CODEWITHSUNDEEP
listpython-2.7indexing
Ting Ping
Ting Ping

Reputation: 1145

How do you find the negative index of a position if you know the index?

I have a string of variable length and I know the index position is 25. As it's variable in his length (>= 25), I need a way to locate the negative index of that same position for easier data manipulation.

Do you have any idea how this can be done?

Upvotes: 2

Views: 92

Answers (2)

Jirilmon
Jirilmon

Reputation: 1944

Lets use a 'list' with following values: '1', '2', '3','4', '5', '6'

Steps to get the negative index of any value:

Step1. Get the 'normal_index' of the value. For example the normal index of value '4' is 3.

Step2. Get the 'count' of the 'list'. In our example the 'list_count' is 5.

Step3. Get Negative index of the requested value. negative_index = (normal_index - list_count) - 1. Which is -3.

Upvotes: 1

RocketDonkey
RocketDonkey

Reputation: 37249

I'm not sure if this is what you're after, but if you have the index of a string, the 'negative' index is just the negative of the length of the string minus the index:

In [1]: import string

In [2]: s = string.ascii_letters

In [3]: s
Out[3]: 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'

In [4]: s[25]
Out[4]: 'z'

In [5]: -(len(s)-25)
Out[5]: -27

In [6]: s[-(len(s)-25)]
Out[6]: 'z'

Or using the example from the comments:

In [7]: s = range(1, 7)

In [8]: s[4]
Out[8]: 5

In [9]: neg = -(len(s)-4) # Here you would replace 4 with your index

In [10]: s[neg]
Out[10]: 5

Upvotes: 2

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