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QUIZ: What does this program do?
1
BR maina: .BLOCK 2main: DECI a, d
LDA a, dSUBA 0x002A, iBREQ yesDECO a, d
fin: STOPyes: CHARO 0x0065, i
CHARO 0x0071, iCHARO 0x0075, iCHARO 0x0061, iCHARO 0x006C, iSTOP.END
Solution
2
The hex ASCII codes are for the letters e, q, u, a and l.
If the number entered is not42, the number itself is output, otherwise the message equal is output.
Note: Instead of SUBA, the instruction CPA can be used – do you see why?
QUIZ on Ch.6:Write the pseudocode for adding two numbers
together; our target language is PEP/8 assembly.
3
4
•Input the first number - into memory location "A" •Input the second number - into memory location "B" •Load A from memory into Accumulator reg. •Add B from memory; result goes into Accumulator reg. •Store the Accumulator into memory location "C" •Output the value in memory location "C"
Solution
For more practice: Write the assembly code!
Problem Solving
How to Solve It: A New Aspect of Mathematical Method by George Polya, 1945
The book is written within the context of solving mathematical problems, but the methods described are applicable to problem solving in general.
6
We can use the methods described by Polya to solve any computer-related problem!
1. Understand the problem– What are the hypotheses? Data? Unknowns?
2. Devise a plan– Can we find a related problem? A sub-problem?– Can we strengthen or relax the hypotheses to obtain an
easier problem?
3. Carry out the plan— Prove that each step is correct!
4. Look back– Check result– Find shortcuts and alternate solutions– Generalize to related problems
7
Polya’s 4 steps
Polya’s StrategiesDo not reinvent the wheel!Similar problems come up again and again in different guises.A good programmer recognizes a task or subtask that has been solved before and reuses the solution.
Can you think of two similar problems we solved in Python?
8
Finding a character in a string and finding an element in a list
Polya’s StrategiesDivide and Conquer!Break up a large problem into smaller sub-problems
and solve each separately – Applies the concept of abstraction
– The divide-and-conquer approach can be applied over and over again until each subtask is manageable
9
Divide and Conquer!Break up a large problem into smaller sub-problems
and solve each separately
10Image source: http://teamapproach.ca/trouble/Methodology.htm
Polya’s 4 steps can be applied to computerproblem solving!
11
Analysis and Specification PhaseAsk questions to understand all the requirementsExplain in detail what needs to be achieved
Algorithm Development PhaseDevelop algorithmTest algorithm
Implementation PhaseCode algorithmTest the code in various ways
Maintenance PhaseUse the code, find bugsFix bugsCode new features, as requested by users
QUIZ: Match the steps in Polya’s method to the ones in the computer method for problem solving
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Analysis and Specification
Implementation
Algorithm Development
Maintenance
Devise a plan
Look back
Understand
Carry out the plan
Interactions among the
phases
13
Algorithms
Algorithm = A set of unambiguous steps for solving a problem in a finite amount of time,using a finite amount of resources
14
Algorithm = A set of unambiguous steps for solving a problem in a finite amount of time, using a finite amount of data
15Image source: http://my-online-log.com/tech/archives/1214
Refining the steps, from ambiguous (abstract) to unambiguous (concrete)
Abstract Step An algorithmic step containing unspecified detailsConcrete StepAn algorithm step in which all details are specified
16
Whether the step is abstract or concrete depends on the programming language we’re using!
7.2 Algorithms with simple variables
Variable = a means of storing intermediate results from one task to the next.
At the hardware level, a simple variable is just one or several adjacent Bytes in the computer memory.
Q: How many Bytes does a simple variable have in PEP/8?
17
QUIZWe have two variables a and b.Create an algorithm to swap their values!
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What is wrong with this algorithm?a = bb = a
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save = aa = bb = save
SolutionThe algorithm proposed results in both a and b storing the value of b!The correct algorithm involves three steps and an extra variable:
Source: AP CS Principles – Course and Exam Descriptions
Similar problem (Do not reinvent the wheel!)
Algorithms with Selection Statements (a.k.a. decision or if)
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Flowchart of if statement
Figure is not in text
22
Algorithm with SelectionProblem: Write the appropriate dress for a given temperature.
