关于 python 列表中错误的快速提问 * 常量，例如 [a, b, c, d, e ] * x

Question

Quick question on a problem was doing previously. This question gives you a list of days from Mon - Sun, and you are supposed to create a list given 'n' (no of days ahead). So if n = 2, Monday will be Wednesday etc.

I tried this in python, and came up with this:

def after_n_days(days, n):
    day = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'] * (int(n/7)+1)
    return [day[day.index(i) + n] for i in days]

after_n_days(["Monday","Friday","Wednesday"], 15)

However, it seems i get an error when i multiply the list 'day' by (int(n/7) + 1 when n= 15 and int(n/7) + 1 =3. It works when I simply multiply the list by 3 though, so I'm wondering what's the issue here?

Many thanks everyone!!

Here is the error message

IndexError                                Traceback (most recent call last)
<ipython-input-124-481771160081> in <module>
      6 
      7 
----> 8 after_n_days(["Thursday", "Monday"], 4)
      9 after_n_days(["Monday","Friday","Wednesday"], 15)

<ipython-input-124-481771160081> in after_n_days(days, n)
      3 def after_n_days(days, n):
      4     day = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'] * (int(n/7)+1)
----> 5     return [day[day.index(i) + n] for i in days]
      6 
      7 

<ipython-input-124-481771160081> in <listcomp>(.0)
      3 def after_n_days(days, n):
      4     day = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'] * (int(n/7)+1)
----> 5     return [day[day.index(i) + n] for i in days]
      6 
      7 

IndexError: list index out of range

Answer 1

The problem is that you don't expand the list far enough. Multiplying a list by 1 is just the list. Any day, n combination that goes off the end of the list fails. For example,

after_n_days(["Thursday", "Monday"], 4)
after_n_days(["Thursday", "Monday"], 7 + 4)
etc...

You could just make the expanded list bigger

day = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 
        'Saturday', 'Sunday'] * (int(n/7)+2)

Or better, use the modulo operator to wrap the index instead.

def after_n_days(days, n):
    print(int(n/7)+1)
    day = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 
            'Saturday', 'Sunday']
    return [day[(day.index(i) + n) % len(day)] for i in days]

after_n_days(["Thursday", "Monday"], 4)
after_n_days(["Monday","Friday","Wednesday"], 15)
after_n_days(["Monday","Friday","Wednesday"], 18)