On Tue, Aug 12, 2014 at 11:35 AM, Warren Weckesser < warren.weckesser@gmail.com> wrote:
I created a pull request (https://github.com/numpy/numpy/pull/4958) that defines the function `count_unique`. `count_unique` generates a contingency table from a collection of sequences. For example,
In [7]: x = [1, 1, 1, 1, 2, 2, 2, 2, 2]
In [8]: y = [3, 4, 3, 3, 3, 4, 5, 5, 5]
In [9]: (xvals, yvals), counts = count_unique(x, y)
In [10]: xvals Out[10]: array([1, 2])
In [11]: yvals Out[11]: array([3, 4, 5])
In [12]: counts Out[12]: array([[3, 1, 0], [1, 1, 3]])
It can be interpreted as a multi-argument generalization of `np.unique(x, return_counts=True)`.
It overlaps with Pandas' `crosstab`, but I think this is a pretty fundamental counting operation that fits in numpy.
Matlab's `crosstab` (http://www.mathworks.com/help/stats/crosstab.html) and R's `table` perform the same calculation (with a few more bells and whistles).
For comparison, here's Pandas' `crosstab` (same `x` and `y` as above):
In [28]: import pandas as pd
In [29]: xs = pd.Series(x)
In [30]: ys = pd.Series(y)
In [31]: pd.crosstab(xs, ys) Out[31]: col_0 3 4 5 row_0 1 3 1 0 2 1 1 3
And here is R's `table`:
x <- c(1,1,1,1,2,2,2,2,2) y <- c(3,4,3,3,3,4,5,5,5) table(x, y) y x 3 4 5 1 3 1 0 2 1 1 3
Is there any interest in adding this (or some variation of it) to numpy?
Warren
While searching StackOverflow in the numpy tag for "count unique", I just discovered that I basically reinvented Eelco Hoogendoorn's code in his answer to http://stackoverflow.com/questions/10741346/numpy-frequency-counts-for-uniqu.... Nice one, Eelco! Warren