2 x *) (3 y *) +) z -) 2 x * 3 y * + z -

MISCELLANEOUS

Remember to sign in! Don’t forget to turn in your check ($15,

payable to Lynbrook ASB) if you haven’t done so already!

Remember to take the USACO (ends today)!

Check http://LynbrookCS.com for updates

Don’t forget to review the material we covered today – it comes up on the ACSL!