[Tutor] Extract several arrays from a large 2D array

[Peter Otten]

Ek Esawi wrote:

> Hi All--

> I have a 2D array (2000, 4); an example is given abelow.  On the 1st
> column I have 15 variables, on the 2nd 4 variables. Ignore column 3 for
> now. I want a code that generate 4 arrays for each variable on the 1st
> column; each array contains all the values from column 4. Then I want to
> find the average of each array.

> For example; pick the 1st variable in column 1 which is 1; then pick the
> 1st variable on the 2nd column which is 5. Now I want an array that
> contains all the values on the 4th column that match variable 1 on the 1st
> and variable 5 on the 2nd column. I need to get the average of each array.
> 
> 
> A             b             c              d
> 
> 1              5              3              4
> 
> 1              3              2              7
> 
> 2              5              7              5
> 
> 3              2              8              5
> 
> 2              3              2              3

With pandas:

>>> df = pandas.DataFrame([
... [1, 5, 3, 4],
... [1, 5, 2, 2],
... [2, 5, 1, 2],
... [2, 3, 1, 2]],
... columns=["a", "b", "c", "d"])
>>> df.groupby(("a", "b"))["d"].mean()
a  b
1  5    3
2  3    2
   5    2
Name: d, dtype: int64