CPS120 Introduction to Computer Science

1. CPS120 Introduction to Computer Science Programming & Debugging Lecture 6

2. Introduction to Programming

3. The Program Development Cycle

4. What Can a Program Do? • A program can only instruct a computer to: • Read Input • Sequence • Calculate • Store data • Compare and branch • Iterate or Loop • Write Output

5. Fundamental Programming Concepts • Assignment of values to a variable • Iteration (Looping) • Over a set of set of statements • With respect to a logical expressions (conditions) • Delegation of sub-tasks to functions / procedures

6. The Structure Theorem The Structure Theorem states that any algorithm can be built from three basic control structures. • One-after-another (Sequence) • Decision-making (Selection) • Making choices between 2 or more alternatives • Repetition (Iteration) • Concerned with repetitive tasks (and the termination conditions of loops)

7. Program Design • Input Data Format • Output Data Format • Procedural Logic • Control Structure

8. Introduction to C++

9. C++ Usages & Conventions • C++ is absolutely case sensitive • For Instance: A is 97 in ASCII and a is 65 • Remember: in ASCII {, [, and ( are not equivalent • No keywords in ANSI standard are even partially uppercase • ‘While’ is not a keyword, ‘while’ is • Be careful if you define new keywords • The most common practice in C+++ is to use small letters of the first part of a variable name and capitals for the rest of it

10. Characteristics of a C++ Program • Comments • Compiler Directives • Functions • Braces • Statements

11. A Simple C++ Program Comments //Simple C++ Program // // Purpose: To demonstrate the // parts of a simple C++ program Compiler Directive #include <iostream.h> Main Function main ( ) Braces { Statements cout << "This is a simple program "; return 0; }

12. Comments • Document what is happening, why it is happening and other issues • Commentary is ignored by the compiler • C++ has inline, block and documentary comments • Inline comments are within line of code • Use the // symbols • Block comments are long comments delimited with /* and */

13. Compiler Directives • Instructions to the compiler rather than part of the C++ language • Most common directive is #include • For Example: #include <iostream.h> • A .h file is a header file. It serves as a link between program code and standard C++ code needed to make programs run

14. Functions • A function is a block of code that carries out a specific task • Every C++ program has a main function that executes when a program initiates • Includes open parenthesis to designate a function • Ends with a return 0; statement

15. Scope Delimiters • A symbol or pair of symbols used to define a region or area which is considered a locale • In programming, many structures need to have their scope defined because they should not affect the entire program • In C++, the symbols ‘{‘ and ‘}’ are used

16. Semicolons • There must be a semicolon after every statement • To tell the compiler that the statement is complete • Function definitions and compiler directives are exempt

17. Columns and White Space • Modern programming languages are free form with delimiters instead of columns to determine the end of instructions • The ; (semi-colon) is the delimiter used in C++ • Use tabs, indents, and blank lines in any manner that makes code easier to understand • Many programming instructions become subordinate to other instructions due to scope and other restrictions. Formatting code to reflect this makes it easier to read

18. Uppercase or Lowercase • Be careful to use the same combination of uppercase or lowercase lettering when you enter source code • Commands and other reserved words are all lower case

19. Variables • Variables or identifiers are used to hold information • Usually mixed case with the first letters small and the rest starting with a capital • e.g. theWeight

20. Literals • Literals are system commands and other pieces of information that the compiler doesn’t understand, and therefore, takes your word for them • In C++, literals are enclosed in straight double quotes "" which is the shift of the apostrophe

21. C++ Control Structures • "Sequence statements" are imperatives • "Selection" is the "if then else" statement • AND, OR, NOT and parentheses ( ) can be used for compound conditions • "Iteration" is satisfied by a number of statements • "while" • " do " • "for" • The case-type statement is satisfied by the "switch" statement. • CASE statements are used for most non-trivial selection decisions

22. Program Design

23. Program Design • Input Data Format • Output Data Format • Procedural Logic • Control Structure Algorithms O U T P U T I N P U T Process Flowcharts Pseudocode

24. Program Design - Input • Record Layout Table Field Name Position Length Data Type EmpName 1-20 20 String EmpID 21-25 5 String EmpAddr 26-45 20 String String BirthDate 46-53 8

