CSC 121 Spring 2017 Howard Rosenthal

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1 CSC 121 Spring 2017 Howard Rosenthal

2 Agenda To be able to define computer program, algorithm, and highlevel programming language. To be able to list the basic stages involved in writing a computer program. To be able to apply an appropriate problem-solving method for developing an algorithmic solution to a problem. To understand the use of flowcharts and pseudocode These notes come from:

3 What Is Programming Some Programming - Planning or scheduling the performance of a task or an event Electronic computer - A programmable device that can store, retrieve, and process data Data - Information in a form a computer can use Information - Any knowledge that can be communicated Data type - The specification of how information is represented in the computer as data and the set of operations that can be applied to it We already have seen the primitive data types that Java supports Computer programming - The process of specifying the data types and the operations for a computer to apply to data in order to solve a problem Computer program - Data type specifications and instructions for carrying out operations that are used by a computer to solve a problem 3

4 What Can We Do With Programming Languages A computer can transfer data from one place to another. A computer can get data from an input device (a keyboard or mouse, for example) and write data to an output device (a screen, for example). A computer can store data into and retrieve data from its memory and secondary storage A computer can compare data values for equality or inequality and make decisions based on the result. A computer can perform arithmetic operations (addition and subtraction, for example) very quickly. A computer can branch to a different section of the instructions. Based on a conditional test (with an if like construct) you can execute or skip an instruction or set of instructions Based on a conditional test you have the ability to loop around and iterate through the same instructions many times, until the condition becomes false You also can jump to and from other programs (or subprograms, which are known as methods in Java) 4

5 The Programming Process (1) PROBLEM-SOLVING PHASE IMPLEMENTATION PHASE Analysis and specification General solution (algorithm) Concrete solution (program) Verify Test Maintenance phase Figure 1.1 Programming process 5

6 The Programming Process (2) Problem-Solving Phase Analysis and Specification. Understand (define) the problem and what the solution must do. General Solution (Algorithm). Specify the required data types and the logical sequences of steps that solve the problem. This is typically done with flow charts or pseudocode Verify. Follow the steps exactly to see if the solution really does solve the problem. This means taking some test cases and verifying that you are getting the expected answers Implementation Phase Concrete Solution (Program). Translate the algorithm (the general solution) into a programming language. Test. Have the computer follow the instructions. Then manually check the results. If you find errors, analyze the program and the algorithm to determine the source of the errors, and then make corrections. Once a program has been written, it enters a third phase: maintenance. Maintenance Phase Use. Use the program. Maintain. Modify the program to meet changing requirements or to correct any errors that show up while using it. The Most Important Part Is Finding A Solution To The Problem, Then The Implementation Is Easy 6

7 Algorithms An algorithm is a written or verbal description of a logical sequence of actions applied to objects. It is a procedure for solving a problem in a specific order that Describes the actions to be performed Describes the order in which these actions are performed We use algorithms every day. Recipes, instructions, and directions are all examples of algorithms that are not programs. An algorithm can be implemented in a programming language(s) as needed to rapidly and repetitively solve a problem A simple example of an algorithm that determines an employee s wages 1. Look up the employee s pay rate. 2. Determine the hours worked during the week. 3. If the number of hours worked is less than or equal to 40, multiply the hours by the pay rate to calculate regular wages. 4. If the number of hours worked is greater than 40, multiply 40 by the pay rate to calculate regular wages, and then multiply the difference between the hours worked and 40 by times the pay rate to calculate overtime wages. 5. Add the regular wages to the overtime wages (if any) to determine total wages for the week. 7

8 From Algorithm to (1) Don t code until your strategy and algorithms are worked out If you start by trying to code without a strategy for solving a problem you will run into trouble Coding is translating your solution from English to a computer language PROBLEM-SOLVING PHASE Problem Algorithm Shortcut? Code IMPLEMENTATION PHASE Figure 1.3 8

From Algorithm to Execu@on (2) You then code your program using the correct syntax If there are errors the compiler will show them to you Once the program is syntactically correct it will execute You

9 From Algorithm to (2) You then code your program using the correct syntax If there are errors the compiler will show them to you Once the program is syntactically correct it will execute You need to test the program to make sure that it is giving you the expected results Multiple test cases are used to ensure program correctness COMPILATION Source code Computer executes compiler code Code listing, possibly with error messages. Machine language version of source code (object code) Loading EXECUTION Input data Computer executes machine language version of source code Results Figure 1.7 Compilation/Execution 9

10 A Simple Set of Steps For Algorithmic Programming (1) 1. Read the problem completely twice If you don t understand the problem, you cannot solve it. The better you understand it the easier it with be to solve 2. Solve the problem manually Nothing can be automated that cannot be done manually (with enough effort) You need to be able to predict the outcome in some test cases in order to ensure that the program is working correctly Understand what you are doing when solving the problem. This will help you better understand the algorithms you are using 10

