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Lecture 2: Developing Procedural Thinking (How to think like a programmer) BJ Furman 06FEB2012

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The Plan for Today Program design process Algorithms, decomposition, and step-wise refinement Example Program design example

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Learning Objectives List and describe the steps in designing a computational solution (computer program) to a problem Articulate what is meant by an algorithm Apply the steps to a particular problem

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Program Design Process Define the problem List the inputs and outputs Design the solution algorithm Check the algorithm by hand Write the program code Test the program code

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Program Design Process – step 1 Define the problem List the inputs and outputs Design the solution algorithm Check the algorithm by hand Write the program code Test the program code State the problem in a clear and concise manner Example Write a program to find the distance between two points P1P2 or P1P2 P1 P2 or Better Write a program to find the straight line distance between two points Is the problem statement okay?

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Program Design Process – step 2 Define the problem List the inputs and outputs Design the solution algorithm Check the algorithm by hand Write the program code Test the program code Write a program to find the straight line distance between two points Inputs Outputs

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Program Design Process – step 3 Define the problem List the inputs and outputs Design the solution algorithm Check the algorithm by hand Write the program code Test the program code DecomposeRefine Write a program to find the straight line distance between two points

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Definition of an Algorithm An algorithm is a well-ordered collection of unambiguous and effectively computable operations, that when executed, produces a result and halts in a finite amount of time. Well-ordered means the steps are in a clear order Unambiguous means the operations described are understood by a computing agent without further simplification A computing agent is the thing that is supposed to carry out the algorithm Effectively computable means the computing agent can actually carry out the operation This definition comes from, An Invitation to Computer Science (Gersting/Schneider) via http://www.cs.xu.edu/csci170/08f/sect01/Overheads/WhatIsAnAlgorithm.html (visited 19JUN2009) http://www.cs.xu.edu/csci170/08f/sect01/Overheads/WhatIsAnAlgorithm.html

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Program Design Process – step 3,cont. Define the problem List the inputs and outputs Design the solution algorithm Check the algorithm by hand Write the program code Test the program code Two approaches are often used to help think through the steps to be carried out by the program code: 1. Pseudocode 2. Flow Charts We’ll use the pseudocode method first.

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Pseudocode Pseudocode (also called Program Design Language, PDL) is English-like statements that precisely describe specific operations 1 : Action statements Focuses on the logic of the program Avoids language-specific elements Written at a level so that code can be generated almost automatically. Will likely need to refine in more and more detail 1 This definition comes from, McConnel, S. (1993). Code Complete, Microsoft Press, Redmond, WA, p. 54.

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Pseudocode – First Pass 1. Prompt user to enter points 2. Get points from user 3. Calculate the straight line distance 4. Display distance to the monitor Write a program to find the straight line distance between two points Comments 1. High level – just the major steps 2. Focus on the logic

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Pseudocode - Refinement 1. Start 2. Declare variables: X1, Y1, X2, Y2, D 3. Prompt user to enter X1 and Y1 4. Display X1 and Y1 to the monitor 5. Prompt user to enter X2 and Y2 6. Display X2 and Y2 to the monitor 7. Calculate the straight line distance, D 8. Display D to the monitor 9. Stop Write a program to find the straight line distance between two points Comments 1. Refine high level ideas down to computable actions What could still be refined?

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Calculating the Distance, D Define the problem List the inputs and outputs Design the solution algorithm Check the algorithm by hand Write the program code Test the program code Write a program to find the straight line distance between two points P1 P2 X1 X2 Y1 Y2 D How do you find D? P1 P2 D X Y

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Program Design Process – step 4 Define the problem List the inputs and outputs Design the solution algorithm Check the algorithm by hand Write the program code Test the program code Write a program to find the straight line distance between two points What values of Xi, Yi (where i=1, 2) would be good to test the algorithm with?

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Program Design Process – step 5 Define the problem List the inputs and outputs Design the solution algorithm Check the algorithm by hand Write the program code Test the program code If you have refined your algorithm sufficiently, writing the code should proceed straightforwardly from it. If not, continue refining the algorithm until you can write the code directly from it. Your pseudocode can be turned into the comments for your code.

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Program Code Note the structure of the program

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Program Design Process – step 6 Define the problem List the inputs and outputs Design the solution algorithm Check the algorithm by hand Write the program code Test the program code Test your code with cases that you know the answer to. Try the ‘boundary’ cases to make sure your code works for them too.

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Algorithm Practice Find the midpoint between two points

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Algorithm – Items to Consider Is the problem statement clear and concise? Could be better: “Find the midpoint on a line between two points What are the inputs? X and Y coordinates of the two points: x1, y1 and x2, y2 What are the outputs? The midpoint between the points: Xm, Ym

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Algorithm – Items to Consider, cont. Solution algorithm Point must be on the line Midpoint means ‘halfway’ between the points Halfway in x and halfway in y This should suggest a solution strategy P1 P2 X1 X2 Y1 Y2 M

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Review

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References

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8.1 8 Algorithms Foundations of Computer Science Cengage Learning.

8.1 8 Algorithms Foundations of Computer Science Cengage Learning.

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