Class 37 — Wednesday, April 20

I know that you know that

Programming basics — Your semester achievement — Bask in the success

Look both ways

• Back or ahead

Agenda

• Taking stock

Downloads

• Module but_its_on_sale.py

• Module naming_names.py

• Module step_case.py

• Module the_average_thing.py

but_its_on_sale.py - functions and optional parameters

• This function takes one required parameter, original_price, and one optional parameter, percent_off.

• The function then determines the final cost of the item, after subtracting the percent off sale from the original price.

• If the item is not on sale, then the percent_off parameter is equal to 100 by default.

• The tester code defined in the same file should run as follows:

A $ 50 item on sale for 40 % off will cost $ 30.0

A $ 10 item on sale for 15 % off will cost $ 8.5

A $ 50 item on sale for 15 % off will cost $ 42.5

A $ 10 item on sale for 40 % off will cost $ 6.0

A $ 10 item on sale for 0 % off will cost $ 6.0

naming_names.py - while looping

• The user will provide a specific desired name, then a list of names that includes the desired name, and a random seed. Then, the program will print a random names one by one from the list until it equals the chosen name/word, which will be the final name to be printed out.

• Some sample runs are as follows:

Enter desired name: Janine

Enter list of names: Laura Michael Elizabeth Isabelle Janine

Enter random seed: 52

Isabelle

Janine

Enter desired name: Michael

Enter list of names: Laura Michael Elizabeth Isabelle Janine

Enter random seed: 52

Isabelle

Janine

Laura

Elizabeth

Isabelle

Elizabeth

Isabelle

Janine

Laura

Laura

Janine

Elizabeth

Laura

Elizabeth

Janine

Laura

Laura

Laura

Michael

step_case.py - functions and strings

• The function will have a string as its only parameter.

• The function will then create a new string, which will be a copy of the parameter, except all characters in the first half of the string will be in upper-case, and all characters in the second half of the string will be in lower case.

• If there are an odd number of characters, then there will be one more lower-cased character than lower-case character.

• The provided tester code in the file will run as follows:

FLORIDA georgia = step_case("Florida Georgia")

MICHIGan ohio = step_case("MICHIGAN Ohio")

VA MAryland = step_case("va maryland")

the_average_thing.py - while loops and user-inputted dictionary

The user inputs pairs of grocery store items and costs, and the program creates a dictionary representation of this user-specified inventory. When the user enters the empty string, the dictionary is fully built. Next, the program will calculate and print out the average cost of all the items. Finally, the program will print out the item that costs closest to the average price, along with that item's price.

Past homework

• Solutions are available

Test taking

• Unless an accomodation has been granted students are expected to take the test in class on Friday.

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.

• Simple image manipulation

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.

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.

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

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.

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

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

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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🦆 © 2022 Jim Cohoon

Resources from previous semesters are available.