uqtredd1 - 2 years ago 105

Python Question

I'm writing a spline class in Python. The method to calculate the the spline interpolated value requires the index of the closest x data points. Currently a simplified version looks like this:

`def evaluate(x):`

for ii in range(N): # N = len(x_data)

if x_data[ii] <= x <= x_data[ii+1]:

return calc(x,ii)

So it iterates through the list of

`x_data`

`ii`

`x`

`calc`

`x_data`

`x`

Note:

`x_data`

`x_data[ii] < x_data[ii+1]`

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Answer Source

this is exactly what bisect is for https://docs.python.org/2/library/bisect.html

```
from bisect import bisect
index = bisect(x_data,x)
#I dont think you actually need the value of the 2 closest but if you do here it is
point_less = x_data[index-1] # note this will break if its index 0 so you probably want a special case for that
point_more = x_data[index]
closest_value = min([point_less,point_more],key=lambda y:abs(x-y))
```

alternatively you should use binary search(in fact im pretty sure thats what bisect uses under the hood) .... it should be worst case `O(log n)`

(assuming your input array is already sorted)

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