Rob - 1 year ago 94

Python Question

If I have a list like [1, 2, 3, 4, 1, 2, 3, 4] and I wanted to sum the values like:

`[6, 9, 8, 6, 7, 4]`

where the formula is

`value + value_one_ahead + value_two_ahead`

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Answer Source

The operation can be interpreted as a (discrete) convolution of the input with [1, 1, 1], so you can use `numpy.convolve`

:

```
In [178]: x = np.array([1, 2, 3, 4, 1, 2, 3, 4])
In [179]: np.convolve(x, np.ones(3, dtype=x.dtype), mode='valid')
Out[179]: array([6, 9, 8, 7, 6, 9])
```

See the docstring for the description of the `mode`

argument. You might have to tweak the return value or add a line or two of additional code if the available `mode`

options don't handle the ends of the input the way you want. For example, to get something closer to what you wrote in the question, you can use `mode='full'`

(which is the default), and then discard the first two elements (by taking the slice `[2:]`

):

```
In [181]: np.convolve(x, np.ones(3, dtype=x.dtype), mode='full')[2:]
Out[181]: array([6, 9, 8, 7, 6, 9, 7, 4])
```

Note: `np`

was defined using `import numpy as np`

. `np.ones(3, dtype=x.dtype)`

creates an array of 1s with the given length and data type:

```
In [180]: np.ones(3, dtype=x.dtype)
Out[180]: array([1, 1, 1])
```

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