Please could someone help me with getting an index of an item in a nested list where a certain condition is met using Python 2.7? I know that there are similar questions on StackOverflow about this but I can't seem to find good examples that deal with "nested" lists.
I have a list of data which is hundreds of thousands of lines long, in the format below:
data =[
["","","","28.04.2015 09:34:38",1.52411,1.52428,17],
["","","","28.04.2015 09:34:40",1.52415,1.52433,18],
["","","","28.04.2015 09:34:42",1.52425,1.52444,19],
["","","","28.04.2015 09:34:44",1.52417,1.52435,18],
["","","","28.04.2015 09:34:46",1.52421,1.52440,19],
["","","","28.04.2015 09:34:48",1.52426,1.52446,20],
["","","","28.04.2015 09:34:50",1.52429,1.52444,15],
["","","","28.04.2015 09:34:58",1.52423,1.52441,18],
["","","","28.04.2015 09:35:00",1.52416,1.52434,18],
["","","","28.04.2015 09:35:02",1.52416,1.52433,17],
["","","","28.04.2015 09:35:04",1.52416,1.52434,18],
["","","","28.04.2015 09:35:06",1.52406,1.52422,16],
["","","","28.04.2015 09:35:10",1.52406,1.52421,15],
["","","","28.04.2015 09:35:14",1.52427,1.52444,17],
["","","","28.04.2015 09:35:16",1.52424,1.52443,19],
["","","","28.04.2015 09:35:18",1.52434,1.52453,19],
["","","","28.04.2015 09:35:20",1.52434,1.52451,17],
["","","","28.04.2015 09:35:22",1.52438,1.52456,18],
["","","","28.04.2015 09:35:24",1.52432,1.52451,19],
["","","","28.04.2015 09:35:28",1.52445,1.52464,19],
["","","","28.04.2015 09:35:34",1.52435,1.52451,16],
["","","","28.04.2015 09:35:36",1.52432,1.52449,17],
["","","","28.04.2015 09:35:38",1.52429,1.52448,19]]
For each row I want to compare the data in "column 5" (the first col of decimal numbers) to a certain value (lets use 1.52440 as an example) and return the index of the first row where the data is greater than my certain value.
I have made code that does this the 'traditional' way using a for-row-in-data type loop, but I would like to use a better (faster) method if possible and cannot seem to produce the expected result.
The rather poor attempt that I have made so far is:
pricedata = [n[4] for n in data]
myindex = (x for x in enumerate(pricedata) if x > 1.5440).next()
The first row extracts the price data col as a new list. I am not sure this is really necessary but as my understanding of list comprehensions is poor I was trying to break things into steps I understand.
I dont really understand what the second line is doing, but it seems to return (0, 1.52411) - the first item in the list - regardless of what comparison value I enter.
I have also tried:
myindex = [x for x in enumerate(pricedata) if x > 1.5440][0]
and it seems to produce the same result.
I thought the comprehension was saying:
"Make a list of price's for each price you look at in the list-of-indexed-prices if the price-you-are-looking-at is greater than 1.5440", but it seems I am mistaken!
Please could someone point out the error of my ways and help me out? Thank you for any assistance!
Problem is, you're comparing a tuple with a float, since enumerate returns tuples of the index and item, however, tuples are deemed greater than floats in Python 2:
>>> () > 4.
True
Therefore, the first tuple produced by enumerate always yields a match.
To solve this, you should instead unpack the tuple first, and return the first matching index from your generator expression using next:
next(i for i, x in enumerate(data) if x[4] > 1.52415)
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