Write "Enter temperature"Read temperatureDetermine Dress
Which statements are concrete?Which statements are abstract?
Algorithm Determine Dress v.1
Computer language is Python from now on!
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Write “Enter temperature”Read temperatureIF (temperature > 90)
Write “Texas weather: wear shorts”ELSE IF (temperature > 70)
Write “Ideal weather: short sleeves are fine”ELSE IF (temperature > 50)
Write “A little chilly: wear a light jacket”ELSE IF (temperature > 32)
Write “Philadelphia weather: wear a heavy coat”ELSE
Write “Stay inside”
Algorithm Determine Dress v.2
Is this concrete enough for implementation in Python?
Algorithms with Loops (a.k.a. repetition)
24
Flow of control of while loop (decision at beginning of loop)
Figure is not in text
QUIZ: There are loops that can execute 0 times and loops that must execute at least 1 time.Which type is this?
25
Answer: It depends on whether the decision (diamond) is at the beginning or at the end of the loop!
26
Figure is not in text
Loops in Python
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Not in text
Extra-credit question:
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Event-controlled Loops, a.k.a. WHILE loops
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They are the most general type of loops!
Set sum to 0Set allPositive to trueWHILE (allPositive)
Read numberIF (number > 0)
Set sum to sum + numberELSE
Set allPositive to falseWrite "Sum is " + sum
Counter-controlled Loops
30
They are a particular case of event-controlled loops: the event is that a counter variable has reached a predetermined limit
Set sum to 0Set limit to 42Set count to 1While (count <= limit)
Read numberSet sum to sum + numberIncrement count
Write "Sum is " + sum
For loops are counter-
controlled!
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Important application of looping: Successive approximation algorithms
Read in squareCalculate the square rootWrite out square and the square root
Algorithm Calculate Square Root v.1
Which steps are abstract and which concrete?
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Set epsilon to 1WHILE (epsilon > 0.001)
Calculate new guessSet epsilon to abs(square - guess * guess)
Which steps are abstract and which concrete?
Algorithm Calculate Square Root v.2
In Python usemath.fabs(…)
A more appropriate name for guess would be approximation
33
Set newGuess to(guess + (square/guess)) / 2.0
Set guess to square/4Set epsilon to 1
What’s the mathematical formula for the new approximation?
How do we get the loop started?
Algorithm Calculate Square Root - Refinements in v.2
34
Read in squareSet guess to square/4Set epsilon to 1WHILE (epsilon > 0.001)
Set guess to (guess + (square/guess)) / 2.0Set epsilon to abs(square - guess * guess)
Write out square and guess
Which steps are abstract and which concrete?
Algorithm Calculate Square Root v.3
35
Set newGuess to(guess + (square/guess)) / 2.0
QUIZ: Square root approximation alg.
We want to calculate = 2.236…
Set your initial guess x0 = 1 and show the next 3 approximations x1, x2, x3
5
36
SolutionWe want to calculate = 2.236…5
37
Set newGuess to(guess + (square/guess)) / 2.0
Quick work for next time (in notebook):
We want to calculate = 2.236…
Set your initial guess x0 = 42 and show the next 3 approximations x1, x2, x3
5
38
QUIZ: Square root algorithm
We want to calculate = 2.645751…
Set your initial guess to x0 = 7/4 = 1.75
and show the next 3 approximations x1, x2, x3
7
39
QUIZ: Square root algorithm
We want to calculate = 2.645751…7
40
QUIZ: Square root algorithm
We want to calculate = 2.645751…7
41
QUIZ: Square root algorithm
We want to calculate = 2.645751…7
42
Remember: The algorithm converges irrespective of the initial guess x0!
= 2.645751…7
43
QUIZ:
Re-write this program using a for loop!
Solution
Problem for lab:
Re-write this program using a while loop!
Hint: What condition should we use to control the loop?
