Sort a Dictionary

[Afanasiy]

This is fairly simple in PHP, how do I do it in Python?

http://www.php.net/manual/en/function.ksort.php

 

[mackstann]

This is fairly simple in PHP, how do I do it in Python?

In PHP, associative arrays are still regular arrays too, you can access
them by index (IIRC), and when you loop through them, they maintain the
order in which you assigned their items. Python seperates associative
arrays (dicts/hashes) from numerically indexed arrays (lists). You
can't sort a dict, because a dict has no order. You could do something
like:

mydict = { ..whatever.. }

sortedkeys = mydict.keys()
sortedkeys.sort()

for key in sortedkeys:
print key, mydict[key]

Then we run into the issue of why we have to do list.sort() in place,
and I'm sure that's been discussed here a billion times (can't say I've
been part of any of those discussions though).

--
m a c k s t a n n mack @ incise.org http://incise.org
After a few boring years, socially meaningful rock 'n' roll died out.
It was replaced by disco, which offers no guidance to any form of life
more advanced than the lichen family.
-- Dave Barry, "Kids Today: They Don't Know Dum Diddly Do"

 

[Andrew Dalke]

Afanasiy:

This is fairly simple in PHP, how do I do it in Python?

http://www.php.net/manual/en/function.ksort.php

def ksort(d, func = None):
keys = d.keys()
keys.sort(func)
return keys

for k in ksort(d):
print k, v

As a bonus, you don't need to tell the sort to sort numerically
vs. lexigraphically --- Python's strong typing knows that by
default. You can pass in an alternate compare function if you
want.

And no, I haven't tested it.

Andrew
(e-mail address removed)

 

[Afanasiy]

Afanasiy:

def ksort(d, func = None):
keys = d.keys()
keys.sort(func)
return keys

for k in ksort(d):
print k, v

As a bonus, you don't need to tell the sort to sort numerically
vs. lexigraphically --- Python's strong typing knows that by
default. You can pass in an alternate compare function if you
want.

Why wouldn't this be a standard function?

 

[Afanasiy]

Afanasiy:

def ksort(d, func = None):
keys = d.keys()
keys.sort(func)
return keys

for k in ksort(d):
print k, v

How about this one?

http://www.php.net/manual/en/function.asort.php

 

[Andrew Dalke]

Afanasiy:

Why wouldn't [ksort] be a standard function?

Because it isn't needed all that often and can be built (when needed)
from the underlying primitives very simply. Because if there are a
lot of similar methods then it becomes harder to remember what each
one does.

The normal practice is

keys = d.keys()
keys.sort()
for k in keys:
....

which isn't all that onerous.

How about this one?

http://www.php.net/manual/en/function.asort.php

The normal idiom for sorting by value then printing
the key/value pairs is

rev_items = [(v, k) for k, v in d.items()]
rev_items.sort()
for v, k in rev_items:
print k, v

If you want that as a function return just the keys
in value order

def asort(d):
rev_items = [(v, k) for k, v in d.items()]
rev_items.sort()
return [k for (v, k) in rev_items]

As you can see, there are many ways you might want
to sort a dict. Why should all of them be present in
the standard dict type when it's really a matter of two
extra lines to get what you need. Seeing the code in
this case is much easier than memorizing the 7 different
sort functions mentioned in the PHP docs.

Additionally, Python's keys can be more complex than
a string or int. Eg,

d = {}
d[ (0,0) ] = "home"
d[ (1,3) ] = "school"
d[ (4,2) ] = "work"

y_items = [(y, name) for ((x, y), name) in d.items()]
y_items.sort()
for y, name in y_items:
print y, name

sorts by y position, ignoring x position. PHP doesn't
have a function for that, but it follows pretty naturally
from the idiomatically Python way to do it.

Andrew
(e-mail address removed)