Exercise in the ordering of tuples

Question

I’m getting beat up in a Python exercise. He asks for the following:

Create a function that:

• Receive a list of tuples (data), an integer (key, zero by default equal) and a boolean (reverse, false by default).
• Return data sorted by the item indicated by the key and in descending order if reverse is true.

The knowledge I have to solve it is the knowledge of how to create functions and the following explanation:

The interpreter has defined some builtin functions, including sorted(), which orders sequences, and cmp(), who makes comparisons between two arguments and returns -1 if the first element is larger , 0 (zero) if they are equal to or 1 if the latter is larger. This function is used by the ordering routine, a behavior that can be modified.

Example:

dados = [(4, 3), (5, 1), (7, 2), (9, 0)]

# Comparando pelo último elemento
def _cmp(x, y):
    return cmp(x[-1], y[-1])

print 'Lista:', dados

# Ordena usando _cmp()
print 'Ordenada:', sorted(dados, _cmp)

Exit:

Lista: [(4, 3), (5, 1), (7, 2), (9, 0)]
Ordenada: [(9, 0), (5, 1), (7, 2), (4, 3)]

Doubts:

I did not understand the phrase "zero by equal default", this "equal" there seems to be left over in the sentence.

I would like to solve by taking advantage of the functions sorted() and cmp() and not implementing ordering from scratch (I think this is the intention of the exercise as well). I understand that I should create a function _cmp() based on cmp() and receiving as optional parameters chave and reverso. But the example calls the function so sorted(dados, _cmp) and I understand that sorted() will call the function internally _cmp() passing two parameters only (the two elements that should be compared). I even tried to solve by passing the two optional parameters to _cmp but that doesn’t make much sense and I stopped there... it was like this:

#!/usr/bin/env python
#-*- coding: utf-8 -*-

def ordenar(dados, chave = 0, reverso = False):
    return (dados, _cmp(chave, reverso))

def _cmp(x, y, chave = 0, reverso = False):
    if reverso == False:
        return cmp(x[chave], y[chave])
    else:
        return cmp(y[chave], x[chave])

dados = [(4, 3), (5, 1), (7, 2), (9, 0)]

print dados
print ordenar(dados, 1, True)

Exit:

[(4, 3), (5, 1), (7, 2), (9, 0)]
Traceback (most recent call last):
  File "apl.py", line 16, in <module>
    print ordenar(dados, 1, True)
  File "apl.py", line 5, in ordenar
    return (dados, _cmp(chave, reverso))
  File "apl.py", line 9, in _cmp
    return cmp(x[chave], y[chave])
TypeError: 'int' object has no attribute '__getitem__'

Answer 1

When to the "equal default zero", I think the word "equal" actually remained there. As for the function cmp, it has been removed from Python 3 and, even if your question cites version 2.7, I see no reason to actually study it or even use such a version. I strongly recommend that you study Python version 3 directly.

The easiest way I see to solve this problem is to use the function sorted together with the function itemgetter module operator.

from operator import itemgetter

def ordenar(dados, chave = 0, reverso = False):
  return sorted(dados, key=itemgetter(chave), reverse=reverso)

Thus, if defined dados as:

dados = [(4, 3), (5, 1), (7, 2), (9, 0)]

You have the results:

>>> ordenar(dados)
[(4, 3), (5, 1), (7, 2), (9, 0)]

>>> ordenar(dados, chave=1)
[(9, 0), (5, 1), (7, 2), (4, 3)]

>>> ordenar(dados, chave=1, reverso=True)
[(4, 3), (7, 2), (5, 1), (9, 0)]

The function itemgetter returns a function that accesses a given key of an object passed via parameter to that second function. For example, do f = itemgetter(5) and then call f(x), the value of x[5].

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