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I'm new to python and need to create a list with a negative index but have not been successful so far.
I'm using this code:
a = []
for i in xrange( -20, 0, -1 ):
a[i] = -(i)
log.info('a[{i}]={v}'.format(i=i, v=a[i]))
else:
log.info('end')
and getting the log output as
end
Incidentally I'm using a site call quantopian so the log.info is from their infrastructure and just print out the output into a web console.
What am I doing wrong?
Thanks in advance for your help.
499
Python3. In this, we reverse the list using slicing, and use ~ operator to get negation, index() is used to get the desired negative index.
Negative Index As an alternative, Python supports using negative numbers to index into a string: -1 means the last char, -2 is the next to last, and so on. In other words -1 is the same as the index len(s)-1, -2 is the same as len(s)-2.
Python also allows you to index from the end of the list using a negative number, where [-1] returns the last element. This is super-useful since it means you don't have to programmatically find out the length of the iterable in order to work with elements at the end of it.
You can't create a list with negative indices. They start with zero and count up by 1. If you use a negative index when accessing an element, Python converts that to a positive index relative to the end of the list.
If you are using Quantopian, it is advisable that you become familiar with numpy and pandas. For example:
>>> import numpy as np
>>> -1*np.arange(20)
array([ 0, -1, -2, -3, -4, -5, -6, -7, -8, -9, -10, -11, -12,
-13, -14, -15, -16, -17, -18, -19])
Then you will have a[1]==-1, a[5]==-5, etc.
181
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