problem with custom sort function .... long

Ken R.

Hello all,
I am relatively new to python but I am having an issue with custom
sort functions..

I am trying to sort a list of lists or tuples with arbitrary ascending
or descending sorts. For example given a list of tuples
('firstname','lastname','age') I want to be able to sort lastname
descending, firstname ascending and age ascending...

I wrote a custom function generator to generate a sort function based
on an input list of column numbers and sort direction. Sort seems to
sort the first column ascending regardless of what the sort function
says. I also googled this group for other solutions and found a more
elegant one than mine but with the same results.

Here is my code and result:
sortList = [('2','D'),('1','D'),('0','D')]
dataList = []
dataList.append(['a','a','b'])
dataList.append(['a','a','a'])
dataList.append(['a','a','c'])
dataList.append(['a','b','a'])
dataList.append(['a','b','b'])
dataList.append(['a','b','c'])
dataList.append(['a','c','a'])
dataList.append(['a','c','b'])
outStr = 'def custSort( a, b):\n'
depth = 1

for sortPair in sortList:
indent = " "
curDent = depth * indent
outStr += curDent + 'if a[' + sortPair[0] + '] == b[' +
sortPair[0] + ']:\n'
depth += 1

depth -= 1
outStr += curDent + indent + 'return 0\n'
for j in range( len(sortList)-1, -1, -1 ):
curDent = depth * indent
print sortList[j][1]
if sortList[j][1] == 'A':
compareSym = '>'
elif sortList[j][1] == 'D':
compareSym = '<'
else:
print 'SORT DIRECTION ERROR ' + sortList[j][1]
outStr += curDent + 'elif a[' + sortList[j][0] + '] ' + compareSym
+ ' b[' + sortList[j][0] + ']:\n'
outStr += curDent + indent + 'return 1\n'
outStr += curDent + 'else:\n'
outStr += curDent + indent + 'return -1\n'
depth -= 1

print outStr
exec( outStr )
dataList.sort( custSort )
print str( dataList )
***************************
results:
D
D
D
def custSort( a, b):
if a[2] == b[2]:
if a[1] == b[1]:
if a[0] == b[0]:
return 0
elif a[0] < b[0]:
return 1
else:
return -1
elif a[1] < b[1]:
return 1
else:
return -1
elif a[2] < b[2]:
return 1
else:
return -1

[['a', 'b', 'c'], ['a', 'a', 'c'], ['a', 'c', 'b'], ['a', 'b', 'b'],
['a', 'a', 'b'], ['a', 'c', 'a'], ['a', 'b', 'a'], ['a', 'a', 'a']]

and Manuel Garcia's solution and results:
sortList = [(0,-1),(1,-1),(2,-1)]
dataList = []
dataList.append(['a','a','b'])
dataList.append(['a','a','a'])
dataList.append(['a','a','c'])
dataList.append(['a','b','a'])
dataList.append(['a','b','b'])
dataList.append(['a','b','c'])
dataList.append(['a','c','a'])
dataList.append(['a','c','b'])

def make_sort_f(list0):
def f(a,b):
for (i,m) in list0:
if a[i] == b[i]: continue
return m * cmp(a[i],b[i])
return 0
return f

dataList.sort( make_sort_f( sortList ) )
print str(dataList)

Results:
[['a', 'c', 'b'], ['a', 'c', 'a'], ['a', 'b', 'c'], ['a', 'b', 'b'],
['a', 'b', 'a'], ['a', 'a', 'c'], ['a', 'a', 'b'], ['a', 'a', 'a']]

is this an issue with sort or is my code screwy? Thanks in advance
for any help!

Ken R.

Jul 18 '05 #1

Terry Reedy

"Ken R." <kr*****@hotmail.com> wrote in message
news:1e*************************@posting.google.co m...

Hello all,
I am relatively new to python but I am having an issue with custom
sort functions..
Athough they seem to be working fine!
I am trying to sort a list of lists or tuples with arbitrary ascending or descending sorts. For example given a list of tuples
('firstname','lastname','age') I want to be able to sort lastname
descending, firstname ascending and age ascending...

I wrote a custom function generator to generate a sort function based on an input list of column numbers and sort direction. Sort seems to
sort the first column ascending regardless of what the sort function
says. I also googled this group for other solutions and found a more elegant one than mine but with the same results.
Given that your example data all have 'a' in the first column, these
statements of ill behavior make no sense!
Here is my code and result:
sortList = [('2','D'),('1','D'),('0','D')]
dataList = []
dataList.append(['a','a','b'])
dataList.append(['a','a','a'])
dataList.append(['a','a','c'])
dataList.append(['a','b','a'])
dataList.append(['a','b','b'])
dataList.append(['a','b','c'])
dataList.append(['a','c','a'])
dataList.append(['a','c','b'])
You could just as well write dataList as a single literal.

[snip]
[['a', 'b', 'c'], ['a', 'a', 'c'], ['a', 'c', 'b'], ['a', 'b', 'b'],
['a', 'a', 'b'], ['a', 'c', 'a'], ['a', 'b', 'a'], ['a', 'a', 'a']]
and columns 2, 1, and 0 are descending (non-increasing) in that order,
just as you asked. What different were you expecting given the input.
and Manuel Garcia's solution and results:
sortList = [(0,-1),(1,-1),(2,-1)] .... Results:
[['a', 'c', 'b'], ['a', 'c', 'a'], ['a', 'b', 'c'], ['a', 'b', 'b'],
['a', 'b', 'a'], ['a', 'a', 'c'], ['a', 'a', 'b'], ['a', 'a', 'a']]
Again, just as requested. Same question.
is this an issue with sort or is my code screwy?

Perhaps your understanding of ascending and descending? or of nested
sorting?

Terry J. Reedy

Jul 18 '05 #2

Ken R.

> Given that your example data all have 'a' in the first column, these

statements of ill behavior make no sense!

Oh my! One would think that I just dashed off a question to the group
without spending any time on the problem (not the case). No excuses
for my oversight but thanks for your gentle reply :).

Jul 18 '05 #3