Class 34 — November 11

Functioning

On Veteran's day — Remember sacrifices — Made on our behalf

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Look both ways

• Back or ahead

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Agenda

• Take stock

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Past homework

• Solutions are available

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Test taking

• Students in continental United States are expected to take the test at 2PM EDT.

• International students taking the test outside of the United States are expected to complete the test by Saturday 2PM EDT.

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What to expect for the test

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

• Ability to write functions using numbers, strings, lists, dictionaries, datasets, and random values.

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Built-in functions

• id( v ): returns indicator where in memory to find v.

• type( v ): returns the type of value stored in v.

• abs( v ): returns absolute value of v.

• len( s ): returns number of things making up s.

• min( s ): returns minimum value in s.

• max( s ): returns maximum value in s.

• sum( s ): returns the sum of s.

• int( s ): returns an integer constructed from s.

• float( s ): returns a decimal constructed from s.

• string( v ): returns a string constructed from s.

• sorted( s ): returns a sorted version of s.

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Functions

• Functions can be invoked.

• The invocation identifies the function to be started up.

• The invocation includes the arguments. The arguments are evaluated. Copies of the values are used to initialize the parameters specified in the function definition.

• Functions must be defined. A function definition consists of a header followed by its body.

• Header: def name ( parameters ) :

• Body: remaining lines in function. How can you tell? They are indented. Body is also known as the statement list

• A return is a special statement in the body of function. When a return statement is reached in a function, the function stops executing its instructions; that is, the function immediately stop its execution.

• A return statement can optionally have a return expression following the keyword return. The expression is evaluated. A copy of that value is the return value of the function. The return value is said to be the value of the evaluation of the function invocation.

• All Python functions return values – if no return statement with a return expression is executed. The function returns None.

• A local variable is a variable initialized/defined inside a function definition.

• Local variables only exist inside the function definition (limited scope)

• The parameters of a function are the variables listed with the parentheses in the function header.

• Parameters are local variables

• The arguments to a function are the values given within the parentheses of an invocation of the function.

• The Python way of handling the arguments is pass by value; that is, the arguments are evaluated to determine their values. Copies of those values are used to initialize the parameters.

• Because Python uses pass by value, the value of an argument is unchanged by a function invocation; that is, the value of an argument is the same after the invocation as it was before the invocation.

• Implication: not possible to write a swapping function in Python

• Because a parameter has the same value as its argument, if that argument is a list, the contents of that list can be modified. Such a change is called a side effect of the function. Functions with side effects generally have a return value of None.

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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.

• List gotchas

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

alist = alist.append( v ) # alist is no longer a list, it is None

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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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FWIW

• 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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© 2020 Jim Cohoon

Resources from previous semesters are available.