Gale and shapley published their paper on the stable matching problem in 1962; but a version of their algorithm had already been in use for ten years by the national resident matching program, for the problem of assigning medical residents to hospitals.
basically, the situation was the following. there were m hospitals, each with a certain number of available positions for hiring residents. there were n medical students graduating in a given year, each interested in joining one of the hospitals. each hospital had a ranking of the students in order of preference, and each student had a ranking of the hospitals in order of preference. we will assume that there were more students graduating than there were slots available in the m hospitals. the interest, naturally, was in finding a way of assigning each student to at most one hospital, in such a way that all
available positions in all hospitals were filled. (since we are assuming a surplus of students, there would be some students who do not get assigned to any hospital.)
we say that an assignment of students to hospitals is stable if neither of the following situations arises.

• first type of instability: there are students s and s′, and a hospital h, so that
a) s is assigned to h, and
b) s′ is assigned to no hospital, and
c) h prefers s′ to s.

• second type of instability: there are students s and s′, and hospitals h and h′, so that
a) s is assigned to h, and
b) s′ is assigned to h′, and
c) h prefers s′ to s, and
d) s′ prefers h to h′.

so we basically have the stable matching problem, except that (i) hospitals generally want more than one resident, and (ii) there is a surplus of medical students.
show that there is always a stable assignment of students to hospitals, and give an algorithm to nd one.

Calculating the "total price" of an item is tedious, so implement a get_item_cost method that just returns the quantity times the price for an item. by the way, the technical term for this kind of instance method is an accessor method, but you'll hear developers calling them getters because they always start with "get" and they get some value from instance attributes. in order to make the items sortable by their total total price, we need to customize our class. search the lectures slides for "magic" to see how to do this. see section 9.8 for an additional reference. the receipt class: this will be the class that defines our receipt type. obviously, a receipt will consist of the items on the receipt. this is called the composition design pattern. and it is very powerful. instance attributes: customer_name : it is very important to always know everything you can about your customers for "analytics", so you will keep track of a string customer name in objects of type receipt. date : the legal team has required that you keep track of the dates that purchases happen for "legal reasons", so you will also keep track of the string date in objects of type receipt. cart_items : this will be a list of the items in the cart and hence end up on the receipt. methods: 1. create a default constructor that can take a customer name as an argument, but if it gets no customer name, it will just put "real human" for the customer_name attribute. it should also accept a date argument, but will just use the value "today" for the date instance attribute if no date is given. the parameters should be named the same as the instance attributes to keep things simple. 2. add_item : self-descriptive. takes a parameter which we hope beyond hope is of type itemtopurchase and adds it to the cart_items. returns none. 3. print_receipt : takes a single parameter isevil, with default value true. returns a total cost of all the items on the receipt (remember to factor in the quantity). prints the receipt based on the following specification: for example, if isevil is true, and customer_name and date are the default values: welcome to evilmart, real human today have an evil day! otherwise, it should print: welcome to goodgo, real human today have an good day! then the receipt should be printed in sorted order like we discussed earlier, but whether or not it starts with the highest cost (think reverse), depends on the value of isevil. if it is evil, then the lowest cost items should print first, but if it is good, then it will print the highest cost items first. (cost meaning price*quantity). remember to return the total cost regardless! your main() function: the main flow of control of your program should go in a main() function or the program will fail all the unit tests. get the name of the customer with the prompt: enter customer name: get the date with the prompt: enter today's date then, ask the question: are you evil? your program should consider the following as true: yeah yup let's face it: yes hint: what do these strings all have in common? your program should consider all the following as false: no nah perhaps but i'm leaning no (just be glad you don't have to handle "yeah no.") okay enough horsing around. (get it? aggies? ! horsing! ) next, in the main() function, you will have to create a receipt object and start adding things into it using an input-while loop. the loop will prompt the user for the item name exactly as in the previous zylab (9.11). but unlike the previous zylab, the loop will terminate only if an empty string is entered for the item name. then, the price and the quantity will be prompted for exactly as in the previous zylab. create the itemtopurchase objects in the same manner as the previous zylab, but don't forget to add them to the receipt using your add_item instance method. then, the items on the receipt should be printed with the same formatting as in the previous zylab, of course with either "good" or "evil" ordering. however, on the last line, the total should be printed as follows: where 10 is replaced by the actual total. sample run here is what a sample run of the final program should look like: enter customer name: nate enter today's date: 12/20/2019 are you evil? bwahahahaha yes enter the item name: bottled student tears enter the item price: 2 enter the item quantity: 299 enter the item name: salt enter the item price: 2 enter the item quantity: 1 enter the item name: welcome to evilmart, nate 12/20/2019 have an evil day! salt 1 @ $2 = $2 bottled student tears 299 @ $2 = $598 total: $600