Finding the nearest value and return the index of array in Python

by

This is similar to using bisect_left, but it’ll allow you to pass in an array of targets 

def find_closest(A, target):
    #A must be sorted
    idx = A.searchsorted(target)
    idx = np.clip(idx, 1, len(A)-1)
    left = A[idx-1]
    right = A[idx]
    idx -= target - left < right - target
    return idx

Some explanation:

First the general case: idx = A.searchsorted(target) returns an index for each target such that target is between A[index - 1] and A[index]. I call these left and right so we know that left < target <= right. target - left < right - target is True (or 1) when target is closer to left and False (or 0) when target is closer to right.

Now the special case: when target is less than all the elements of A, idx = 0. idx = np.clip(idx, 1, len(A)-1) replaces all values of idx < 1 with 1, so idx=1. In this case left = A[0], right = A[1] and we know that target <= left <= right. Therefor we know that target - left <= 0 and right - target >= 0 so target - left < right - target is True unless target == left == right and idx - True = 0.

There is another special case if target is greater than all the elements of A, In that case idx = A.searchsorted(target) and np.clip(idx, 1, len(A)-1)
 replaces len(A) with len(A) - 1 so idx=len(A) -1 and target - left < right - target ends up False so idx returns len(A) -1. I’ll let you work though the logic on your own.

For example:

In [163]: A = np.arange(0, 20.)

In [164]: target = np.array([-2, 100., 2., 2.4, 2.5, 2.6])

In [165]: find_closest(A, target)
Out[165]: array([ 0, 19,  2,  2,  3,  3])

More Related Contents:

  1. Change sign of elements with an odd sum of indices
  2. Python Lists and tuples
  3. How do you fix “runtimeError: package fails to pass a sanity check” for numpy and pandas?
  4. Rolling window for 1D arrays in Numpy?
  5. Windows Scipy Install: No Lapack/Blas Resources Found
  6. FutureWarning: elementwise comparison failed; returning scalar, but in the future will perform elementwise comparison
  7. What is vectorization? [closed]
  8. Calculate average of every x rows in a table and create new table
  9. Conditionally fill column values based on another columns value in pandas
  10. Import Error: No module named numpy
  11. Installing scipy in Python 3.5 on 32-bit Windows 7 Machine
  12. Transform a set of numbers in numpy so that each number gets converted into a number of other numbers which are less than it
  13. Python’s sum vs. NumPy’s numpy.sum
  14. numpy is already installed with Anaconda but I get an ImportError (DLL load failed: The specified module could not be found)
  15. Why np.hypot and np.subtract.outer very fast compared to vanilla broadcast ? Using Numba for speedup numpy in parallel for distance matrix calculation
  16. scipy curve_fit doesn’t like math module
  17. Is there anything faster than dict()?
  18. TypeError: only integer scalar arrays can be converted to a scalar index with 1D numpy indices array
  19. Are numpy’s basic operations vectorized, i.e. do they use SIMD operations?
  20. Type hinting / annotation (PEP 484) for numpy.ndarray
  21. Error “Import Error: No module named numpy” on Windows
  22. How to share numpy random state of a parent process with child processes?
  23. pandas.read_csv from string or package data
  24. How to replicate array to specific length array
  25. Python pandas: how to remove nan and -inf values
  26. Loading text file containing both float and string using numpy.loadtxt
  27. AttributeError: module ‘numpy’ has no attribute ‘__version__’
  28. In sklearn.decomposition.PCA, why are components_ negative?
  29. Is this the best way to add an extra dimension to a numpy array in one line of code?
  30. Adding a data file in Pyinstaller using the onefile option

Leave a Comment