11 A Simple Set of Steps For Algorithmic Programming (2) 3. Optimize the manual solution It is much easier to rearrange and reconstruct an idea or algorithm in plain English than it is in code Sometimes optimization takes place through consolidation of loops, if statements, and other techniques you became familiar with in Create pseudocode or flow diagrams Pseud0-code uses English like phrases to describe the logical steps to carry out a task Pseudocode Java add grade to total total = total + grade; Print grade System.out.println( The grade is + grade); A flowcharts graphically depicts the logical steps to carry out a task and shows how the steps relate to one another You can lay out the program steps before worrying about the syntax Sometimes the pseudocode may be very close to the syntactically correct code When writing pseudocode write enough comments so that you can understand what you are doing 11

12 A Simple Set of Steps For Algorithmic Programming (3) 5. Replace pseudocode with real, syntactically correct code Compile and debug 6. Execute the code and test it Use multiple test cases to ensure it works properly Examples If searching a list make sure it can find a number at the beginning, somewhere in the middle, and at the end Make sure to test for cases where a value is 0 and for negative as well as positive numbers whenever appropriate 7. Review the code and optimize it, then repeat steps 5 and 6 Have confidence in the process and practice of creating your programs this ways 12

13 Applying The Process (1) Read and analyze the problem Reverse a string of characters (let s use the string Zebra ) When we are done we have created the new String arbez Solve the problem manually - Write Zebra down. - Start a new word, and put a as the first letter. (Why > because it is the last letter, we want to start here) - Put r down as the 2nd letter. (Why > because it is the next letter backwards from the last letter we copied) - Put b down as the 3rd letter. (Why > same as above) - Continue with subsequent letters until done 13

14 Applying The Process (2) Optimize the manual solution 1 Write Zebra down. 2 Start at the last letter in the word and create a new empty word. 3 Append the current letter to the new word 4 If there is a previous letter, make the previous letter the current letter and return to step 3. 14

15 Applying The Process (3) Write the pseudocode or comments - //Read in String to reverse - // NewWord = - // Loop backwards through word to reverse - // NewWord += CurrentLetter Replace comments with real code System.out.println( Enter a String to reverse ); String oldword = keyboard.next(); String newword = for(int index = oldword.length() 1; index >= 0; index--) newword = newword +oldword.charat(index); 15

16 Problem Solving Techniques (1) Ask Questions!!! What do I have to work with that is, what are my data/ inputs? What do the data items look like What is my output and what should it look like? You can work backwards from the output once you understand what it looks like What are the operations to be performed on the data to convert the inputs to outputs. How much data is there? How will I know when I have processed all the data What special error conditions might come up? The Important Thing Is To Analyze the Problem and Develop a Strategy Before You Begin Programming 16

17 Problem Solving Techniques (2) Look For Familiar Patterns Many problems are similar (even between the labs and the homework In the examples below we are simply looking for the highest and lowest values We can use the same algorithm in both cases Find analogies with other types of problems If you organize files in a cabinet that might give you some ideas on how to organize computer files LIST OF TEMPERATURES Use the same method to find these values in both cases. LIST OF TEST SCORES HIGHEST = 95 LOWEST = 18 HIGHEST = 98 LOWEST = 12 17

18 Problem Solving Techniques (3) Means-Ends Analysis Figure out what the end-state is and then determine how to get there from the initial state There are often alternative ways to get to your end state As you go along there may be intermediate end states that are easier to reach Start: Boston Goal: Austin Start: Boston Goal: Austin Start: Boston Intermediate Goal: Newark Goal: Austin Means: Fly, walk, hitchhike, bike, drive, sail, bus Revised Means: Fly to Chicago and then Austin; fly to Newark and then Austin: fly to Atlanta and then Austin Means to Intermediate Goal: Commuter flight, walk, hitchhike, bike, drive, sail, bus Solution: Take commuter flight to Newark and then catch cheap flight to Austin Figure 1.15 Means-ends analysis 18

19 Problem Solving Techniques (4) Divide and Conquer The most popular technique Break up your hard problem into smaller, more easily solvable problems Develop a strategy for solving the easier problems (which can themselves be divided into smaller problems) Hard problem Easy subproblem Hard subproblem Easy subproblem Easy subsubproblem Easy subsubproblem Figure 1.16 Divide and conquer 19

20 Problem Solving Techniques (5) Building Blocks and Merging Solutions Take pieces from existing programs and use them in a larger solution We do this almost from the beginning in Java, using IO solutions that are provided as part of Java in order to read in data and write out results. Java has a huge number of libraries with programs that can be used to simplify our programming In the beginning we often recreate some of these tools to better understand computing principles 20

