CPS120 Introduction to Computer Science

Programming & Debugging

Lecture 6

Introduction to Programming

The Program Development Cycle

Get the program into machine-readable form

Test and debug the program

Translate the program

Understand the problem

Code the program

Plan the program's logic

Document the program

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

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

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)

Program Design

Input Data Format Output Data Format Procedural Logic Control Structure

Introduction to C++

C++ Usages & ConventionsC++ 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

Characteristics of a C++ Program

Comments Compiler Directives Functions Braces Statements

A Simple C++ Program

Comments //Simple C++ Program // // Purpose: To demonstrate the // parts of a simple C++

programCompiler Directive #include <iostream.h>Main Function main ( )

Braces {Statements cout << "This is a simple program ";

return 0;}

Comments

Document what is happening, why it is happening and other issues

Commentary is ignored by the compilerC++ has inline, block and documentary comments

–Inline comments are within line of code• Use the // symbols

–Block comments are long comments delimited with /* and */

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

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

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

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

Columns and White SpaceModern 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

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

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

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

C++ Control Structures

1. "Sequence statements" are imperatives2. "Selection" is the "if then else" statement

– AND, OR, NOT and parentheses ( ) can be used for compound conditions

3. "Iteration" is satisfied by a number of statements– "while" – " do " – "for"

4. The case-type statement is satisfied by the "switch" statement.

– CASE statements are used for most non-trivial selection decisions

Program Design

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

I N P U T

O U T P U T

Flowcharts Pseudocode

Algorithms

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

BirthDate 46-53 8 String

Program Design- Output Report Sample

Employee Name ID Birth Day

Robert Williams A4687 04/08/1976

J3567 02/01/1983

K2467 07/04/1978

L8909 03/06/1966

Ronald Wilson

Larry Jackson

Mary Roosevelt

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++

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!

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

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 witha flow line in and the other with a flow line out.

Off-page connector. Even fairly small programs can have flowcharts that extend severalpages. 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

Common Flowchart Symbols

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

Start

Sum =0

Count =0

Input data

Sum = Sum+data

Count = Count+1

Count =3?

NOYESA

A

Avg = Sum/Count

Output Avg

End

Flowcharting Example

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).

Start Get f C=5/9*(f-32) Output c End

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

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.

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

Using Pseudocode

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 * width

Pseudocode 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

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

Rules for Pseudocode1. Make the pseudocode language-independent

2. Indent lines for readability

3. Make key words stick out by showing them capitalized, in a different color or a different font

4. Punctuation is optional

5. End every IF with ENDIF

6. Begin loop with LOOP and end with ENDLOOP

7. Show MAINLINE first; all others follow

8. TERMINAE all routines with an END instruction

Compiling and Debugging

Compilation Process

1. Get the set of instructions from you2. Review the instructions to see if they violate the

rules (syntax) of the language3. If all the rules are obeyed, create a working file

in the language of the computer (machine language)

4. Attach to the working file full instructions for any shortcuts you may have used (linkage)

5. Assemble a final file in machine language

Compiling

Source Code

Compiler

Object Code

LinkerAdditional Code

Executable Code

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

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

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

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

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.

Debugging Steps

1. Proofread before compiling

2. Compile

3. 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

4. Recompile when you have fixed what you recognize

Debugging Steps

5. Repeat 3 & 4 until no further errors are obvious

6. Attempt to solve the remaining errors in a top-down fashion

7. Solve whatever errors you can without spending long periods of time on any given error

8. Recompile whenever you feel you don’t see any further solutions

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

Debugging Checklist1. 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

2. Verify syntax with a reference book, not just visually-- Don’t trust your eyes; you see what is supposed to be there

3. Try to find an example in the reference book that does something similar and compare the code

4. Verify that the necessary delimiters used for that line are there

-- Check the lines above and below as well

Debugging Checklist

5. Without looking at your source code or notes, rewrite the instruction on a piece of paper and then compare it to your actual code; don’t cheat

6. Verify that the line is really the source of the error by commenting the line using //

a) Don’t worry about other errors that result from this

b) If the error disappears, it probably results from the line you are working on

c) If it moves to the next line it is probably caused earlier in the code

d) Remember that the compiler cannot be trusted to pinpoint lines

Warnings

Even though an executable has been generated, you may not be done with syntax errors

Compilers generate syntax warning messages which are not fatal errors but represent special error checking functions for certain common programming errors

Linker Errors

Not all syntax errors are detectable by the compiler– These errors do not become apparent until files

are put together to create an executable– These errors are not linked to a specific line of

code• Look for the name of the variable and see what lines

of code it occurs on using EDIT and FIND– LNIK2001: unresolved external – LNK1120: unresolved externals

Logic/Semantic Errors

If the data is good and a program does not do what it is supposed to do, there must be at least one logic error present– The syntax rules of C++ have been correctly

followed, but the meaning of the code is incorrect

Semantic Errors

A semantic error is a violation of the rules of meaning of a programming language– E.g. My refrigerator just drove a car to Chicago– Overt logic errors

• Something is obviously wrong even though the program is running

– Covert logic errors• Not so obvious something is wrong

Run things various ways to highlight these errors

Approaches to Correction

Desk-checking Inserting Tracing Statements

– Used when program "crashes"– Runs to completion with incorrect output

Using an interactive debugger

Common Semantic Errors

1. Infinite Loop– Created when a loop in which the expression tested never

becomes false

2. Misunderstanding operator precedence3. Dangling else4. Off-By-One Error

– Loop that iterates one fewer or one more than is correct

5. Code inside a loop that doesn’t belong there6. Not using a compound statement when one is required7. Array index bounds error

Color Coding in Visual C++ Editor

Comments are green and are ignored by the compiler

All ANSI keywords are coded in blue Other code is in plain black

– Compiler keywords like cin and cout are also shown in black

C2065: Undeclared Identifier

1. Several things may produce this error• Misspelling a keyword

• Misspelling a programmer defined name (identifier)

• Misuse of case in a keyword or identifier

• Failure to declare an identifier