代写data留学生、代做Python，c/c++程序设计、代做Java/Python

日期：2019-10-09 10:52

A Counter class

Your first task is to define a class Counter for a counter that counts down. When

it gets down to 0 and counts down again, it wraps back to a limit, minus 1. For

example, if the limit is 60, the counter's value is 0, and it counts down, its next

value is 59.

The Counter class must support the following methods:

__init__(self, limit, initial = 0, min_digits = 1) is automatically called by the

Counter constructor. The parameters are the counter's limit (60 in the example

above), an initial value, and a minimum number of digits (explained below). If

the initial value supplied as a parameter is between 0 and limit-1, then that's the

counter's initial value. Otherwise, an error message is printed and the counter's

initial value is limit-1.

get_value(self) just returns the counter's value, as an int.

__str__(self) returns the counter's value, as a string. If the counter's value would

print with fewer than the min_digits parameter supplied to the __init__ method,

then the string contains min_digits characters, padded on the left with zeros.

For example, if min_digits is 4 and the counter's value is 15, then the __str__

method should return the string 0015. (Hint: you can determine the length of the

counter's value as a string (by calling the str function on the counter's value),

and then concatenate as many zeros as necessary (the difference between

min_digits and the length of this string).)

tick(self) decrements the counter's value, but if the value would go negative,

then it becomes limit-1. For example, if the counter's limit is 12 and its current

value is 0, then after calling tick, the counter's value should be 11. This method

also returns to its caller a boolean value indicating whether it wrapped around,

i.e., did the value wrap from 0 back to limit-1. True means that it wrapped, False

means that it did not. We'll see below why we want this extra boolean value.

A Counter driver

A driver is a piece of code that exists solely to exercise some other piece of

code. A driver is typically not interesting in and of itself.

Write a driver for the Counter class. It should create two or more Counter

objects and demonstrate that all the methods in the Counter class work properly

by printing to the console.

A Timer class

Your next task is to define a class Timer for a 24-hour hours:minutes:seconds

timer that counts down. It must have instance variables that reference Counter

objects for the hours, minutes, and seconds. Your Timer class must support the

following methods:

__init__(self, hours, minutes, seconds) creates a timer whose initial values for

its hours, minutes, and seconds are given by the parameters. This method must

create three Counter objects and save the references given back by the

Counter constructors in instance variables for the Timer object. If any of the

values given for hours, minutes, or seconds is out of range, then the initial value

used should be the value that the corresponding Counter object would use.

__str__(self) returns a string giving the timer's current time, in the format

hh:mm:ss. That is, two digits for the hours, followed by a colon, followed by two

digits for the minutes, followed by a colon, followed by two digits for the

seconds. For example, if the timer currently reads 1 hour, 7 minutes, and 52

seconds, __str__ should return the string 01:07:52.

tick(self) ticks down the timer by one second. For example, if a timer reads

01:07:52, then after calling tick, it should read 01:07:51. If a timer reads

01:07:00, then after calling tick, it should read 01:06:59. If a timer reads

01:00:00, then after calling tick, it should read 00:59:59. Finally, if a timer reads

00:00:00, then after calling tick, it should read 23:59:59.

is_zero(self) returns a boolean that is True if the timer currently reads 00:00:00.

These methods should call the appropriate methods on the appropriate Counter

objects.

A Timer driver

Finally, write a driver for the Timer class. It should start a timer at 01:30:00 and

count it down to 00:00:00, printing to the console all the times between and

including 01:30:00 and 00:00:00.

Important note about the tick method of the Timer class

Remember that the tick method of Counter not only ticks the Counter down, but

it also returns a boolean telling you whether the Counter wrapped. If you call the

tick method of Counter on the seconds counter, and this call returns True, then

you need to call the tick method on the minutes counter. And if you call the tick

method of Counter on the minutes counter, and this call returns True, then you

need to call the tick method on the hours counter. What about if you call the tick

method of Counter on the hours counter, and this call returns True? It doesn't

matter; you don't have to do anything when the hours counter wraps.

Because each call of the tick method of Counter does two things—decrements

the Counter's value and returns a boolean telling you whether the Counter

wrapped--you need exactly one call of tick for each of the three Counter objects

each time you tick the Timer. I was able to write the body of the tick method of

Timer in three lines, each containing a call of tick on a Counter (and possibly

some other actions as well).