Introduction to Algorithms and Computer Science

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Introduction to Algorithms and Computer Science
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CS in a nutshell

• Computer scientists are on Calvins side
• Do it once, then never again.
• Can do more difficult versions of same task.
• The technology could be adapted to other tasks.
• Building a robot is just more fun!

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CS Vocabulary

• Algorithm A detailed, unambiguous set of
  instructions to carry out a given task.
• Alphabet A set of characters.
• Ex English alphabet, ASCII alphabet, binary
  alphabet (0,1), DNA alphabet (A,T,C,G), amino
  acid alphabet.
• String A set of characters from an alphabet in a
  particular order.
• Empty string A string containing no characters

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CS Vocabulary

• Identifier a string associated with a value. The
  value can be either constant or a variable.
• Constant a value for something that remains the
  same throughout an algorithm.
• Variable a value associated with an identifier
  that can change as we execute and algorithm.

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CS Vocabulary

• Substring A set of characters that appear in
  order and consecutively in another string.
• Ex cat is a substring of concatenate
• Subsequence A set of characters that appear in
  order but not necessarily consecutively in
  another string.
• Ex cat is a subsequence of correlate but not
  a substring.

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CS Vocabulary

• Prefix A substring entirely at the beginning of
  a string.
• Suffix A substring entirely at the end of a
  string.
• Concatenate To combine two strings by attaching
  the end of one to the beginning of the other to
  create a third string.
• Ex Concatenating cat and bird gives catbird

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Building An Algorithm

• Computers are dumb
• Human instructions Pour me a cup of coffee.
• Computer Instructions
• Walk to the coffee pot.
• Is there a cup? If not get a cup out of the
  cupboard.
• Is the cup right side up? If not, turn it over.
• Is the cup clean? If not, get another cup out of
  the cupboard and make sure it is right side up
  and clean.
• Pour coffee into cup.
• Return to me with cup containing coffee.

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Flowcharts

• A way of organizing an algorithm visualy
• Rectangles are instructions
• Diamonds are decisions
• Ovals represent end points
• Arrows move between different shapes
• A rectangle has 1 arrow from it, a diamond has 2.
• A shape can have any number of arrows leading to
  it.
• To follow the algorithm, just follow the arrows

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Flowchart for coffee algorithm
Walk to coffee pot.
Is it right-side up?
Y
Y
Is there a cup?
Is the cup clean?
Pour coffee into cup.
Y
N
N
N
Get out a cup.
Turn cup over.
Return to me.
Give me cup.
Done
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Algorithm to Do a Punnett Square

• Task Given maternal and paternal genotypes, give
  the resulting punnett square of genotypes for the
  offspring and calculate the phenotype ratio for
  the offspring.
• We will be looking at pea plant height with T
  tall and t short, with T dominant.
• Make an algorithm to allow a computer to do this,
  and write your algorithm in the form of a
  flowchart.

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Algorithm to Do a Punnett Square

• Our computer can do the following tasks
• Give a name to a note card
• Put 1 number or 1 string on a named card
• Read the value off a named card
• If value is a string, can read a single character
  if we are given the number of that character in
  the string
• Change the value on a named card
• Do anything involving the 4 basic math functions
• Check all the comparisons , ?, lt, , gt,