EoHF 1
QUIZ True/FalseA. Count-controlled loops repeat a specific task a
number of times.B. Event-controlled loops repeat a specific
number of timesC. Count-controlled loops are controlled by a
counterD. Count-controlled loops are more general than
event-controlled loops
46
47
Set newGuess to(guess + (square/guess)) / 2.0
QUIZ: Square root algorithm (approximations)
We want to calculate = 3.16227…
Set your initial guess x0 = 10/4 and show the next 3 approximations x1, x2, x3
10
48
Solution
We want to calculate = 3.16227…
Set your initial guess x0 = 10/4 and show the next 3 approximations x1, x2, x3
10
x0 = 2.5 x1 = 3.25 x2 = 3.16346… x3 = 3.16227…
7.3 Composite Data Types and their Algorithms
They can be classified according to many criteria:
• homogeneous vs. heterogeneous
• accessed by index vs. accessed by name
• mutable vs. immutable
• linear vs. non-linear
• etc.
49
Composite Data Types
RecordsA named heterogeneous collection of items in which individual items are accessed by name. For example, we could bundle name, age and hourly wage items into a record named Employee
ArraysA named homogeneous collection of items in which an individual item is accessed by its position (index) within the collection
50
Are these the lists from Python? Why not?
Python strings are arrays of characters
Composite Data Types (contd.)
Lists (will be covered in next chapter)A named heterogeneous collection of items in which individual items are accessed by position (index).
We have them in Python, e.g.>>> myList = [“dog”, 42, 51.375, [1,2]]>>> myList[1]42
51
Composite Data Types - RecordsEmployee
nameagehourly/Wage
Algorithm to store values into the fields of record:
Employee employee // Declare an Employee variableSet employee.name to “Frank Jones”Set employee.age to 32Set employee.hourlyWage to 27.50
52
Composite Data Types - Arrays
53
numbers[0]
numbers[4]
numbers
Some items in an array may be unused at a given time
54
first
last
numbers
??
??
??
??
Useful Algorithms on Arrays
• Initializing all items• Printing all items• Searching for an item• Sorting the array
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Algorithm for initializing an array
integer data[20]Write “How many values?”Read lengthSet index to 0WHILE (index < length)
Read data[index]Set index to index + 1
Task: Fill an array numbers with length values that are being input from the keyboard.
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QUIZ
integer data[20]Write “How many values?”Read lengthSet index to 0WHILE (index < length)
Read data[index]Set index to index + 1
Modify this pseudocode to print the values after initializing them.
An Unsorted
Array
58
data
A Sorted Array
59
data
Sorted Arrays
• The values stored in an array have unique keysof a type for which the relational operators are defined.
• Sorting rearranges the elements into either ascending or descending order within the array.
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Reality check: In a real-life problem it’s very common
to encounter repeated keys!
7.4 Search algorithms
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Sequential Search in anUnsorted Array
A sequential search examines each item in turn and compares it to the one we are searching for.
If it matches, we have found the item. If not, we examine the next item in the array.
We stop either when we have found the item or when we have looked at all the items and not found a match.
Thus, we have a loop with two ending conditions.
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Set position to 0Set found to FALSEWHILE (position < length AND NOT found )
IF (numbers[position] equals searchItem)Set found to TRUE
ELSE Set position to position + 1
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The array’s name is numbersThe value we’re searching for is stored in searchItem
Set position to 0Set found to FALSEWHILE (position < length AND NOT found )
IF (numbers[position] equals searchItem)Set found to TRUE
ELSE Set position to position + 1
64
QUIZ: When the loop exits, what do we need to do?
Set index to 0Set found to FALSEWHILE (index < length AND NOT found )
IF (data[index] equals searchItem)Set found to TRUE
ELSE Set index to index + 1
65
QUIZ: Desk-check this algorithm for the arrayThe item searched is:A. 35B. 43
42100351
Sequential Search in a Sorted ArrayIdea:If items in an array are sorted, we can stop
looking when we pass the place where the item would be if it were present in the array
66
Is this (always) better?
67
Sequential Search in a Sorted ArraySet index to 0Set found to FALSEWHILE (index < length AND NOT found)
IF (data[index] equals searchItem)Set found to TRUE
ELSE IF (data[index] > searchItem)Set index to length
ELSESet index to index + 1
This is the new part!