25. Program Design- Output • Report Sample Employee Name ID Birth Day Robert Williams A4687 04/08/1976 Ronald Wilson J3567 02/01/1983 Larry Jackson K2467 07/04/1978 L8909 03/06/1966 Mary Roosevelt

26. What is an Algorithm? • An algorithm is merely the sequence of steps taken to solve a problem • Two parts • Actions to be executed • Order in which those actions are to be done • Computational steps that transform the input data into useful output data. • Algorithms are not programs • They need to be coded in a programming language like C++

27. Pseudocode & Flowcharts are Important • Pseudocode – • Make a detailed description of your algorithm’s logic before worrying about C++ syntax and data layout. • An algorithm you develop using pseudocode should be capable of implementation in any procedural programming language • Pseudocode is generally independent of the implementation language • Flowcharts – • A graphical layout of the algorithm is often very useful in spotting “illogical” logic!

28. Reasons Programmers Draw Flowcharts • Drawing a flowchart gives the programmer a good visual reference of what the program will do • Flowcharts serve as program documentation • Flowcharts allow a programmer to test alternative solution to a problem before coding • Flowcharts provide a method for easy desk checking

29. Common Flowchart Symbols Terminator. Shows the starting and ending points of the program. A terminator has flow lines in only one direction, either in (a stop node) or out (a start node). Data Input or Output. Allows the user to input data and results to be displayed. Processing. Indicates an operation performed by the computer, such as a variable assignment or mathematical operation. With a heading – an internal subroutine Decision. The diamond indicates a decision structure. A diamond always has two flow lines out. One flow lineout is labeled the “yes” branch and the other is labeled the “no” branch. Predefined Process. One statement denotes a group of previously defined statements. Such as a function or a subroutine created externally Connector. Connectors avoid crossing flow lines, making the flowchart easier to read. Connectors indicate where flow lines are connected. Connectors come in pairs, one with a flow line in and the other with a flow line out. Off-page connector. Even fairly small programs can have flowcharts that extend several pages. The off-page connector indicates the continuation of the flowchart on another page. Just like connectors, off-page connectors come in pairs. Flow line. Flow lines connect the flowchart symbols and show the sequence of operations during the program execution. Common Flowchart Symbols

30. Rules for Drawing Flowcharts • Top to bottom and left to right • Draw the flowchart the way you like to read • Use arrowheads on flow lines whenever the flow is not top to bottom, left to right • Be neat ! Use graphics software • Avoid intersecting lines

31. Flowcharting Example Start A Sum =0 Count =0 Avg = Sum/Count Input data Output Avg Sum = Sum+data Count = Count+1 End Count =3? A YES NO

32. Get f C=5/9*(f-32) Output c Start End Program Design: Flowcharts Problem: Compute a Centigrade temperature from a Fahrenheit temperature, which has been entered through the keyboard. The Centigrade value is then output. A centigrade temperature is computed as 5/9 * (Fahrenheit temp -32).

33. Disadvantages to Flowcharts • Time consuming • A program flowchart shows how the input becomes output, but it does not show why a particular step is done • Flowcharts are subjective

34. Pseudocode • This device is not visual but is considered a “first draft” of the actual program. • Pseudocode is written in the programmer’s native language and concentrates on the logic in a program—not the syntax of a programming language.

35. General Rules for Pseudocode • There is no standard pseudocode • The rules of Pseudocode are generally straightforward • Should be easily read and understood by non-programmers • All statements showing "dependency" are to be indented. • These include while, do, for, if, switch

36. Problem Solving Example:Write a program that allows the user to calculate the area of a rectangle as many times as they want.A) Input: length, width, continue/quitB) Restrictions: inputs must be positiveC) Output: areaD) Formula: area = length * widthPseudocode for algorithm (there are many different ways to write pseudocode, two are shown): Pseudocode Example 1 Pseudocode Example 2 1. Ask user to input length 1. Repeat until the user wants to stop 2. Read in length 2. Repeat until the length is positive 3. If length is not positive, return to step 1 3. Ask user to input length 4. Ask user to input width 4. Read in length 5. Read in width 5. Repeat until the width is positive 6. If width is not positive, return to step 4 6. Ask user to input width 7. Calculate area   7. Read in width 8. Output area   8. Calculate area 9. Ask user if they want to do it again 9. Output area 10. Read in answer 10. Ask user it they want to do it again 11. If the answer is to continue, return to step 1 11. Read in user's response Using Pseudocode

