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How can I create a 2-way table in python? I have 2 categorical variables in a data set and would like to look at the relationship between the 2 variables by creating a 2-way table. Thank you.

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  • Possible Duplicate stackoverflow.com/questions/6129693/python-creating-a-table Commented Jun 19, 2013 at 21:19
  • 3
    Do you mean a bidirectional map? Commented Jun 19, 2013 at 21:21
  • 5
    Can you give us a sample data set? Commented Jun 19, 2013 at 21:21
  • will creating two dictionaries, each one the opposite of the other, i.e. the key in one is the value in the other solve your problem? Commented Jun 19, 2013 at 21:32
  • I thought it was obvious what you wanted, but I was obviously wrong. Can you please elaborate on what you mean by "two-way table"? Do you mean a table where you look up a value by row and column, or a bidirectional mapping as brice suggested? Commented Jun 19, 2013 at 21:34

6 Answers 6

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There's a bidict package:

>>> from bidict import bidict
>>> husbands2wives = bidict({'john': 'jackie'})
>>> husbands2wives['john']  # the forward mapping is just like with dict
'jackie'
>>> husbands2wives[:'jackie']  # use slice for the inverse mapping
'john'

You can install it using pip install bidict.

EDIT: For your actual problem - if I understand you correctly - I would use pandas:

# data.csv
Gender Height GPA HS GPA Seat WtFeel Cheat 
Female 64 2.60 2.63 M AboutRt No 1 
Male 69 2.70 3.72 M AboutRt No 2 
Female 66 3.00 3.44 F AboutRt No 3 
Female 63 3.11 2.73 F AboutRt No 4 
Male 72 3.40 2.35 B OverWt No 0

In [1]: import pandas as pd

In [2]: df = pd.read_csv('data.csv', sep = '\s')

In [3]: grouped = df.groupby(['Gender', 'Seat'])

In [4]: grouped.size()
Out[4]: 
Gender  Seat   
Female  AboutRt    3
Male    AboutRt    1
        OverWt     1
dtype: int64
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5 Comments

The 2 columns are Gender (M, F) and Body Figure (AboutRt, OverWt, UnderWt). All I want to do is count how many males are AboutRt, Overwt and underWt, and do the same for the females. I was hoping that there is a package in python similar to the gmodels library in R where you could create a cross table which is found in R. Thanks again.
@TarekDeeb -- please edit your question populating it with a small sample of your data...
@TarekDeeb -- You should probably take a look at pandas. Added an example...
data.head() Gender Height GPA HS GPA Seat WtFeel Cheat 0 Female 64 2.60 2.63 M AboutRt No 1 Male 69 2.70 3.72 M AboutRt No 2 Female 66 3.00 3.44 F AboutRt No 3 Female 63 3.11 2.73 F AboutRt No 4 Male 72 3.40 2.35 B OverWt No
@TarekDeeb -- Please, update your question, with well formatted data - don't paste it in the comments. This makes it much easier for both the people who answer your questions and for further readers, who may have a similar problem.
1

You may be able to use a DoubleDict as shown in recipe 578224 on the Python Cookbook.

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0

Probably the best solution in the standard library, if your data is moderately large, is to use sqlite, the in-memory database: http://docs.python.org/2/library/sqlite3.html

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0

Assuming you don't have to do any interpolation, you can use a dictionary. Use (x, y) tuples as the keys, and whatever your values are as the values. For instance, a trivial 2x2 table like this:

   ___0___1___
0 |   0   0.5
1 |   0.5 1

Would look like this in code:

two_way_lookup = {
                  (0, 0) : 0,
                  (0, 1) : 0.5,
                  (1, 0) : 0.5,
                  (1, 1) : 1
                 }
print(two_way_lookup.get((0, 1))) # prints 0.5
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5 Comments

This doesn't look like it actually responds to the question.
Dicts provide only one way lookup, OP asked for 2-way table.
@Marcin (& Ashwini) I understand "2-way table" to mean "table where you look up x and y to get a value representing something about that combination of elements." It is not, at least in my mind, the same thing as 2-way lookup, where key/value pairs can be used in either direction.
"would like to look at the relationship between the 2 variables by creating a 2-way table. Thank you." Suggests that OP wants two-way lookup.
@Marcin That's exactly the line that I think supports my interpretation. The asker presumably has two keys already, and wants to look up the relationship between them.
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You can create something like a two-level dict (that is, a dict which comprehends two dicts that map the same data in reverse order:

>>> mappings=[(0, 6), (1, 7), (2, 8), (3, 9), (4, 10)]
>>> view = dict(view1=dict(mappings), view2=dict(reversed(k) for k in mappings))
>>> view
{'view2': {8: 2, 9: 3, 10: 4, 6: 0, 7: 1},
'view1': {0: 6, 1: 7, 2: 8, 3: 9, 4: 10}}
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1 Comment

This works well for a one-to-one function, but gets a bit tricky when you don't assume this condition. Ex: x = 4, y = (-2, 2).
0

If you want a home-brewed, wonky solution, you could do something like this:

import collections

class BDMap:
    def __init__(self):
        self.x_table = {}
        self.y_table = {}

    def get(self, x = None, y = None):
        if (x != None) and (y != None):
            y_vals = self.x_table[x]
            if (y in y_vals):
                return (x, y)
        elif x != None:
            return self.x_table[x]
        elif y != None:
            return self.y_table[y]

    def set(self, x, y):
        if isinstance(x, collections.Hashable) and isinstance(y, collections.Hashable):
            self.x_table[x] = self.x_table.get(x, list()) + [y]
            self.y_table[y] = self.y_table.get(y, list()) + [x]
        else:
            raise TypeError("unhashable type")

For anything other than a one-off script with a small data set, you're undoubtedly better off with one of approaches mentioned, though :)

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