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CSci 107 Introduction to Computer Science Lecture 1

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Administrativia Class webpage http://www.bowdoin.edu/~ltoma/teaching/cs107/fall04/ –Office hours –Grading policy –Syllabus –Lab assignments –Readings This class is an introduction to CS. The goal is to find out what CS is about and find out about its applications and impact in other disciplines.

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What is Computer Science? Computer Science is the study of computers (??) This leaves aside the theoretical work in CS, which does not make use of real computers, but of formal models of computers A lot of work in CS is done with pen and paper! Actually, the early work in CS took place before the development of the first computer Computer Science is no more about computers than astronomy is about telescopes, biology is about microscopes, or chemistry is about test tubes. Science is not about tools. It is about how we use them, and what we find out we can do.

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What is Computer Science? Computer Science is the study of how to write computer programs (programming) (??) Programming is a big part of CS....but it is not the most important part. Computer Science is the study of the uses and applications of computers and software (??) Learning to use software packages is no more a part of CS than driver’s education is part of automotive engineering. CS is responsible for building and designing software.

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What is computer science? The study of algorithms: –their formal properties correctness, limits efficiency/cost (Chapters 2, 3, 10) –their hardware realizations computer design (Chapters 4-6) –their linguistic realizations programming languages (Chapters 7-9) –their applications network design, ocean modeling, bioinformatics,...

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What is an algorithm? … a well-defined procedure that allows an agent to solve a problem. Note: often the agent is a computer or a robot… Example algorithms Cooking a dish Making a peanut-butter jelly sandwich Shampooing hair Programming a VCR Making a pie

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Example Is this an algorithm? Step 1: Wet hair Step 2: Lather Step 3: Rinse Step 4: Repeat Would you manage to wash your hair with this algorithm? How about a robot? Why (not)?

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Algorithms An algorithm must: 1. Be well-ordered and unambigous 2. Each operation must be effectively computable 3. Terminate.

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Example Problem: Find and print the 100th prime number –A prime number is a whole number not evenly divisible by any other number other than 1 and itself Algorithm (?): 1. Generate a list of all primi numbers L1, L2, L3,… 2. Sort the list into ascending order 3. Print out the 100th element in this list Is this an algorithm?

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Algorithm for Programming a VCR Step 1: If the clock and calendar are not correctly set, then go to page 9 of the instructionmanual and follow the instructions before proceeding Step 2: Place a blank tape into the VCR tape slot Step 3: Repeat steps 4 through 7 for each program that you wish to record, up to a maximum of 10 shows Step 4: Enter the channel number that you wish to record, and press the button labeled CHAN Step 5: Enter the start time and press TIME-START Step 6: Enter the end time and press END-TIME Step 7: This completes the programming of one show. If you do not wish to program anything else press END-PROG Step 8: Press te button labeles TIMER. Your VCR is ready to record.

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Types of Operations Basic operations –Wet hair –Rinse –Turn on VCR Conditional operations –If batter is too dry add water Repeat/looping operations –Repeat step 1 and 2 three times –Repeat steps 2,3,4,…10 until batter becomes soft.

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Algorithm How to come up with an algorithm? –That is, how to discover an algorithm underlying a problem? –Problem solving How to represent an algorithm? –In English?? –In a programming language??

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Example Problem: Given two positive integers, compute their greatest common divisor Euclid’s algorithm: –Step 1: Get two positive integer values from the user –Step 2: Assign M and N the value of the larger and smaller of the two input values, respectively –Step 3: Divide M by N, and call the remainder R –Step 4: If R is not 0, then assign M the value of N, assign te value of R, and return to step 2; otherwise, the greatest common divisor is the value currently assigned to N

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Coming up with algorithms.. How do people think???? Puzzle: –Before A, B, C and D ran a race they made the following predictions: A predicted that B would win B predicted that D would be last C predicted that A would be third D predicted that A’s prediction would be correct. –Only one of these predictions was true, and this was the prediction made by the winner. In what order did A, B, C, D finish the race?

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Example Problem: Adding two n-digit numbers 7597831 + 1287525 ------------------- 8885356 How would you write an algorithm to solve this problem? Assume the basic operation is adding one-digit numbers.

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Expressing algorithms Is natural language good? –For daily life, yes…but for CS is lacks structure and would be hard to follow –Too rich, ambiguous, depends on context How about a programming language? –Good, but not when we try to solve a problem..we want to think at an abstract level –It shifts the emphasis from how to solve the problem to tedious details of syntax and grammar.

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Pseudocode Pseudocode = English but looks like programming Good compromise –Simple, readable, no rules, don’t worry about punctuation. –Lets you think at an abstract level about the problem. –Contains only instructions that have a well-defined structure and resemble programming languages

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Pseudocode Basic (primitive) operations –Read the input from user –Print the output to the user –Cary out basic arithmetical computations Conditional operations –Execute an operation if a condition is true Repeat operations –Execute a block of operation multiple times until a certain condition is met

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Variables Variable –A named memory location that can store a value –Think of it as a box into which you can store a value, and from which you can retrieve a value Examples: Example of operations Set the value of i to 3 Set the value of M to i*3 + 12 Set the value of i to i+10 i M

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A model for visualizing an algorithm Algorithm Variables Operations An algorithm consists of operations that involve variables

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Primitive operations Get input from user –Get the value of x from user Assign values to variables using basic arithmetic operations –Set the value of x to 3 –Set the value of y to x/10 –Set the value of z to x +25 Print output to user –Print the value of y, z to the user

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Example 1 Problem: For any three numbers input by the user, compute their sum and average and output them Example of algorithm in pseudocode: Variables: a,b,c, sum, avg –Get the values of a, b, c from user –Set avg to (a+b+c)/3 –Set sum to (a+b+c) –Print sum and avg

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Example 2 Problem: Given any value of radius from the user, compute and print the circumference of a circle with that radius Algorithm in pseudocode: variables: r, c 1. Get the value of r from user 2. Set c to 2 * pi * r 3. Print “The circumference of your circle is “ c

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For next time Read Chaper 1 (textbook) Be ready to answer the following question: Who was Alan Turing and what is he famous for? Read Chapter 2.2.1 and 2.2.2

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