How can I reorder a list? [closed]

Answer 1

>>> a = [1, 2, 3]
>>> a[0], a[2] = a[2], a[0]
>>> a
[3, 2, 1]

Answer 2

You can provide your own sort function to list.sort():

The sort() method takes optional arguments for controlling the comparisons.

• cmp specifies a custom comparison function of two arguments (list items) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument: cmp=lambda x,y: cmp(x.lower(), y.lower()). The default value is None.

• key specifies a function of one argument that is used to extract a comparison key from each list element: key=str.lower. The default value is None.

• reverse is a boolean value. If set to True, then the list elements are sorted as if each comparison were reversed.

In general, the key and reverse conversion processes are much faster than specifying an equivalent cmp function. This is because cmp is called multiple times for each list element while key and reverse touch each element only once.

Answer 3

newList = [oldList[3]]
newList.extend(oldList[:3])
newList.extend(oldList[4:])

Answer 4

Is the final order defined by a list of indices ?

>>> items = [1, None, "chicken", int]
>>> order = [3, 0, 1, 2]

>>> ordered_list = [items[i] for i in order]
>>> ordered_list
[<type 'int'>, 1, None, 'chicken']

edit: meh. AJ was faster... How can I reorder a list in python?

Answer 5

If you do not care so much about efficiency, you could rely on numpy's array indexing to make it elegant:

a = ['123', 'abc', 456]
order = [2, 0, 1]
a2 = list( np.array(a, dtype=object)[order] )

Answer 6

One more thing which can be considered is the other interpretation as pointed out by darkless

Code in Python 2.7

Mainly:

1. Reorder by value - Already solved by AJ above

2. Reorder by index

   mylist = ['a', 'b', 'c', 'd', 'e']
   myorder = [3, 2, 0, 1, 4]
   
   mylist = sorted(zip(mylist, myorder), key=lambda x: x[1])
   print [item[0] for item in mylist]
   

This will print ['c', 'd', 'b', 'a', 'e']