(Compare with previous alg. for unsorted array)
This alg. is called sequential search with early termination
QUIZ: End-of-chapter question 66 a
68
SearchItem = 4SearchItem = 49SearchItem = 50
Binary Search in Sorted Array
Search begins at the middle and finds the item or eliminates half of the unexamined items; the process is then repeated on the half where the item might be
69
24 30 31 42 44 90 92 94 99
Example: searchItem = 42
Binary Search
Algorithm
70
Set first to 0Set last to length-1Set found to FALSEWHILE (first <= last AND NOT found)
Set middle to (first + last)/ 2IF (item equals data[middle]))
Set found to TRUEELSE
IF (item < data[middle])Set last to middle – 1
ELSESet first to middle + 1
RETURN found
Grok why integerdivision works
here!
Binary Search example
71Figure 7.10 Trace of the binary search
rat
QUIZ Binary Search
72
Search for deer
Search for the key 42 in this array, using binary search:
73
Trick QUIZ
EoHW 2
0] Define arrays
1] Define records
2] How are arrays and records:
(a) similar?
(b) different?
74
QUIZ
3] What are the 4 fundamental types of algorithms used to manipulate arrays?
4] What control structure is normally used to access all the elements of an array?
75
QUIZ
5] Explain the following search algorithms in your
own words:
(a) Sequential search
(b) Sequential search with early termination
(c) Binary/logarithmic search
76
QUIZ
QUIZ Binary Search
-50 -28 -23 -10 -4 0 3 41 28 37 60 155 200
How many comparisons are needed to find the key 60 in this array?Show first, last, and middle for each iteration.
7.5 Sorting algorithms
78
Sorting
SortingArranging items in a collection so that there is an ordering on one (or more) of the fields in the items
Sort KeyThe field (or fields) on which the ordering is based
Sorting algorithmsAlgorithms that order the items in the collection based on the sort key
79
Selection Sort
80
Figure 7.11 Example of a selection sort (sorted elements are shaded)
81
10010111235200132
Show the swapped elements with arrows.Show the sorted elements with shading.
QUIZ Selection SortHandout at the end
Solution
82
83
24101112352012
Show the swapped elements with arrows.Show the sorted elements with shading.
Quick work for next timeHandout at the end
84
Solution
85
Selection Sort PseudocodeSelection SortSet firstUnsorted to 0WHILE (not sorted yet)
Find smallest unsorted itemSwap firstUnsorted item with the smallestSet firstUnsorted to firstUnsorted + 1
Not sorted yetcurrent < length – 1
Red steps are abstract (not concrete)
86
Find smallest unsorted itemSet indexOfSmallest to firstUnsortedSet index to firstUnsorted + 1WHILE (index <= length – 1)
IF (data[index] < data[indexOfSmallest])Set indexOfSmallest to index
Set index to index + 1Set index to indexOfSmallest
Swap firstUnsorted with smallestSet tempItem to data[firstUnsorted]Set data[firstUnsorted] to data[indexOfSmallest]Set data[indexOfSmallest] to tempItem
87
SKIP Bubble Sort and Insertion Sort, just remember that they are as fast
(efficient) as Selection Sort
88
Is it possible to devise a more efficient sorting algorithm?
89
Is it possible to devise a more efficient sorting algorithm?
Yes, but, in order to do this, we have to understand subprograms and
recursion.
7.6 Recursive Algorithms
To understand Quick Sort, we need:
• a new control structure: subprogram
• a new concept: recursion
90
Subprogram StatementsWe can give a section of code a name and use that name as a statement in another part of the programWhen the name is encountered, the processing in the other part of the program halts while the named code is executedWhen execution is finished, the first part of the program resumes execution
That section of code is called a subprogram
91
92
Figure 7.14 Subprogram flow of control
We have already used subprograms in Python!
Ee called them
• Functions– int() float() ord() chr() str() etc.
• Methods– list.append() string.upper() string.find() etc.
93
Do you remember what each of them
does?
Input to SubprogramsWhat if the subprogram needs data from the calling unit? This data is called input.
ParametersIdentifiers listed in parentheses beside the subprogram declaration; sometimes called formal parametersArgumentsIdentifiers listed in parentheses on the subprogram call; sometimes called actual parameters
94
Formal and actual params in Python
def disp(a, b):print a, 'and', b
x, y = 1, 2disp(x, y)
95
What if the subprogram needs to give data back to the calling unit? This data is called output.
