starting at the beginning of a list when index is out of range?

Question

I have made a list in python and I want to find an item in that list by an index number. However, I also have to use index numbers larger than the list. So for example:

list = [a, b, c, d]
for i in list:
   number_that_i_want_returned = list.index(i) + int(5)
   print(list[number_that_i_want_returned])

I'd like this to return the corresponding value in the list, and if the index number is larger than the remaining items in the list to start over at the beginning. However, this gives me a list index out of range. Is there any way I can make this work? Thanks in advance.

EDIT: This is my full program so far. I'm trying to create a ceasar cipher encrypter.

program_name = 'ceasar cipher\n'
print (program_name.upper())

org_line = input('enter the line to encrypt here: ')

decrypt_num = input('''by how many characters do you want to shift?
                    enter here: ''')

alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 
            'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
for char in org_line:
    new_pos = alphabet.index(char) + int(decrypt_num)
    print(alphabet[new_pos])

so now if I input for example 30 it says: list index out of range I'm new to python...

Use `% len(list)` to wrap around the list (but don't call it `list` because that overwrites a builtin). — ggorlen, Jul 10 '20 at 16:26
I still don't understand. Maybe some more background information will help. I'll edit it into the post. — Jordhi Obdam, Jul 10 '20 at 16:33
Beside the point, but you can use [`string.ascii_lowercase`](https://docs.python.org/3/library/string.html#string.ascii_lowercase) instead of `alphabet`. — wjandrea, Jul 10 '20 at 16:40

score 3 · Answer 1 · answered Jul 10 '20 at 16:28

3

Use modulo (% operator), for example:

>>> x = ['a', 'b', 'c', 'd']
>>> n = len(x)
>>> x[5 % n]
'b'

answered Jul 10 '20 at 16:28

wjandrea

28,235
9
60
81

score 0 · Answer 2 · edited Jul 10 '20 at 16:34

0

Use:

list = [a,b,c,d]
for i in list:
   number_that_i_want_returned = list.index(i) + int(5)
   if number_that_i_want_returned > len(list)-1:
       number_that_i_want_returned %= len(list)  # add this
   print(list[number_that_i_want_returned])

Output:

b
c
d
a

edited Jul 10 '20 at 16:34

wjandrea

28,235
9
60
81

answered Jul 10 '20 at 16:28

Sanket Singh

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26

starting at the beginning of a list when index is out of range?

2 Answers2