0001 from random import randrange
0002 from timeit import Timer
0003 from sys import stdout
0004 
0005 ###############################################################################
0006 def show(L): print( str(sum(L)) + '/' + str(len(L)) + ':' + str(L) ); return
0007 
0008 ###############################################################################
0009 def reverse_1(L): # reverse a list in manner of CALL-BY-RANK
0010    lo, hi = 0, len(L)-1 # starting from the first and the last element
0011    while lo < hi: # for each pair of symmetric elements
0012       L[lo], L[hi] = L[hi], L[lo] # swap them and
0013       lo, hi = lo+1, hi-1 # consider the next pair
0014    return L
0015 
0016 def reverse_1_test(n): reverse_1([ randrange(n) for i in range(n) ]); return
0017 
0018 ###############################################################################
0019 def reverse_2(L): # reverse a list in manner of CALL-BY-POSITION
0020    for i in range( len(L) ): # for each i in [0, n)
0021       L.insert(i, L.pop()) # move the last element to position i
0022    return L
0023 
0024 def reverse_2_test(n): reverse_2([ randrange(n) for i in range(n) ]); return
0025 
0026 ###############################################################################
0027 if __name__ == '__main__':
0028    n = 13
0029    L = [ randrange(n) for i in range(n) ]; show(L) # create a random list of size n
0030    L = reverse_1(L); show(L) # reverse it using algorithm #1
0031    L = reverse_2(L); show(L) # reverse it using algorithm #2
0032    L.sort(); show(L) # and sort it for verification
0033    print('\n\t   i        n :       T1     T1/n  |     T2    T2/n/n')
0034    print(22*'-' + '+' + 20*'-' + '+' + 26*'-' )
0035    for i in range(19): # for each i in [0, 18) (slows down on typical PC for i > 18)
0036       n = 2**i; # test the two algorithms with a list of size n = 2^i
0037       t1 = 1000*Timer('__main__.reverse_1_test(__main__.n)', 'import __main__').timeit(1);
0038       t2 = 1000*Timer('__main__.reverse_2_test(__main__.n)', 'import __main__').timeit(1);
0039       print('\t2^%2d = %6d : %8.2f %8.5f  | %8.2f %8.5f' %(i, n, t1, t1/n, t2, t2/n/n*1000));
0040       stdout.flush()