Void subprograms They do not return a value, just perform certain actions>>> print(“Enter a positive integer”)>>> printer(3, 4)
Value-returning subprograms >>> a = input(“Enter a positive integer”)The keyword RETURN is used in many programming languages
96
What if the subprogram needs to give data back to the calling unit? This data is called output.
Void subprograms They do not return a value, just perform certain actions>>> print(“Enter a positive integer”)>>> printer(3, 4)
Value-returning subprograms >>> a = input(“Enter a positive integer”)The keyword RETURN is used in many programming languages
97
Do not confuse returning values with printing or displaying values. Returning means that a value is
given back to the main program!
Value-returning function in Python
def sum(a, b):return a + b
x, y = 1, 2print sum(x, y)
98
Compare to this previous function, which returns void (does not return anything)
Value-returning vs. void Subprograms
99
Source: AP CS Principles –Course and Exam Descriptions
QUIZIs MoveAndTurn a value-returning or void subprogram?
Subprograms are very important tools for abstraction
Other popular names for subprograms:• sub• subroutine• function• procedure• module• method
101
Recursion RecursionThe ability of a subprogram to call itselfBase caseThe case to which we have an answer General caseThe case that expresses the solution in terms of a call to itself with a smaller version of the problem
102
Recursion – Factorial function The factorial of a positive integer is defined (non-recursively) as the integer times the product of all the integers between itself and 1:
N! = 1⋅2⋅3⋅ … ⋅NBut an alternate recursive definition is possible:
N! = N⋅(N − 1)!
103
Two definitions for the factorial Iterative: N! = 1⋅2⋅3⋅ … ⋅NRecursive: N! = N⋅(N − 1)!
Base caseFact(0) = 1 (0! is 1)
General CaseFact(N) = N ⋅ Fact(N-1) (for N ≥ 1)
104
Is this a valid recursion?
Source: http://www.urbandictionary.com/define.php?term=recursion105
Base caseFact(1) = 1
General CaseFact(N) = N ⋅ Fact(N-1)
106
How does the genie calculate Fact(3)?
Your turn!N! = N * (N − 1)!Base case
Facto(0) = 1 (0! is 1)General Case
Fact(N) = N * Fact(N-1) (for N ≥ 1)
Calculate:0! = 1! = 2! = 5! =
107
108
Recursive Factorial algorithm
Write “Enter n”Read nSet result to Factorial(n)Write result + “ is the factorial of “ + n
Factorial(n)IF (n equals 0)
RETURN 1ELSE
RETURN n * Factorial(n-1)
109
QUIZ: Write this recursive function in Python
Write “Enter n”Read nSet result to Factorial(n)Write result + “ is the factorial of “ + n
Factorial(n)IF (n equals 0)
RETURN 1ELSE
RETURN n * Factorial(n-1)
solution
110
111
Recursive Binary Search BinarySearch (first, last)IF (first > last)
RETURN FALSEELSE
Set middle to (first + last)/ 2IF (item equals data[middle])
RETURN TRUEELSE
IF (item < data[middle])BinarySearch (first, middle – 1)
ELSEBinarySearch (middle + 1, last)
SKIP QUICKSORT …Just remember that it is faster
(more efficient) than Selection Sort
112
SKIP 7.7 Important Threads
Read and take notes: Who am I?
113
I am a mathematician.Why is my picture in abook about computerscience?
Read and take notes:Ethical Issues - Open-Source Software
What are the advantages and disadvantages of open-source software?
What are the FSF and the GPL? Are they related?What does the success of Linux suggest about the
future of open-source software?Is open-source software exempt from copyright laws?
(Hint: What happened in 2008?)
114
Homework, due Tue, Nov.13:
End-of-chapter questions 30, 31, 34, 35, 36, 66 b, c
Not from text: Calculate 7! by hand, showing all the recursive steps, as done in the lecture examples.
115
Chapter review questions• Describe the computer problem-solving process and relate it
to Polya’s How to Solve It list• Distinguish between a simple type and a composite type• Distinguish between a void subprogram and a value-returning
subprogram• Recognize a recursive problem and write a recursive algorithm
to solve it • Distinguish between an unsorted array and a sorted array• Describe the Quicksort algorithm • Apply the linear search, binary search, selection sort and
Quicksort to an array of items by hand• What are the advantages and disadvantages of open-source
software?
116