How to find the pattern in a list

Question

For example,

# list A
A = [1, 2, 3, 3, 4, 4, 4, 5, 6]

# list B
B = [1, 2, 3, 3, 4, 5, 5, 5, 6]

are given, and we have to check if elements of the lists have the pattern of [x, x, y, y, y].

list A should return "True":
[1, 2, 3, 3, 4, 4, 4, 5, 6]

list B should return "False" as ‘4’ intercepts in the middle: [1, 2, 3, 3, 4, 5, 5, 5, 6]

Asked By: CescoDesu

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Source

Answer 1

Just check every possible subarray of length 5 for the pattern. Straightforward way would be:

a = [1, 2, 3, 3, 4, 4, 4, 5, 6]
b = [1, 2, 3, 3, 4, 5, 5, 5, 6]


def is_pattern(alist):
    for i in range(0, len(alist) - 4):
        sub = alist[i:i + 5]
        if sub[0] == sub[1] and (sub[2] == sub[3] == sub[4]):
            return True
    return False


assert is_pattern(a) is True
assert is_pattern(b) is False

Answered By: funnydman

Answer 2

O(n) solution is to iterate the list and count X and Y appearances. Once it breaks the pattern, restore the values accordingly.

Once you find 2 or more appearances you can count Y appearances and return true once it reaches 3.

Otherwise, if count x is less than 2, restore it. If count x is 2 or more but current value doesn’t equal to the earlier y, set count x to be count y and set count y to be 1.

a = [1, 2, 3, 3, 3, 4, 4, 4, 5, 6]
b = [1, 2, 3, 3, 4, 5, 5, 5, 6]
c = [1,1,1,1,1]
d = [1, 2, 3, 3, 3, 4, 4, 5, 5, 5, 6]

def is_x_y(arr):
    count_x, count_y = 0, 0
    x, y = None, None
    
    for val in arr:
        if not x:
            x = val
        if x == val:
            count_x += 1
        elif count_x >= 2:
            if not y:
                y = val
            if y == val:
                count_y +=1
            else:
                count_x = count_y
                x = y
                count_y = 1
                y = val
        else:
            count_x = 0
            x = None
        if count_x >= 2 and count_y == 3:
            return True
    return False


print(is_x_y(a))
print(is_x_y(b))
print(is_x_y(c))
print(is_x_y(d))

True
False
False
True

if x, x, x, x, x also match your pattern, change the return True condition to also return True on count_x = 5:

if count_x == 5 or (count_x >= 2 and count_y == 3):
    return True

Answered By: Yoav Sheetrit

How to find the pattern in a list

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