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Problem Solving (design of programs) CS140: Introduction to Computing 1 8/26/13
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Last time Design before code Computers are stupid, you are smart Variable = a name for a place to store a value Activity diagram elements Pseudo code 2
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Next time If you want to take notes, get/print the slides before you come to class BRING YOUR CLICKERS 3
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4 Programming is more than writing code Programming – representing a general solution to problem set in a way a machine can use, which consists of 1.Design of an Algorithm a step by step solution to a problem that completes in finite time 2.Coding – translate an algorithm into a program 3.Testing – ensure the correctness of the program 4
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5 Job 1: Design developing a solution to the problem If you can’t solve the problem, you can’t write the code Use graphical representations (UML) to solve 5
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6 Programs are like airplanes Airplanes On the ground (still) – Static – Easy to know what it looks like (structure) In the air – Dynamic – Flexes with wind, gravity, pilot controls, physics Programs Code (not running) – Static – Easy to know what it looks like (structure) Executing (a process) – Dynamic – Changes with input data, user control, system, rules of languages
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OO Programming Break the problem into entities (space, board, game) Model the entities with classes Declare and use objects of the classes (9 spaces) 7 entity classes boundary class
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8 OO Programming Code Programs are composed (written by humans) – Structural Code – Action Code – Comments – because a meaningful program is revised at a later date
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Structural Code Establishes the organizational structure of programs and classes Designed using UML class diagrams 9
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Action Code Implements algorithms that actually do things Mostly written inside methods Designed using flow charts / activity diagrams 10
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11 UML is used to design UML = “unified modeling language” includes several diagram types, including – Class diagrams represents the structure of the program – Activity diagrams represents the activity of the program
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12 UML Class Diagram Robot X Y direction forward() turn() Name of the class Values (instance variables) Actions (methods)
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13 Class and Object Class is the pattern (cookie cutter) – Structure: defines the Values (instance variables) Actions (methods) – Structure: the programmer writes a class (code not executing) Object is the instance (cookie) – Action – Each object has its own copy of instance variables – Each can do the actions defined in the class in a running program
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14 Big Picture NetBeans Project program Package – a collection of classes Class – a name for a collection of values (attributes) and a collection of methods Method – the actions in a class Statements – the individual actions of a method
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15 Big Picture Package Robot X Y direction forward() turn() Room length width Obstacle length width RProgram Robot Room Obstacle1 Obstacle2 Obstacle3 Begin() Each class is a file on the computer Class Method
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Activity Diagram Elements ElementSymbol Start Stop Statements Branching Repetition Subprograms Order of activities 16
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17 Loops: top-tested 17 Loop condition checked BEFORE the loop body Loop may execute 0 times
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18 Loops: bottom-tested 18 Loop condition checked AFTER loop body execution Loop will execute AT LEAST once
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19 Write a program that … Simulates the movement of a robot in a room. The robot can do these things: move forward one step (one foot), turn to the right (90°). In the room are some number of rectangular obstacles. If the robot runs into one of these objects, it can’t move forward. …
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20 Paper airplanes! Step 0: partner Step 1: write the instructions for making a paper airplane on a piece of paper Step 2: give the instructions to your partner Step 3: do exactly as the instructions you received say to do (just as a computer would) Step 4: test 20
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21 Find (and fix) design problems before they become code problems Play the computer – Simulate the program you have designed by following the design you wrote – Follow the design you wrote, not the one in your head Tracing / Desk-checking 21
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22 Problems from Chapter 4 Write a program to read a list of nonnegative integers and to display the largest integer, the smallest integer, and the average of all integers. The user indicates the end of the input by entering a negative sentinel value that is not used in finding the largest, smallest, and average values. The average should be a value of type double so that it is computed with a fractional part. 22
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23 Summary Problem solving does not involve code We’ve discussed (and practiced) the basic problem solving concepts and skills Don’t panic if this didn’t all make sense … it’s only week 2 23
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