CS 1400

Jan 10, 2007

What is it?

• Computer Science is the art and science of writing programs– Users use programs– Programmers create these programs

• A program is a logical sequence of instructions that leads to a correct solution to a problem.

• Programmers are story-problem solvers!

Successful Problem SolversSuccessful Problem Solvers

• have or acquire the necessary subject matter knowledge • have a positive, objective attitude, viewing the activity as a learning

opportunity rather than a “test” of their intelligence• are active• begin with what they understand • draw on other information and sources of knowledge• refer to the problem statement frequently• employ sequential analysis when a question is initially unclear• ask themselves questions and think out loud – (we call this "self

talk")• brainstorm• use physical aids to thinking, such as drawing pictures• relate problems to familiar / concrete experiences or to previous

problems• carefully proceed through a series of steps to reach a conclusion

Less Successful Problem SolversLess Successful Problem Solvers

• lack necessary subject-matter knowledge and are willing to allow gaps of knowledge to exist

• have a poor attitude: an attitude of indifference towards achieving a complete, accurate comprehension of situations and relations

• demonstrate a lack of objectivity in dealing with problems • are passive in their thinking• have great difficulty in ascertaining what they are required to do• grope blindly toward a solution• fail to complete a chain of reasoning• use one-shot thinking, rather than building an extended, sequential

path to understanding• rush through or skip instructions• take little time to consider a question or break it down into its

component parts• are mentally careless and superficial• select an answer on the basis of a feeling or guess

Example Program… washing hair

• Wet hair• Pick up shampoo bottle• Open the bottle• Pour some shampoo on your hand• Close the shampoo bottle• Set bottle down• Put shampoo on hair• Rub shampoo into hair until lather covers hair• Rinse hair until all lather is removed• Repeat the previous eight steps one more time

Programs vs. Algorithms

• A program contains an algorithm

• A program must be in a certain format– -- think of a business letter!

Algorithms in Computer Science

1. The instructions must be well ordered.

2. The algorithm must be unambiguous -- clear and incapable of being misunderstood.

3. The process must eventually end.

4. The requested action must be "doable.“

5. The algorithm must produce a result.

Program DevelopmentStep 1 - Analyze the problem• InputInput: : Identify what information the program will have

to retrieve or will be provided to the program by the user.

• Values given or known values: Identify any values that appear in the problem statement. These may or may not be used in your solution. Also watch for values or formulas that are not given but that will need to be used to complete the solution.

• Processing: What will need to be done to produce the desired output? It is important at this stage to focus on what needs to be done, not how it will be accomplished.

• Output : Identify what is to be produced by this program. This will be a list of the desired results.

Example…

“Write a program to calculate the volume of a box, using dimensions provided by the user.”

Input: height, width, depth of box

Given values: none

Processing: calculate volume

Output: volume of the box

Step 2 – Outline the solution

• List steps to be performed in correct order – not concerned with details at this point.

Get height, width and depth of the box from user

Calculate the volume of the box

Output the dimensions entered and the volume of the box

Step 3 – Develop an algorithm in pseudocode

Print “Enter the height of the box: “

Get height from user

Print “Enter the width of the box: “

Get width from user

Print “Enter the depth of the box: “

Get depth from user

volume = height * width * depth

Print “The volume of the box is “

Print volume

Step 4 – Test the algorithm

• Pretend you are the computer – hand tracing

Step 5 – Code the algorithm into C++

Step 6 – Run and test the program on the computer

Step 7 – Document the program

Programming Language ElementsProgramming Language Elements

• Punctuation and spelling

• Syntax and grammar

• Semantics

Pseudocode allows us to start writing Pseudocode allows us to start writing algorithms without worrying about these algorithms without worrying about these issues.issues.

English-like Pseudocode

Key Words – these words have special meaningKey Words – these words have special meaning• Get • Print• And• Or• If• Else• End If• Repeat• End Repeat• While• End While

English-like Pseudocode

• VariablesVariables – one-word name representing a value or a place where a value can be stored

Think of a mailbox: – The name on the mailbox represents a

variable name, – The contents of the mailbox represent the

variable value.A variable only holds one value at a time!A variable only holds one value at a time!

