High-level software (Ch.9)

Algorithms and data structures

Low-level software

Components

Circuits

Gates

Transistors

Remember: Decision-making instructions

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BR Set PC to operand unconditionally

BRLT i Set PC to operand if A < 0

BREQ i Set PC to operand if A = 0

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We use decision-making instructions to build:

• branches

• loops

Flowcharts!

6.5 Algorithms and Pseudocode

Algorithm = A sequence of steps for solving a problem

Muḥammad ibn Mūsā al-Khwārizmī (780-850A.D.)

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How to describe an algorithm in an intuitive way?

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Flowcharts!

Not in text

Problems with flowcharts: • They’re hard to follow when they get complex

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Not in text

Problems with flowcharts: • They’re hard to draw in electronic documents

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Not in text

Problems with flowcharts: • They’re hard to draw in electronic documents

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Not in text

Our text uses only pseudocode

6.5 Algorithms and Pseudocode

Pseudocode = A mixture of English and formatting to make the steps in an algorithm explicit

There are no syntax rules in pseudocode!

Pseudocode is not case sensitive!

Example: Repeated-division algorithm (convert base-10 number to other bases):

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While ( the quotient is not zero )

Divide the decimal number by the new base

Make the remainder the next digit to the left in the answer

Replace the original decimal number with the quotient

Algorithms can also be described in natural language!

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… but pseudocode is more precise

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IF concerned about cholesterol

Put butter substitute in a pot

ELSE

Put butter in a pot

Turn on burner

Put pot on the burner

WHILE (NOT bubbling)

Leave pot on the burner

Put other ingredients in the blender

Turn on blender

WHILE (more in pot)

Pour contents into lender in slow steam

Turn off blender

Pseudocode functionality

Pseudocode has all of the concepts encountered in any high-level programming language, only the syntax is informal:

– Variables

– Assignment

– I/O

– Selection / decision

– Repetition / loop

– Boolean expressions

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To do for next time

Read pp.175-179 of the text, referring to your Python experience

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We can test a pseudocode algorithm w/pencil & paper, a.k.a. “desk checking”

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What is 93 in base 8?

93/8 gives 11 remainder 5

11/6 gives 1 remainder 3

1/ 8 gives 0 remainder 1

answer 1 3 5

While ( the quotient is not zero )

Divide the decimal number by the new base

Make the remainder the next digit to the left in the answer

Replace the original decimal number with

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Organizing the solution in a computer-like way gives us

better idea of what is required for the computer to

execute it, e.g. two numbers need to be entered, etc.

Pseudocode for Complete Computer Solution (in Python)

Write "Enter the new base"

Read newBase

Write "Enter the number to be converted"

Read decimalNumber

Set quotient to 1

WHILE (quotient is not zero)

Set quotient to decimalNumber DIV newBase

Set remainder to decimalNumber REM newBase

Make the remainder the next digit to the left in the answer

Set decimalNumber to quotient

Write "The answer is "

Write answer

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How to Develop an Algorithm in Pseudocode

Problem: Read in pairs of positive numbers and print each pair in order.

First version:

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WHILE (not done)

Write "Enter two values separated by blanks"

Read number1

Read number2

Print them in order Do you see any problems with this algorithm?

In order to be able to write a computer program to implement this algorithm, several extra questions need to be settled.

We say that the algorithm is not concrete yet.

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WHILE (not done)

Write "Enter two values separated by blanks"

Read number1

Read number2

Print them in order

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Pseudocode – Refining the algorithm

How do we know when we’re done?

Let the user tell us how many

Where do we store the user’s response?

In a variable. Let’s call it numberOfPairs

How do we keep track of how many pairs have been entered?

With another variable! Let’s call it numberRead

How do we print the numbers in order?

If first number is smaller

print first, then second

If first number if larger

print second, then first

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Write "How many pairs of values are to be entered?"

Read numberOfPairs

Set numberRead to 0

WHILE (numberRead < numberOfPairs)

Write "Enter two values separated by a blank; press return"

Read number1

Read number2

IF(number1 < number2)

Print number1 , " " , number2

ELSE

Print number2 ," " , number1

Make numberRead equal to itself plus 1

Second version:

Is it concrete yet?

Desk checking

Data Fill in values during each iteration 3 numberRead number1 number2

55 70

2 1

33 33

numberOfPairs

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What is the output?

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Translating Pseudocode into a Computer Program

What kind of program?

Machine language – Nooooooo!

Assembly language

Very detailed and time consuming, but

sometimes it needs to be done

High-level language

Python!

SKIP 6.6 Testing

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Read and take notes in notebook:

Software Piracy and Copyrighting

Have you every "borrowed" software

from a friend?

Have you ever "lent" software to a

friend?

Did you know that about 100,000 jobs are

lost in the US every year due to such

"borrowing" and "lending?"

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Chapter Review Questions

• List the operations that a computer can perform

• Describe the important features of the Pep/8 virtual machine

• Distinguish between immediate addressing mode and direct addressing mode

• Write a simple machine-language program

• Distinguish between machine language and assembly language

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Chapter Review Questions

• Describe the steps in creating and running an assembly-language program

• Write a simple program in assembly program

• Distinguish between instructions to the assembler (a.k.a. directives) and instructions to be translated into machine code (a.k.a. executable instructions)

• Distinguish between following an algorithm and developing one

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Chapter Review Questions

• Describe the pseudocode constructs used in expressing an algorithm

• Use pseudocode to express and algorithm

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Second homework for ch.6

44, 45

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