cf2 - 1 month ago 5x

Python Question

I'm hoping anybody could help me with the following.

I have 2 lists of arrays, which should be linked to each-other. Each list stands for a certain object.

`arr1`

`arr2`

For example:

`import numpy as np`

arr1 = [np.array([1, 2, 3]), np.array([1, 2]), np.array([2, 3])]

arr2 = [np.array([20, 50, 30]), np.array([50, 50]), np.array([75, 25])]

The arrays are linked to each other as in the

`1`

`arr1`

`20`

`arr2`

Example:

`array([[ 0, 20, 50, 30],`

[ 0, 50, 50, 0],

[ 0, 0, 75, 25]])

How would I link these two list of arrays and reshape them in the desired format as shown in the above example?

Many thanks!

Answer

Here's an almost* vectorized approach -

```
lens = np.array([len(i) for i in arr1])
N = len(arr1)
row_idx = np.repeat(np.arange(N),lens)
col_idx = np.concatenate(arr1)
M = col_idx.max()+1
out = np.zeros((N,M),dtype=int)
out[row_idx,col_idx] = np.concatenate(arr2)
```

*: Almost because of the loop comprehension at the start, but that should be computationally negligible as it doesn't involve any computation there.

Source (Stackoverflow)

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