How to store an array in a dictionary using python

Question

I am currently attempting to modify a series of programs by utilizing dictionaries as opposed to arrays. I have columns of raw information in a file, which is then read into an ASCII csv file. I need to convert this file into a dictionary, so that it can be fed into another program.

I used a numpy.genfromtxt to pull out the information i need from the csv file, following this format:

a,b,c,d = np.genfromtxt("file",delimiter = ',', unpack = true)

this step works completely fine.

I then attempt to build a dictionary:

ouputDict = dict([a,a],[b,b],[c,c],[d,d])

As i understand it, this should make the key "a" in the dictionary a correspond to the array "a".

thus if:

a = [1,2,3,4]

then:

outputDict[a][0] = 1

However, when i attempt to create this dictionary i receive the following error:

TypeError: unhashable type: 'numpy.ndarray'

Why can't I construct an array in this fashion and what is the workaround, if any? Any help will be greatly appreciated!

trying to build a dict that maps an array to the same array makes very little sense. I think you may have to rethink what inputs the other program requires. It seems that a list of arrays may make more sense. — cel
– cel, Commented Nov 11, 2015 at 21:47
i understand that you can't map an array to itself, and i apologize for any confusion. i was trying to make the keys have the same name as the arrays for convenience's sake — Jean Sylvian
– Jean Sylvian, Commented Nov 11, 2015 at 21:49
Did you try: {'a':a, 'b':b,...}, or dict(a=a, b=b,...)? dict([('a', a), ('b', b)]) should also work. — hpaulj
– hpaulj, Commented Nov 11, 2015 at 21:49
I agree with what cel said, trying to implement dictionary (which is not necessary) where the problem can be solved with an array is not a good way to code. — Harwee
– Harwee, Commented Nov 11, 2015 at 21:50
The problem is right now it's trying to set a numpy array as a key at some point and this won't work because python doesn't allow mutable objects as a key in a dict. That's where the unhashable exception is coming from — TayTay
– TayTay, Commented Nov 11, 2015 at 21:51

Aman Chawla · Accepted Answer · 2015-11-11 21:54:54Z

5

You can do this even with using collections

Declare your dictionary as:

Dictionary = {}; // {} makes it a key, value pair dictionary

add your value for which you want an array as a key by declaring

Dictionary[a] = [1,2,3,4]; // [] makes it an array

So now your dictionary will look like

{a: [1,2,3,4]}

Which means for key a, you have an array and you can insert data in that which you can access like dictionary[a][0] which will give the value 1 and so on. :)

Btw.. If you look into examples of a dictionary, array and key value pairs, nested dictionary, your concept will get clearer.

edited Nov 11, 2015 at 21:54

answered Nov 11, 2015 at 21:52

Aman Chawla

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2 Comments

Aman Chawla Over a year ago

Oh ya i get it now, well I just started explaining the basic format.. lol.. old habits

AaronJPung Over a year ago

This was really helpful, I hadn't been exposed to storing arrays in dictionaries before! You've solved my problem, thank you for posting @AmanChawla

hpaulj · Accepted Answer · 2015-11-11 21:54:28Z

2

Copied from my comment:

Correct dictionary formats:

{'a':a, 'b':b,...}, or 
dict(a=a, b=b,...)
dict([('a', a), ('b', b),...])

The goal is to make the strings 'a','b',etc the keys, not the variable values.

answered Nov 11, 2015 at 21:54

hpaulj

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1 Comment

Jean Sylvian Over a year ago

Feel like a rookie, spent like an hour staring at dictionary documentation... thanks!

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