Stéphane Laurent - 8 months ago 194

Python Question

Is there a Python function similar to the expand.grid() function in R ? Thanks in advance.

(EDIT) Below are the description of this R function and an example.

`Create a Data Frame from All Combinations of Factors`

Description:

Create a data frame from all combinations of the supplied vectors

or factors.

> x <- 1:3

> y <- 1:3

> expand.grid(x,y)

Var1 Var2

1 1 1

2 2 1

3 3 1

4 1 2

5 2 2

6 3 2

7 1 3

8 2 3

9 3 3

(EDIT2) Below is an example with the rpy package. I would like to get the same output object but without using R :

`>>> from rpy import *`

>>> a = [1,2,3]

>>> b = [5,7,9]

>>> r.assign("a",a)

[1, 2, 3]

>>> r.assign("b",b)

[5, 7, 9]

>>> r("expand.grid(a,b)")

{'Var1': [1, 2, 3, 1, 2, 3, 1, 2, 3], 'Var2': [5, 5, 5, 7, 7, 7, 9, 9, 9]}

`>>> a = [1,2,3]`

>>> b = [5,7,9]

>>> expandgrid(a, b)

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

File "<stdin>", line 2, in expandgrid

NameError: global name 'itertools' is not defined

However the itertools module seems to be installed (typing

`from itertools import *`

Answer

Here's an example that gives output similar to what you need:

```
import itertools
def expandgrid(*itrs):
product = list(itertools.product(*itrs))
return {'Var{}'.format(i+1):[x[i] for x in product] for i in range(len(itrs))}
>>> a = [1,2,3]
>>> b = [5,7,9]
>>> expandgrid(a, b)
{'Var1': [1, 1, 1, 2, 2, 2, 3, 3, 3], 'Var2': [5, 7, 9, 5, 7, 9, 5, 7, 9]}
```

The difference is related to the fact that in `itertools.product`

the rightmost element advances on every iteration. You can tweak the function by sorting the `product`

list smartly if it's important.