Metal - 1 year ago 113

Python Question

If I want to extract a vector of certain size from a matrix, how do I do it?

I'm looking for something that does exactly the same thing as

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Answer Source

It turns out that a lot of the use-cases of `wkeep`

can be written more idomatically:

```
X[1:3,1:4] # wkeep(X, [2, 3])
```

If you don't actually need it to be centered, you could use:

```
X[:2, :4] # wkeep(X, [2, 3], 'l')
X[-2:, -4:] # wkeep(X, [2, 3], 'r')
```

Or if the real reason you're using wkeep is to trim off a border:

```
X[2:-2,2:-2] # wkeep(X, size(X) - 2)
```

*If you really want a direct translation* of `wkeep(X,L)`

, here's what `wkeep`

seems to do:

```
# Matlab has this terrible habit of implementing general functions
# in specific packages, and naming after only their specific use case.
# let's pick a name that actually tells us what this does
def centered_slice(X, L):
L = np.asarray(L)
shape = np.array(X.shape)
# verify assumptions
assert L.shape == (X.ndim,)
assert ((0 <= L) & (L <= shape)).all()
# calculate start and end indices for each axis
starts = (shape - L) // 2
stops = starts + L
# convert to a single index
idx = tuple(np.s_[a:b] for a, b in zip(starts, stops))
return X[idx]
```

So for example:

```
>>> X = np.arange(20).reshape(4, 5)
>>> centered_slice(X, [2, 3])
array([[ 6, 7, 8],
[11, 12, 13]])
```

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