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--- other:python:misc_by_jyp [2022/02/21 16:31]
jypeter [numpy related stuff] Added ufuncs
+++ other:python:misc_by_jyp [2022/02/21 16:46]
jypeter [numpy related stuff] Added reduceat
@@ Line 275: / Line 275: @@
 >>> sorted_vals[0:3].sum(), sorted_vals[3:6].sum(), sorted_vals[6:10].sum()
 (3.0, 4.5, 8.0)</code>
+=== Applying a ufunc over specified sections of an array ===
+The [[https://numpy.org/doc/stable/reference/generated/numpy.ufunc.reduceat.html#numpy.ufunc.reduceat|reduceat]] function can be used to avoid explicit python loops, and improve the speed (but not the readability...) of a script. The example below //improves// what has been shown above
+<code># Define a list with the boundaries of the intervals we want to apply the 'add' function to
+# We need to add the beginning index (0), AND remove the last index
+# (reduceat will automatically go to the end of the input array
+>>> nb_unique
+array([3, 3, 4])
+>>> slices_indices = [0] + list(np.add.accumulate(nb_unique))
+>>> slices_indices.pop() # Remove last element
+>>> slices_indices
+[0, 3, 6]
+# Compute the sums over the selected intervals with just one call
+>>> np.add.reduceat(np.sort(vals), slices_indices)
+array([3. , 4.5, 8. ])</code>
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