solution_057.py

#!/usr/bin/python3
# ===================================================================
# given a random list of values, rearrange the list values into
# high/low pairs. use the initial list; do not create a new list.
#
# example results:
#    [ <highest-value>,<lowest-value>,
#      <next-highest_value>, <next-lowest,value>,
#      <next-high-value>,<next-low-value>,
#      ... ]
#
# ===================================================================


# -------------------------------------------------------------------
# ---- given a list and two list indexes, swap the values if one
# ---- value is less than the other value
# ---- make sure the values exist before swapping
# -------------------------------------------------------------------

def swap_values(lst,i,j):
    if i < len(lst) and j < len(lst) and i != j and lst[i] < lst[j]:
        lst[i],lst[j] = lst[j],lst[i]


# -------------------------------------------------------------------
# ---- rearrainge the list into high/low pairs (in place)
# -------------------------------------------------------------------

def rearrange_list(lst):

    # ---- are there enough elements in the list to swap?

    if len(lst) < 2:
        return

    # ---- rearrange the list, there are enough elements in the list

    for high_idx in range(0,len(lst)-1,2):

        # ---- find highest value
  
        for idx in range(high_idx+1,len(lst)):
            swap_values(lst,high_idx,idx)

        # ---- find lowest value

        for idx in range(high_idx+2,len(lst)):
            swap_values(lst,idx,high_idx+1)
 

# -------------------------------------------------------------------
# ---- main
# -------------------------------------------------------------------

if __name__ == '__main__':

    # random lists for testing

    testlists = [
                  [56,21,1,5,87,65,2,10],    # even
                  [87,56,21,1,5,65,10,2],    # even
                  [56,21,1,5,87,65,2],       # odd
                  [1,21,56],                 # 3 elements
                  [56,21],                   # 2 elements
                  [21,56],                   # 2 elements
                  [199],                     # 1 element
                  []                         # 0 elements
                ]
    
    for lst in testlists:
        print()
        print(f'start: {lst}')
        rearrange_list(lst)
        print(f'final: {lst}')

    print()