21 The Flowchart A Flowchart Shows logic of an algorithm Emphasizes individual steps and their interconnections Shows control flow from one action to the next 21

22 Flowchart Symbols Name Symbol Use in Flowchart Oval Denotes the beginning or end of the program Parallelogram Denotes an input operation Rectangle Denotes a process to be carried out e.g. addition, subtraction, division etc. Diamond Denotes a decision (or branch) to be made. The program should continue along one of two routes. (e.g. IF/THEN/ELSE) Hybrid Denotes an output operation Flow line Denotes the direction of logic flow in the program 22

23 Example 1 Write an algorithm and draw a flowchart to convert the length in feet to centimeters. Pseudocode: Input the length in feet (lengthinfeet) Calculate the length in cm (lengthincentimeters) by multiplying lengthinfeet with 30.0 Print length in cm (lengthincentimeters) 23

24 Example 1 Algorithm Step 1: Input lengthinfeet Step 2: lengthincentimeters lengthinfeet * 30 Step 3: Print lengthincentimeters Flowchart START Input lengthinfeet lengthincentimeters lengthinfeet * 30.0 Print lengthincentimeters STOP 24

25 Example 2 Write an algorithm and draw a flowchart that will calculate the roots of a quadratic equation 2 ax + bx + c = 0 Hint: d = sqrt ( b 2 4*a *c ), and the roots are: x1 = ( b + d)/2*a and x2 = ( b d)/2*a 25

26 Example 2 Pseudocode: Input the coefficients (a, b, c) of the quadratic equation Calculate d Calculate x1 Calculate x2 Print x1 and x2 26

27 Example 2 Algorithm and Flowchart Algorithm: Step 1: Input a, b, c Step 2: d sqrt ( ) Step 3: b*b 4*a *c x1 ( b + d) / (2 *a) Step 4: x2 ( b d) / (2 * a) Step 5: Print x1, x2 START Input a, b, c d sqrt(b * b 4*a * c) x 1 ( b + d) / (2 * a) X 2 ( b d) / (2 * a) Print x 1,x 2 STOP 27

28 IF THEN ELSE STRUCTURE The structure is as follows If condition then true alternative else false alternative endif 28

29 IF THEN ELSE STRUCTURE The algorithm for the flowchart is as follows: If A>B then print A else print B endif Y Print A is A>B N Print B 29

30 Looping Flowchart 30

31 Example 3 Write an algorithm to determine a student s final grade and indicate whether it is passing or failing. The final grade is calculated as the average of four marks. 31

32 Example 3 Pseudocode & Algorithm (1) Pseudocode: Input a set of 4 marks Calculate their average by summing the grades and then dividing by 4.0 if average is below 50 Print FAIL else Print PASS 32

33 Example 3 Pseudocode & Algorithm (2) Detailed Algorithm Step 1: Input M1,M2,M3,M4 Step 2: GRADE (M1+M2+M3+M4)/4.0 Step 3: if (GRADE < 50) then Print FAIL else Print PASS endif 33

34 Example 3 Flowchart START Input M1,M2,M3,M4 GRADE (M1+M2+M3+M4)/4.0 Step 1: Step 2: Step 3: Input M1,M2,M3,M4 GRADE (M1+M2+M3+M4)/4 if (GRADE <50) then Print FAIL else Print PASS endif N IS GRADE<50 Y PRINT PASS PRINT FAIL STOP 34

35 In Class Problem 1 Direction of Numbered NYC Streets Algorithm Problem: Given a street number of a one-way street in New York City, decide the direction of the street, either eastbound or westbound Discussion: in New York City even numbered streets are Eastbound, odd numbered streets are Westbound Write the Pseudocode and Flowchart for this Problem Do you know how you can determine if an integer is even or odd? 35

36 Class Problem 1 Pseudocode Program: Determine the direction of a numbered NYC street Get street If street number is even Then Display Eastbound Else Display Westbound End If 36

37 Class Problem 1 Flowchart 37

38 Class Problem 2 Class Average Algorithm Problem: Calculate and report the grade-point average for a class Discussion: The average grade equals the sum of all grades divided by the number of students Output: Average grade Input: Student grades Processing: Find the sum of the grades; count the number of students; calculate average Write the Pseudocode and Flowchart for this Problem 38

39 Class Problem 2 Pseudocode Program: Determine the average grade of a class Initialize Counter and Sum to 0 Do While there are more data Get the next Grade Add the Grade to the Sum Increment the Counter Loop Average = Sum / (double)counter Display Average 39

40 Class Problem 2 Flowchart 40

ALGORITHMS AND FLOWCHARTS

ALGORITHMS AND FLOWCHARTS ALGORITHMS AND FLOWCHARTS A typical programming task can be divided into two phases: Problem solving phase produce an ordered sequence of steps that describe solution of problem