我哪里出错了 Sentinel 或破坏了这个 Python 代码

Question

Okay this program is almost done but it keeps running. Need to figure out where the sentinel or break needs to go. This is what I have so far in Python. The program is suppose to read all the Pres on the list and print them. Then there is a slice removing the first two and last two.Then it prints the list size is 6 and then proceeds to give me the 6 Pres in alpha reverse order. I have everything working other than its repeating the end indefinitely. There has to be a while loop to display the elements in the list returned by playlist.

Pres = ['Kennedy','Johnson','Nixon','Ford','Carter','Reagan',\
        'Bush','Clinton','Bush','Obama'] 
Pres2 = Pres[2:8]

def main():

    for names in Pres:

        print(names)


    PR3=playlist(Pres2)
    while playlist !='6':
            for PR3 in Pres2:
                    print(PR3)


def playlist(PR):

    size = len(PR)
    print('list size is now', size)

    PR.sort()
    PR.reverse()
    return PR

main()

Now this is what I should get when I run it.

Kennedy
Johnson
Nixon
Ford
Carter
Reagan
Bush
Clinton
Bush
Obama
list size is now 6
Reagan
Nixon
Ford
Clinton
Carter
Bush

But instead after List size is now 6, the last 6 Presidents just keeps repeating. And it needs to read vertically on its own line.

Answer 1

It appears that you have some problems understanding even the basics of programming. I strongly encourage you to read again the courses that you had already read.

Nevertheless, here are some explanations:

• playlist is actually a function. This while playlist !='6' loop is just verifying that this function object is not a string. It is always different, so. If you want to compare the result of the function, you have to call it: playlist(PR) , this will execute the function and return a list that you can store in a variable.

• Your playlist function is returning a list object. Why do you try to compare it with '6' ? Moreover, '6' is not the length of your list. It is a string. 6 is the length of your list as it is an int.

• Why do you use a while loop since you only want the six Presidents to being displayed once? This does not make sense. A loop is for actions which need to be repeated an unknown number of times.

• As Pres2 is sliced from Pres at the beginning, its length is 6. Its length has never been higher, and even assuming that your loop had been properly drafted, the code inside would never have been executed.

• When you use for loop to iterate trough a list, the variable written after for is used as an alias which represents the current object iterated from your list. for PR3 in Pres2: is weird as you already defined a variable called PR3 . If you want to display president inside the list, then you just as to use for name in PR3 as you did before.

Let me show you an enhanced version of the function, hoping that you manage to understand better how it works:

def main():

    # Print the 10 Presidents
    for name in Pres:
        print(name)

    # Print the size of the list which already contains 6 Presidents
    # Then, sort the list, reverse it and return it
    # The new list is store into PR3
    PR3 = playlist(Pres2)

    # Print the 6 reverse sorted Presidents
    for name in PR3:
        print(name)

Did you get it? You have absolutely no need to use a loop.

Answer 2

1. Write a single program named mylist.py as follows. Follow instructions carefully to avoid point deductions.

In the main function:

create a list that holds the surnames of recent USA presidents, starting with Kennedy and ending with Obama, in chronological order. use a for loop to iterate over the entire list, printing each president's name on its own line. make a slice by removing the first two presidents and the last two presidents from the list. pass the new slice as an argument to a custom function named playlist. use a while loop to display the elements in the list returned by playlist. In the playlist function:

print the size of the sliced list. Use a list function. sort the sliced list in reverse alphabetical order. return this list to main. Sample Output Original list in main: Kennedy Johnson Nixon Ford Carter Reagan Bush Clinton Bush Obama Not in main: list size is now 6 Back in main, list in reverse alpha order Reagan Nixon Ford Clinton Carter Bush

It calls for a while loop in the main function