[Tutor] arrays

[John Fouhy]

On 15/09/06, federico ramirez <fedekiller at gmail.com> wrote:
> an array to order the keys and then display it in order but..python orders
> the array like this
>
> ['_1', '_10', '_11', '_12', '_13', '_2', '_3', '_4', '_5', '_6', '_7', '_8',
> '_9']
>
> and i want
>
> ['_1', '_2', '_3', '_4', '_5', '_6', '_7', '_8', '_9','_10', '_11', '_12',
> '_13']

Hi,

First, just a terminology note -- in python, these are called lists, not arrays.

>         arr = []
>         for key in db.keys():
>             arr += [key]
>         arr.sort()
>         arr.reverse()

db.keys() returns a _new_ list, so there is no need to do this step;
you could just write:
    arr = db.keys()

Now, since your keys are strings, python is sorting them
lexicographically, which is why (for instance) '_10' comes before
'_2'.  You need to tell python to change its sort order.

You can do this in a couple of ways.  If you have python2.4 or
greater, you can use the key= keyword argument to sort, like this:

def keyToInt(s):
    """Convert '_10' to 10, '_2' to 2, etc. """
    return int(s[1:])
arr.sort(key=keyToInt)

If you have an earlier version of python, you can define your own
comparison function:

def myCmp(x, y):
    return cmp(keyToInt(x), keyToInt(y))
arr.sort(myCmp)

You can also use the decorate-sort-undecorate idiom, which may be
faster than a custom comparison function if the list is very large.

decorated = [(keyToInt(s), s) for s in arr]
decorated.sort()
arr = [x[1] for x in decorated]

>         for i in range(len(db)):
>             print db[arr[i]],'<br />'

In python, you can iterate over lists directly.  ie, since arr is your
list of keys:

for key in arr:
    print db[key], '<br />'

HTH!

-- 
John.