37. Pseudocode Statement Rules • Statements are written in a simple English-like language • Each instruction is started on a separate line • Logic-showing keywords are written in UPPER CASE or typed in BOLD UPPERCASE • (e.g. IF, THEN, FOR, DO etc.) • These are the only uppercase words in this form of pseudocode. • Indentation is used to show structure • Instructions are written from top to bottom, with only one entry point and one exit point • Logically related groups of instructions can be formed into modules and given a name

38. Rules for Pseudocode • Make the pseudocode language-independent • Indent lines for readability • Make key words stick out by showing them capitalized, in a different color or a different font • Punctuation is optional • End every IF with ENDIF • Begin loop with LOOP and end with ENDLOOP • Show MAINLINE first; all others follow • TERMINAE all routines with an END instruction

39. Compiling and Debugging

40. Compilation Process • Get the set of instructions from you • Review the instructions to see if they violate the rules (syntax) of the language • If all the rules are obeyed, create a working file in the language of the computer (machine language) • Attach to the working file full instructions for any shortcuts you may have used (linkage) • Assemble a final file in machine language

41. Compiling Source Code Compiler Object Code Executable Code Additional Code Linker

42. Compiling and Debugging • Executable code will not be created until you correct all of the syntax errors in your source code • Then the fun (with logic errors) begins

43. Syntax & Logic Errors • A syntax error is simply the violation of the rules of a language; misuse of structure and form in programming or a violation of the compiler’s rules. These errors are detected by the compiler • Also know as 'fatal compilation errors' • A logic error is a mistake that complies with the rules of the compiler that causes the program to generate incorrect output

44. Error Prevention & Testing • Use good design and programming style • Don't use global variables • Study your code before typing and running it • Have someone else look at it • Make your program self-documented • Program defensively – put in assertions and self-checking code and comment them out • Test your code at boundary values for variables • Log your bugs • Test code in pieces using "stubs" • Consider – correctness, reliability, utility and performance

45. Semantic Error Detection • Use the tracing method to display the value of critical variables • Make the error reproducible • Get a stack trace of function calls to verify sequencing • Correct the error immediately when you find it and check if you made it somewhere else • Examine your last code changes for errors • Ensure that you have saved and run the corrected programs

46. Debugging • Debugging is the process of locating and fixing or bypassing bugs (errors) in computer program code or the engineering of a hardware device. • To debug a program is to start with a problem, isolate the source of the problem, and then fix it.

47. Debugging Steps • Proofread before compiling • Compile • Correct all the obvious errors • Start at the beginning of the list of errors and warnings • A single syntax error may cause the compiler to believe numerous other syntax errors are occurring • Look at the error lines and if you see the error, fix it. Otherwise, leave it for later. It may vanish when you fix something else • Don’t worry if more errors appear. Some errors mask other errors • Recompile when you have fixed what you recognize

48. Debugging Steps • Repeat 3 & 4 until no further errors are obvious • Attempt to solve the remaining errors in a top-down fashion • Solve whatever errors you can without spending long periods of time on any given error • Recompile whenever you feel you don’t see any further solutions

49. A Debugging Mindset • Assume your syntax is wrong. Look it up! • Add working comments as you change things • If you are 100% sure a line is correct, then search for a syntax error in the lines ABOVE that line • Start with the immediately previous line and work backward • Never make a change you can’t explain

50. Debugging Checklist • Visually verify the spelling and case of keywords and identifiers -- Remember, in the editor, keywords are blue, literals are black and comments are green -- Look for problems with l and 1 and o and 0 • Verify syntax with a reference book, not just visually -- Don’t trust your eyes; you see what is supposed to be there • Try to find an example in the reference book that does something similar and compare the code • Verify that the necessary delimiters used for that line are there -- Check the lines above and below as well