Syntheses of past Q & A

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Miscellaneous

• Nested for-loops usually wrong unless using a list-of-lists; e.g., a dataset

• Two lists? probably do not want one list inside another.

• Name of func with no return value? -> there is no general name in Python (they return None)

• Do not use input() in a function unless explicitly told to do so

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Argument versus parameter

• Defining: e.g., in def sqrt( x ) variable x is the parameter

• Parameters are variables defined in the parentheses of a function definition

• Using/Invoking: e.g., in math.sqrt( 3 ) literal 3 is the argument

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Parts of function

• First line: keyword def, then function name, then (, then parameter list, then ) :

• After first line:

• Indented

• The body of the function definition, which is a statement list

• return: a special statement in the body of the function that both ) gives a value and stops the function

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Local variable

• Variable that is defined inside a function

• Parameters are local variables that are initialized at the start of a function by copying the values of arguments

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

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When do functions have error versus None as output?

• To get None:

• Do not have a return statement

• To have a return with no value after it

• To get an error:

• Do something illegal inside the function

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print() versus return

• print:() display, output -- shows up in the console, not in the invoking python code

• return: produce, hand-back, give -- shows up in the invoking python code, not in the console

• We usually want return, not print() (unless we ask for print/display/output)

• Consider the following:

def f( x ) :

  print( x )

This function displays the value of x and returns None

• Consider the following:

def g( x ) :

  return x

This function does no display and returns the value of its parameter

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How can a function change the contents of argument memory?

• Examples

def h ( my_list, my_dictionary ) :

  my_list[ index ] = ...

  my_list.remove( ... )

  my_list.append( ... )

  my_list.pop( ... )

  my_list.sort()

  my_dictionary[ key ] = value

• If we do not tell you to modify a parameter, don’t

• This means do not use the above statements on the parameter

• Does that mean we can

def f( x ) :

  y = x

  y.pop()

No! The assignment of x to y means modifying y affects x

• Does that me we can

def f( x ) :

  y = x[ : ] # or y = list( x ) or y = x.copy()

  y.pop()

Yes! The assignment to y makes it a copy of x and not x itself

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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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Consider

def g( x ) :

  x = [ 1112 ]

x = [3,1,4]

g( x )

Neither x nor its contents have changed

• If we run

def g( x ) :

  x = [ 1112 ]

  return x

x = [3,1,4]

y = x

x = g( x )

Then because of the return, x has changed; i.e., it points to a different list

Also, y has not changed

• If we run

def g( x ) :

  x = [ 1112 ]

x = [ 3, 1, 4 ]

y = x

x = g( x )

Then x is None

Also, y has not changed

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Invocation (examples)

• if you see identifier( and it is not after a def, it is an invocation

len( x )

math.sqrt( 4 )

sorted( x )

math.sqrt( len( x ) )

f( x )

f()

def f(): # not an invocation

  return 3 # not an invocation

g = f # not an invocation (and probably a typo)

g = f() # an invocation

s = 'hi there' # not an invocation

s.lower # not an invocation

s.lower() # an invocation, but does not change anything...

s = s.lower() # an invocation, and probably what you meant to write

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Invocation versus argument

• Invocation uses a function to do work

• Argument is the information passed into the invocation

• math.sqrt( 4 ) is an invocation of function math.sqrt(), with argument 4

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While loops examples

x = 3

while ( x > 0 ) :

  x = x - 1

text = input( 'Type the third positive integer: ' )

while ( text != '3' ) :

  text = input( 'Type the third positive integer: ' )

while ( 2 < 3 ) :

  print( 'this will print again and again forever' )

while 2 > 3:

  print( 'this will never be printed' )

• Can we use while in a function?

• Yes.

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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 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 (change find key for value 18)

found = None

for key in d.keys():

  value = d[ key ]

  if value == 18:

  found = key

 

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© 2019 Jim Cohoon

Resources from previous semesters are available.