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I need to find the longest contiguous subsequence in a rising sequence in Python.

For example if I have A = [1, 2, 3, 5, 8, 9, 11, 13, 17, 18, 19, 20, 21, 25, 27, 28, 29, 30]

The answer would be [17, 18, 19, 20, 21] because it's the longest contiguous subsequence with 5 numbers (whereas [1, 2, 3] is 3 numbers long and [27, 28, 29, 30] is 4 numbers long.)

My code is stuck in an endless loop

num_list = [1, 2, 3, 5, 8, 9, 11, 13, 17, 18, 19, 20, 21, 23, 25, 26, 27]
longest_sequence = {}
longest_sequence_length = 1
for num in num_list:
    sequence_length = 1
    while True:
        if (num + sequence_length) in num_list:
            sequence_length += 1
        else:
            if sequence_length > longest_sequence_length:
                longest_sequence_length_length = sequence_length
                longest_sequence = {"start": num, "end": num + (sequence_length - 1)}
        break
print(f"The longest sequence is {longest_sequence_length} numbers long"
      f" and it's between {longest_sequence['start']} and {longest_sequence['end']}")
swapman
  • 11
  • 2

2 Answers2

2

You can use numpy to solve it in one line:

import numpy as np
A = [1, 2, 3, 5, 8, 9, 11, 13, 17, 18, 19, 20, 21, 25, 27, 28, 29, 30]
out = max(np.split(A, np.where(np.diff(A) != 1)[0] + 1), key=len).tolist()

You can also find the same outcome by running 3 iterations.

(i) First you need to find the differences between consecutive elements in A; that's found in diff (with zip(A,A[1:]), you can access consecutive elements).

(ii) Then you split A on indices where the difference is not 1; that's being done in the second iteration. Basically, if a difference is 1, append the value in A to the running sublist, if not, create a new sublist and put the corresponding value to this new sublist.

(iii) Finally, using max() function, you can find the longest sublist using key=len.

This exact same job is done by the numpy code above.

diff = [j-i for i,j in zip(A, A[1:])]
splits = [[A[0]]]
for x,d in zip(A[1:], diff):
    if d == 1:
        splits[-1].append(x)
    else:
        splits.append([x])
out = max(splits, key=len)

Output:

[17, 18, 19, 20, 21]

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In line 13 you need a break instead of a continue statement.

Also, in line 11 you had a little mistake, added an extra "_length" to you variable name.

Ztork
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