Computer Science 101 A Survey of Computer Science

1
Computer Science 101A Survey of Computer Science

• Pseudocode,Searching

2
Pseudocode

• Informal language used to present algorithms.
• Compromise between verbose natural language and
  precise programming language.
• Has statements like most programming languages,
  but not quite as picky.

3
Three major control constructs of programming

• Sequential Simply do steps one after the other
  in order they are listed.
• Conditional Decide which statement to do next
  based on some true/false test.
• Iterative A set of statements is repeated over
  and over until some condition is met.

4
Language terminology

• Syntax The formal rules for legal statements in
  the language.
• Semantics The meaning of the statements - what
  happens when the statement is executed.

5
Sequential Operations
Step i

• Computation
• Input
• Output

Step i1
Step i2
6
Variables and expressions

• Variable A named storage location whose stored
  value can be changed as the algorithm is
  executed.
• Arithmetic expression A legal expression
  constructed of constants, variables, parentheses,
  and arithmetic operations. Its value can be
  computed using current values of the variables.

7
Computation operation

• Syntax Set variable to expression
• Semantics
• Compute value of expression
• Store this as new value of the variable
• Example Set Pay to PayRate x Hours

8
Input/Output

• Input
• Syntax Get variable, variable,
• Semantics User enters value(s) for variable(s)
• Output
• Syntax Print variable, variable,
• Semantics Value(s) of variable(s) displayed

9
Conditional Operation if-then

• Syntax if condition then list of
  statements
• Exampleif Age 65 then Set Price to .85 x
  Price Set SrCnt to SrCnt1
• StyleIndent the list of statements

10
Conditional Operation if-then-else

• Syntax if condition then list1 of
  operations else list2 of operations
• Exampleif Hourly1 then Set Pay to Rate x
  Hourselse Set Pay to Salary/12
• StyleIndent the lists of statements

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Iteration Do-while

• Do Step I Step J
  while condStyleIndent the list of statements

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Iteration While

• SyntaxWhile cond do Step
  end-of-loop
• StyleIndent the list of statements

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Pseudocode Summary

• Sequential
• Set .
• Get ...
• Print ...
• Conditional
• If-then
• If-then-else
• Iteration
• Do-while
• While-do

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Searching

• Searching is one of the most common operations of
  computing
• Search for student record based on id
• Search for best item based on criteria
• Search for airline reservations
• Clearly we know all there is to know about
  searching

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Or do we?
16
Or do we?
17
Or do we?
18
Or do we?
19
Sequential Search Algorithm

• Given List of values v1,v2,,vn
  Target value, T
• Output Information about T if in list
  and message if T is not in list
• Strategy
• Use variables
• i for current location in list
• Found to indicate whether T located or not
• Initialize i1, Foundfalse
• Pass through the list sequentially

20
Sequential Search-Names/Phone Nos

• Given Names N1,N2,Nn Target NAME Phone
  Nos T1,T2,,Tn
• Output Phone number of NAME

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Names/Phone Nos Algorithm

• Set Found to false
• Set i to 1
• While not found and i last position Do
• if Ni NAME then
• Print Ti
• Set Found to true
• else
• Set i to i1 end-of-loop
• if Foundfalse then
• Print Sorry, not in directory
• Stop

22
Algorithm to Find Largest

• Given List of numbers A1,A2,,An
• Output Largest value in list and its position
• Strategy
• Use variables
• i for current location in list
• Largest for largest value found so far
• LargeLoc for position of largest found so far
• Initialize LargestA1,LargeLoc1,i2
• Pass through the list sequentially

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Algorithm to Find Largest (cont)

• Set Largest to A1
• Set LargeLoc to 1
• Set i to 2
• While i ? n do
• if Ai gt Largest then
• Set Largest to Ai
• Set LargeLoc to i
• Set i to i1
• end-of-loop
• Print Largest, LargeLoc
• Stop

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Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
8
25
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
9 8
26
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
9 1 8
27
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
9 1 2 8
28
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
9 1 2 8
29
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
9 1 2 8
30
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
9 1 3 8
31
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
9 1 3 8
32
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
9 1 3 8
33
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
9 1 3 8
34
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 1 3 8
35
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 3 8
36
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 4 8
37
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 4 8
38
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 4 8
39
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 4 8
40
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 5 8
41
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 5 8
42
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 5 8
43
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 5 8
44
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 6 8
45
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 6 8
46
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 6 8
47
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
12 3 6 8
48
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 3 6 8
49
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 6 8
50
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 7 8
51
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 7 8
52
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 7 8
53
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 7 8
54
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 8 8
55
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 8 8
56
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 8 8
57
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 8 8
58
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 9 8
59
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 9 8
60
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 9 8
61
Example
1 2 3 4 5 6 7 8 9 5 12 6 3 15
10 14

• 1. Set Largest to A1
• 2. Set LargeLoc to 1
• 3. Set i to 2
• 4. While i ? n do
• 5. if Ai gt Largest then
• 6. Set Largest to Ai
• 7. Set LargeLoc to i
• 8. Set i to i1
• 9. end-of-loop
• 10. Print Largest, LargeLoc
• 11. Stop

Largest LargeLoc i n
15 6 9 8
OUTPUT 15, 6
62
News alert
63
News Alert!
64
It's a dirty job, but someone's gotta do it!