Adding numbers in a loop python

Question

Here is my conundrum.Imagine you have a loop and a list with 100 members in python like the following:

myList=range(100)

for i in range(100):
    print(myList[i]) 
    print(myList[i+5])
    print(myList[i-5])

How would one prevent the index of myList from exceeding the length of the list or going under 0?

For example, if the index in the loop was 99 then:

print(myList[i+5])

would return an index error.

To clarify, I mean so that I could find out what [i+5] as if the list was on a loop.

So if you imagine that the index was 99 (again) I would like to go to the 4th index (after adding 5 and looping back to the start).

I should have made this clearer.I am using this for a game which loops the last pixel with the start pixel so I want to be able to move the index past the end of the list and start at the start again. — willmac
– willmac, Commented Apr 2, 2014 at 15:45

thefourtheye · Accepted Answer · 2014-04-02 15:45:50Z

2

The straight forward way would be to start from 5 and end at the length of the list - 5, like this

for i in range(5, len(myList) - 5):
    print(myList[i]) 
    print(myList[i+5])
    print(myList[i-5])

Or you can make sure that the index is valid before accessing the element at that location, like this

for i in range(100):
    print(myList[i])
    if i + 5 < len(myList):
        print(myList[i+5])
    if i - 5 >= 0:
        print(myList[i-5])

Edit: If you like to wrap around and start from the other end, once you reached an end, then you can use modulo operator, like this

for i in range(100):
    print(myList[i])
    print(myList[(i + 5) % len(myList)])
    print(myList[(i - 5) % len(myList)])

edited Apr 2, 2014 at 15:45

answered Apr 2, 2014 at 15:40

thefourtheye

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Selcuk · Accepted Answer · 2014-04-02 15:45:14Z

1

You could also define a "safe" print function:

EDIT: After your clarification, I modified my answer to be circular

def safe_print(mylist, myindex):
    print mylist[myindex % len(mylist)]

myList=range(100)

for i in range(100):
    safe_print(myList, i)
    safe_print(myList, i+5)
    safe_print(myList, i-5)

answered Apr 2, 2014 at 15:45

Selcuk

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