English-like Pseudocode

• OutputOutputPrint “message”

Print variable

• InputInputGet variable from user

English-like Pseudocode

• OperatorsOperatorsArithmetic: Arithmetic: **, , ++, , --, , //

Assignment: Assignment: ==

• CalculationsCalculationsVariable = arithmetic expression

(Arithmetic expressions may only use known values and variables)

Example:

“Write a program to calculate the volume of a box, using dimensions provided by the user.”

Print “Enter height, width, and length of box:”

Get height from user

Get width from user

Get length from user

volume = height * width * length

Print “Volume is: “

Print volume

More Examples:

• “Write a program to calculate a paycheck, using hours worked and a payrate provided by the user”

• “Write a program to convert a temperature provided by the user in degrees Fahrenheit into degrees Centigrade”

• “Write a program to calculate the circumference of a circle provided by the user”

Control Structures

• Sequence

• Selection (or decision)

• Repetition

Selection

If condition

Statements…

ElseStatements…

End If

Conditions

• A condition is a comparison question whose answer is either true or false (Boolean).

• The comparison operators are;< less than > greater than

<= less than or equal >= greater than or equal

== equal != not equal

Example Selections

If age < 18

Print “you cannot vote”

Else

Print “you can vote!”

End If

If quantity >= 26

Print “sufficient quantity is available: ”

Print quantity

End If

Example algorithm• Write a program to calculate an employee

paycheck using time-and-a-half for overtime

Input: hours, payrate

Given values: over 40 hours is overtime, overtime rate is 1.5 of

payrate

Processing: calculate regular pay, overtime pay, and total pay

Output: total pay

OutlineGet hours_worked and payrate from user

If there was overtime, calculate pay using payrate for first 40 hours and 1½ payrate for additional hours

Otherwise, calculate pay at payrate

Print pay

PseudocodePrint “Enter hours worked: “Get hours from userPrint “Enter pay rate: “Get payrate from userOvertime_rate = payrate * 1.5If hours > 40

pay = 40 * payrate + (hours-40) * over_timerateElse

pay = hours * payrateEnd IfPrint pay

Repetition

• The counted loopRepeat N times

Statements…End Repeat

• The condition-controlled loopWhile condition

Statements…End While

Examples

• “Write a program to output the squares of each number input by the user until the user inputs a negative number”

• “Write a program to accept 10 test scores from the user and output the counts of passing (>=60) and failing scores”

• “Write a program to output the squares of the integers 1 through 5”

“Write a program to output the squares of the integers 1 through 5”

Inputs: none

Given values:integers 1 through 5

Processing: calculation squares

Outputs: squares of integers 1 through 5

Outline…

Start N as 1

As long as N is less than or equal to 5, repeat;

Calculate and print the square of n

Add 1 to n

N = 1

While N <= 5

square = N * N

Print “The square of “, N, “ is “, square

N = N+1

End While

“Write a program to output the squares of the integers 1 through 5”

Print “The square of 1 is 1”

square2 = 2 * 2

Print “The square of 2 is “, square2

square3 = 3 * 3

Print “The square of 3 is “, square3

square4 = 4 * 4

Print “The square of 4 is “, square4

square5 = 5 * 5

Print “The square of 5 is “, square5

Why not this?

“Write a program to accept 10 test scores from the user and output the counts of

passing (>=60) and failing scores”

Inputs: a list of 10 test scores

Give values: a count of 10, a passing score of 60

Processing: find a count of passing and failing test scores

Outputs: count of passing/failing scores

Outline…

Start the counts of passing and failing scores as 0

Repeat the following 10 times;

prompt the user and get a score

if this score is passing

add 1 to the passing count

otherwise

add 1 to the failing count

Print the counts of passing and failing scores

passing = 0failing = 0Repeat 10 times

Print “Enter a test score: “Get score from userIf score >= 60

passing = passing + 1Else

failing = failing + 1End If

End RepeatPrint “Passing: “, passing, “ Failing: “, failing

“Write a program to output the squares of each number input by the user until the user inputs a

negative number”

Inputs: a list of positive numbers followed by a negative number

Given values:none

Processing: square of each positive number

Outputs: calculated squares

Outline…

Prompt the user and get the first number

Repeat as long as the number is positive;

calculate the square of this number

Print the square

Prompt the user and get the next number

Print “Enter the first number: “

Get number from user

While number > 0

Square = number * number

Print “Square is: “, square

Print “Enter the next number: “

Get number from user

End While

“Write a program to output the squares of each number input by the user until the user inputs a

negative number”