CSE 102 Introduction to Computer Engineering

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CSE 102Introduction to Computer Engineering

• What is an Algorithm?

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What is an algorithm?

• An algorithm is a well-defined procedure that
  allows the computer to solve a problem.

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What is an algorithm?

• Example algorithms
• Cooking a dish
• Making a peanut-butter jelly sandwich
• Shampooing hair
• Programming a VCR
• Making a pie

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Example

• Is this an algorithm?
• Step 1 Wet hair
• Step 2 Lather
• Step 3 Rinse
• Step 4 Repeat
• Would you manage to wash your hair with this
  algorithm? How about a robot? Why (not)?

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Algorithms

• An algorithm must
• be well-ordered and unambiguous,
• each operation must be effectively computable,
• terminate

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Example

• Problem Find and print the 100th prime number
• A prime number is a whole number not evenly
  divisible by any other number other than 1 and
  itself
• Algorithm (?)
• 1. Generate a list of all prime numbers n1, n2,
  n3,
• 2. Sort the list in ascending order
• 3. Print out the 100th element in this list
• Is this an algorithm?

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Algorithms

• An algorithm consists of
• the actions to be executed, and
• the order in which the actions are to be executed

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Example

• Problem Find the sum of the first 5 numbers in a
  list.
• Actions
• Set the initial value of SUM to 0
• Take the first number from the list
• Add the number to SUM
• Take the next number from the list
• Algorithm
• Set the initial value of SUM to 0
• Take the first number from the list
• Add the number to SUM
• Take the next number from the list
• Repeat steps 3-4 five times

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Pseudocode

• Pseudocode is an informal language that helps
  programmers develop algorithms.

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Example

• Problem Find the sum of the first 5 numbers in a
  list.
• Pseudocode
• SUM ? 0
• N ? first number in the list
• SUM ? SUM N
• N ? next number in the list
• Repeat steps 3-4 five times

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Flowchart

• Flowchart is a graphical representation of an
  algorithm.

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Example

• Problem Find the sum of the first 5 numbers in a
  list.

start
SUM ? 0
N ? first number in the list
SUM ? SUM N
N ? next number in the list
five times
False
True
exit
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Flowchart

• Flowchart symbols
• Sequence structure
• Selection structure
• Repetition structure

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Flowchart

• Sequence structure

SUM ? 0
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Flowchart

• Selection structure

True
False
Ngt50
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Flowchart

• Repetition structure

repeated action
True
condition
False
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Example

• Problem Read the grades of 20 students and print
  passed if the grade is greater than or equal to
  50, otherwise print failed.

COUNT ? 0
Read grade
True
False
grade gt 50
Print passed
Print failed
COUNT ? COUNT1
COUNT20
False
True
exit
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Variables

• Variable
• A named memory location that can store a value
• Think of it as a box into which you can store a
  value, and from which you can retrieve a value
• Examples
• Example of operations
• Set the value of i to 3
• Set the value of M to i3 12
• Set the value of i to i10

i
M
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A model for visualizing an algorithm
Algorithm
Variables
Operations
An algorithm consists of operations that involve
variables
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Primitive operations

• Get input from user
• Get the value of x from user
• Assign values to variables using basic arithmetic
  operations
• Set the value of x to 3
• Set the value of y to x/10
• Set the value of z to x 25
• Print output to user
• Print the value of y, z to the user

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Example

• Problem For any three numbers input by the user,
  compute their sum and average, and output them
• Example of algorithm in pseudocode
• Variables a, b, c, sum, avg
• Get the values of a, b, c from user
• Set avg to (abc)/3
• Set sum to (abc)
• Print sum and avg

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Example 2

• Problem Given any value of radius from the user,
  compute and print the circumference of a circle
  with that radius
• Algorithm in pseudocode
• variables r, c
• 1. Get the value of r from user
• 2. Set c to 2 pi r
• 3. Print The circumference of your circle is c

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Basic operations

• Read the input from user
• Get x
• Get a, b, c
• Print the output to the user
• Print x
• Print Your mileage is x
• Cary out basic arithmetical computations
• Set x to 10
• Set y to xx/3

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• Conditional statements
• Specify a statement that may or may not be done
• if ltconditiongt then
• ltstatement to be donegt
• else ltstatement to be done otherwisegt
• Example
• if the value of A is greater than 5
• then set the value of B to 1
• else set the value of B to 0

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• Loop statements
• specify a group of statements that may be done
  several times (repeated)
• repeat until ltconditiongt
• lt statements to be repeated gt
• How does this work?
• Condition is evaluated
• If it is true than the loop terminates and the
  next instruction to be executed will be the
  instruction immediately following the loop
• If it is false, then the algorithm executes the
  ltstatements to be repeatedgt in order, one by one

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Example

• Variables count
• Step 1 set count to 1
• Step 2 repeat step 3 to step 5 until count is gt
  10
• Step 3 set square to count count
• Step 4 print value of square and value of count
• Step 5 add 1 to count
• Step 6 end
• What does this algorithm do?
• Note indentation
• Not necessary, but makes reading/understanding
  algorithms easier

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Pseudocode examples

• Equivalent
• Set the value of a to 1
• Set a to 1
• a1
• a?1
• Equivalent
• Add 1 to count
• Set count to count 1
• Increment the value of count by 1
• count count 1
• count ? count 1
• Writing in pseudocode gives you the freedom to
  choose any of these!

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• Incorrect
• Set 1 to a
• Add a b (what do you do with the result?)
• Set ab 3
• Note not the same
• set a to b
• set b to a
• Example what is the output of the following
  algorithms?
• set a to 2 set a to 2
• set b to 4 set b to 4
• set a to b set b to a
• print a, b print a, b

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A model for visualizing an algorithms behavior
Computer
Algorithm
Input (keyboard)
Output (screen)
Variables
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Designing Algorithms A Methodology

• Read the problem, identifying the input and the
  output.
• What variables are needed?
• What computations are required to achieve the
  output?
• Usually, the first steps in your algorithm bring
  input values to the variables.
• Usually, the last steps display the output
• So, the middle steps will do the computation.
• If the process is to be repeated, add loops.

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More algorithms

• Write algorithms to find
• the largest number in a list of numbers (and the
  position where it occurs)
• the smallest number in a list of numbers (and the
  position where it occurs)
• the range of a list of numbers
• Range largest - smallest
• the average of a list of numbers
• the sum of a list of numbers