def combine_adjacent(arr):
    """Sum like signed adjacent elements
    arr : starting array

    Returns
    -------
    output: new summed array
    indexes: indexes indicating the first 
             element summed for each group in arr
    """
    output, indexes = [], []
    curr_i = 0
    while len(arr) > 0:
        curr_sign = arr[0]/abs(arr[0])
        index = 0
        while index < len(arr) and arr[index]/abs(arr[index]) == curr_sign:
            index += 1
        output.append(sum(arr[:index]))
        indexes.append(curr_i)
        curr_i += index
        
        for _ in range(index):
            arr.pop(0)
    return output, indexes