Class 38 — Wednesday April 22

Functioning

The end is coming — Soon time to show mastery — Expect to do well

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Look both aways

• Back or ahead

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Zoom

• Link

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Agenda

• Take stock

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Art exhibition

• Best in show

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Download

• Module duck.py

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Expectation

• You will take test 2 as close to 2 PM EDT as possible. No matter what you will complete the test before 10 AM EDT on Saturday April 25.

• Please submit duck.py today.

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What to expect for the test

• Knowledge of if and for statements; along with the ability to use them when problem solving.

• Ability to write functions using numbers, strings, lists, dicts, datasets, random, and image manipulation akin to the image flipping and bluing examples.

• No short answer questions

• If you have an accomodation, then you get your accommodation multiplier * 90 minutes for test taking time.

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List patterns — suppose alist, blist, and clist are lists and that value v is not a list

• To analyze a list use

result = ...

for element in alist :

  ...

return result

• To modify a list use

n = len( alist )

for i in range( 0, n ) :

  element = alist[ i ]

  ...

  alist[ i ] = element

• Important list operations

• alist.append( v )

• Appends v to end of alist.

• alist.remove( v )

• Removes first occurence of v from alist.

• alist.index( v )

• Index of first occurrence of v in alist.

• clist = alist + blist

• Concatenates alist and blist to make clist.

• v = alist.pop( )

• Removes the last element from alist. The value of that former element is the return value assigned to v.

• v = alist.pop( i )

• Removes the element at index i from alist. The value of that former element is the return value assigned to v.

• Major gotcha

blist = alist + v # cannot add a list and a non-list

blist = alist + [ v ] # can add a list with another list

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Warnings

• A print() statement in a function is not supplying a return value. It is sending a message to the program user.

• An input() statement is wrong to include in a function body, information comes into a function through its parameters.

• No function you will be asked to implement should have a print() or input() statement.

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Gotchas

• Nested for-loops usually wrong unless using a list-of-lists (e.g., a dataset) or looking at one list in relation to another list or image manipulation

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Function sort() versus sorted():

• x.sort() modifies the contents of x to be in non-descending order and returns None

• sorted( x ) creates a new list that has the elements of x in non-descending order, returns that new list, and does not modify x

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Dictionaries

• Unordered collection of key-value pairs

• Initialization examples

d = {} # empty dict

d = { 18 : 'voting', 67 : 'retirement', 'eighteen' : 18}

• Access values by key: d[ 18 ]

• Means the same thing as 'voting'

• Access values by key: d[ 21 ]

• Will be an error because 21 is not a key

• Access values by key: d[ 'voting' ]

• Will be an error because 'voting' is not a key

• Access values by key: d.get( 18 )

• Means the same thing as 'voting'

• Access values by key: d.get( 21 )

• Means the same thing as None (not an error)

• Access key by value: not easy, probably need a loop

• Collection of all keys: d.keys()

• Collection of all values: d.values()

• Indices: there are none (just keys)

• List of all keys: list( d.keys() )

• Set the value of a key:

d[ 100 ] = 'centarian' # makes association for key 100 to be 'centarian'

• How do we look at the keys a dict

for key in d.keys():

  # do something with key

• How do we look at the values of a dict

for value in d.values():

  # do something with value

• Trying to find and change a particular entry

• By key; e.g., change key 18 to 'adult'

d[ 18 ] = 'adult'

• By value (find key for value 18)

found = None

for key in d.keys():

  value = d[ key ]

  if value == 18:

  found = key

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First question from Test 2

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© 2020 Jim Cohoon

Resources from previous semesters are available.