MAS - 1 year ago 74
Python Question

# find the start position of the longest sequence of 1's

I want to find the start position of the longest sequence of 1's in my array:

``````a1=[0,0,1,1,1,1,0,0,1,1]
#2
``````

I am following this answer to find the length of the longest sequence. However, I was not able to determine the position.

Inspired by `this solution`, here's a vectorized approach to solve it -

``````# Get start, stop index pairs for islands/seq. of 1s
idx_pairs = np.where(np.diff(np.hstack(([False],a1==1,[False]))))[0].reshape(-1,2)

# Get the island lengths, whose argmax would give us the ID of longest island.
# Start index of that island would be the desired output
start_longest_seq = idx_pairs[np.diff(idx_pairs,axis=1).argmax(),0]
``````

Sample run -

``````In [89]: a1 # Input array
Out[89]: array([0, 0, 1, 1, 1, 1, 0, 0, 1, 1])

In [90]: idx_pairs # Start, stop+1 index pairs
Out[90]:
array([[ 2,  6],
[ 8, 10]])

In [91]: np.diff(idx_pairs,axis=1) # Island lengths
Out[91]:
array([[4],
[2]])

In [92]: np.diff(idx_pairs,axis=1).argmax() # Longest island ID
Out[92]: 0

In [93]: idx_pairs[np.diff(idx_pairs,axis=1).argmax(),0] # Longest island start
Out[93]: 2
``````
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