On using a string as an integer counter (aka index) in a for loop

Question

I have a folder with n csv files. They are accessed and read in a for loop like this:

#do stuff
for file in os.listdir(directoryPath):
    if file.endswith(".csv"):
       #do stuff

Outside of the for loop are a number of numpy arrays initialized to zero. Each array has to carry a specific value located in the current csv file.

My question: is it possible to use file, which I assume is a string, as an integer, in order to fill my arrays for instance like this: array1[file]=numpy.genfromtxt(file,delimiter=',')[:,2]?

I am afraid this very line does not work as file cannot be treated like an integer, so how would you deal with this? Thank you!

Yes, an index. In my view, a[file] should work as a[i] but of course i was not "declared" in the for loop so I got stuck. — FaCoffee
– FaCoffee, Commented Nov 4, 2015 at 10:32

Will Vousden · Accepted Answer · 2015-11-04 22:22:06Z

1

You could use enumerate, which generates tuples that pair the index of an item with the item itself:

for i, file in enumerate(os.listdir(directoryPath)):
    if file.endswith(".csv"):
       array1[i] = numpy.genfromtxt(file, delimiter=',')[:,2]

Or you could store the Numpy arrays in a dictionary that is indexed directly by the associated file name:

arrays = {}
for file in os.listdir(directoryPath):
    if file.endswith(".csv"):
       arrays[file] = numpy.genfromtxt(file, delimiter=',')[:,2]

Or with an OrderedDict:

from collections import OrderedDict

arrays = OrderedDict()
for file in os.listdir(directoryPath):
    if file.endswith(".csv"):
       arrays[file] = numpy.genfromtxt(file, delimiter=',')[:,2]

edited Nov 4, 2015 at 22:22

answered Nov 4, 2015 at 10:32

Will Vousden

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8 Comments

FaCoffee Over a year ago

I assume the resulting arrays, if created as dicts, will not be sorted smallest to largest in terms of key. It's important for me to have them sorted. Should I create a new for which sorts the dicts or should I make sure their keys are sorted smallest to largest upon filling them? Thanks!

FaCoffee Over a year ago

But since the file names are used as keys, isn't it true that if the former are sorted event_SMALLEST to event_LARGEST, the latter will automatically follow this order?

Will Vousden Over a year ago

@FrancescoCastellani Python dictionaries are unordered; you can't preserve a given ordering of key/value pairs when storing them in a dictionary. You can use OrderedDict instead, which remembers the order in which you added items (which will be useful if they're sorted already).

Will Vousden Over a year ago

@FrancescoCastellani Also see this: stackoverflow.com/questions/9001509/…

Will Vousden Over a year ago

@FrancescoCastellani Alternatively, just use sorted(arrays) to get a sorted list of keys, and use them to access the items of arrays in order.

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