Determining a sublist containing elements with alternant sign for which the sums of absolute value of the elements is maximal

Problem: Let us consider an array with real values (positive and negative) Determine a subarray containing elements with alternating signs (positive followed negative, negative followed by positive) for which the sum of the absolute value of the elements is maximal.

After running the code with this list it doesn’t give me the right output the output is [6, -3, 7, -5, 13, 12, 8, -9] but it should be [6, -3, 7, -5, 13, -9, 8]

a_list = [2, 6, -3, 7, -2, -5, 10, 13, 13, 13, 13, 12, 12, 12, 12, 12, 8, 8, 8, 8, 8, 8, -9, 8, -9, -9, -9, -9, -9, -9, -9, -9, 12, 12, 12, 12, 12] # [6, -3, 7, -5, 12]
sum_list = []
i=0
while i <len(a_list)-1:
    if (a_list[i] < 0 and a_list[i+1] > 0):
        if a_list[i] in sum_list:
            pass
        else:
            if sum_list[-1]<0:
                pass
            else:
                sum_list.append(a_list[i])
    if (a_list[i] > 0 and a_list[i+1] < 0):
        if a_list[i] in sum_list:
            pass
        else:
            if sum_list[-1]>0:
                pass
            else:
                sum_list.append(a_list[i])
    else:
         if a_list[i]>0 and  a_list[i+1]>0:
             if max(a_list[i],a_list[i+1]) in sum_list:
                 pass
             else:
                sum_list.append(max(a_list[i],a_list[i+1]))
         if a_list[i]<0 and  a_list[i+1]<0:
             if min(a_list[i],a_list[i+1]) in sum_list:
                 pass
             else:
                sum_list.append(min(a_list[i], a_list[i + 1]))
    i=i+1
print(sum_list)```

Answer

Update: the code is not correct in general, I’m at it …

Update: corrected version see below

This gives the expected result (with a few inline comments):

#https://stackoverflow.com/questions/65673470/

# test lists for a few edge cases
list_1 = []
list_2 = [0]
list_3 = [0, 0]
list_4 = [0, 0, 0]
list_5 = [-2, 4, -2, 4]
list_6 = [2, -4, 2, -4]

t_list = [ 2, 6
          , -3
          , 7
          , -2, -5
          , 10, 13, 13, 13, 13, 12, 12, 12, 12, 12, 8, 8, 8, 8, 8, 8
          , -9
          , 8
          , -9, -9, -9, -9, -9, -9, -9, -9
          , 12, 12, 12, 12, 12
           ]  # expected result: [6, -3, 7, -5, 13, -9, 8]

all_lists = [list_1, list_2, list_3, list_4, list_5, list_6, t_list]

def sign_differs(n1, n2):
    '''return True if n1 and n2 have different signs.'''
    return n1 < 0 and n2 >= 0 or n1 >= 0 and n2 < 0

for a_list in all_lists:
    print(f'a_list: {a_list}')

    sum_list = []
    prev_idx = 0
    for idx in range(1, len(a_list)):
        n1, n2 = a_list[idx-1], a_list[idx]
        if n1 in sum_list:
            continue
        if sign_differs(n1, n2):  # +/- or -/+ detected
            if a_list[prev_idx] >= 0:
                max_abs = max(a_list[prev_idx:idx])  # largest positive
            else:
                max_abs = min(a_list[prev_idx:idx])  # smallest negative

            if not max_abs in sum_list:
                sum_list.append(max_abs)
            prev_idx = idx

    print(f'sum_list: {sum_list}n')

Output including a few test cases:

a_list: []
sum_list: []

a_list: [0]
sum_list: []

a_list: [0, 0]
sum_list: []

a_list: [0, 0, 0]
sum_list: []

a_list: [-2, 4, -2, 4]
sum_list: [-2, 4]

a_list: [2, -4, 2, -4]
sum_list: [2, -4]

a_list: [2, 6, -3, 7, -2, -5, 10, 13, 13, 13, 13, 12, 12, 12, 12, 12, 8, 8, 8, 8, 8, 8, -9, 8, -9, -9, -9, -9, -9, -9, -9, -9, 12, 12, 12, 12, 12]
sum_list: [6, -3, 7, -5, 13, -9, 8]

Corrected version:

#https://stackoverflow.com/questions/65673470/

# test lists for a few (edge) cases
list_1 = []
list_2 = [0]
list_3 = [0, 0]
list_4 = [0, 0, 0]
list_5 = [-2, 4, -2, 4]
list_6 = [2, -1, 2, -2, -3, -4, 3]
list_7 = [-99, 2, -1, 3, -1, 4, 5, -3, -3, 4, -1, -3, -4, -5, 2, 3, -5, 6, 4, 7]

t_list = [ 2, 6
          , -3
          , 7
          , -2, -5
          , 10, 13, 13, 13, 12, 12, 12, 8, 8, 8
          , -9
          , 8
          , -9, -9, -9
          , 12, 12, 12
           ]  # expexted result: [6, -3, 7, -5, 13, -9, 8]

all_lists = [list_1, list_2, list_3, list_4, list_5, list_6, list_7, t_list]

def alternating_nums(a_list):
    sum_list = []
    if not a_list:
        return sum_list
    number = a_list[0]
    sum_list = [number]
    srch_neg = number >= 0  # look for next pos./neg. number
    other_ok = True

    for number in a_list:
        if srch_neg:  # searching for a neg. number
            if number >= 0:
                if number in sum_list:
                    continue
                if number <= sum_list[-1]:
                    continue
                if not other_ok:
                    continue
                sum_list[-1] = number  # found a "better" one
                continue
            if number < 0:
                if number in sum_list:
                    other_ok = False
                    continue
                sum_list.append(number)
                srch_neg = False  # now searching for a pos. number
                other_ok = True  # a consecutive smaller neg. will be ok, too
                continue
        else:  # searching for a pos. number
            if number < 0:
                if number in sum_list:
                    continue
                if number >= sum_list[-1]:
                    continue
                if not other_ok:
                    continue
                sum_list[-1] = number  # found a "better" one
                continue
            if number >= 0:
                if number in sum_list:
                    other_ok = False
                    continue
                sum_list.append(number)
                srch_neg = True  # now searching for a neg. number
                other_ok = True  # a consecutive larger pos. will be ok, too
                continue
    return sum_list

for a_list in all_lists:
    print(a_list)
    sum_list = alternating_nums(a_list)
    print(sum_list, 'n')