Franz - 1 year ago 68

Python Question

I try to create a meshgrid with n dimensions.

Is there a nicer way to call meshgrid with n column vectors than with the if clause I am using?

Edit: The goal is to use it for user-defined n (2-100) without writing 100 if clauses.

The second line in the if clauses reduces the grid so column(n) < column(n+1)

Example:

`import numpy as np`

dimension = 2

range = np.arange(0.2,2.4,0.1)

if dimension == 2:

grid = np.array(np.meshgrid(range,range)).T.reshape(-1,dimension)

grid = np.array(grid[[i for i in range(grid.shape[0]) if grid[i,0]<grid[i,1]]])

elif dimension == 3:

grid = np.array(np.meshgrid(range,range,range)).T.reshape(-1,dimension)

grid = np.array(grid[[i for i in range(grid.shape[0]) if grid[i,0]<grid[i,1]]])

grid = np.array(grid[[i for i in range(grid.shape[0]) if grid[i,1]<grid[i,2]]])

Edit: The solution was posted below:

`dimension = 2`

r = np.arange(0.2,2.4,0.1)

grid=np.array(np.meshgrid(*[r]*n)).T.reshape(-1,n)

for i in range(0,n-1):

grid = [g for g in grid if g[i]<g[i+1]]

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Answer Source

I haven't fully absorbed your approach and goals, but here's a partial simplification

```
In [399]: r=np.arange(3) # simpler range for example
In [400]: grid=np.meshgrid(*[r]*2) # use `[r]*3` for 3d case
In [401]: grid=np.array(grid).T.reshape(-1,2)
In [402]: np.array([g for g in grid if g[0]<g[1]]) # simpler comprehensions
Out[402]:
array([[0, 1],
[0, 2],
[1, 2]])
```

`itertools.product`

makes that 2 column grid easier:

```
In [403]: from itertools import product
In [404]: np.array([g for g in product(r,r) if g[0]<g[1]])
Out[404]:
array([[0, 1],
[0, 2],
[1, 2]])
```

That is, your `grid`

before filtering is

```
In [407]: grid
Out[407]:
array([[0, 0],
[0, 1],
[0, 2],
[1, 0],
[1, 1],
[1, 2],
[2, 0],
[2, 1],
[2, 2]])
```

and `product`

is

```
In [406]: list(product(r,r))
Out[406]: [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]
```

`product`

has a `repeat`

parameter that makes this even easier:

```
In [411]: list(product(r,repeat=2))
Out[411]: [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]
```

You still need the `if`

clause to apply the 2 step filtering for dim=3. I guess the could written iteratively

```
for i in range(0,dimension-1):
grid = [g for g in grid if g[i]<g[i+1]]
```

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