dictionary initialization

[Weiguang Shi]

Hi,

With awk, I can do something like
$ echo 'hello' |awk '{a[$1]++}END{for(i in a)print i, a[i]}'

That is, a['hello'] was not there but allocated and initialized to
zero upon reference.

With Python, I got

b={}
b[1] = b[1] +1

Traceback (most recent call last):
File "<stdin>", line 1, in ?
KeyError: 1

That is, I have to initialize b[1] explicitly in the first place.

Personally, I think

a[i]++

in awk is much more elegant than

if i in a: a[i] += 1
else: a[i] = 1

I wonder how the latter is justified in Python.

Thanks,
Weiguang

[Caleb Hattingh]

Hmm :)

"b[1]" looks like a List (but you created a Dict)
"b['1'] looks more like a Dict (but this is not what you used).

If lists are your thing:

a = []
a.append(1)
a [1] a[0] += 1
a [2]

If dicts are your thing:
b = {}
b['1'] = 1
b {'1': 1} b['1'] += 1
b

{'1': 2}

Lists are ordered, Dicts are not.
Dict entries accessed with 'string' keys, List entries accessed with a
position integer.

Which feature specifically do you want justification for?

thx
Caleb

With Python, I got >>> b={}
>>> b[1] = b[1] +1

Traceback (most recent call last):
File "<stdin>", line 1, in ?
KeyError: 1

That is, I have to initialize b[1] explicitly in the first place.

Personally, I think

a[i]++

in awk is much more elegant than

if i in a: a[i] += 1
else: a[i] = 1

I wonder how the latter is justified in Python.

Thanks,
Weiguang

[Bengt Richter]

On Thu, 25 Nov 2004 18:38:17 +0000 (UTC), wg***@namao.cs. ualberta.ca (Weiguang Shi) wrote:

Hi,

With awk, I can do something like
$ echo 'hello' |awk '{a[$1]++}END{for(i in a)print i, a[i]}'

That is, a['hello'] was not there but allocated and initialized to
zero upon reference.

With Python, I got

>>> b={}
>>> b[1] = b[1] +1 Traceback (most recent call last):
File "<stdin>", line 1, in ?
KeyError: 1

That is, I have to initialize b[1] explicitly in the first place.

Personally, I think

a[i]++

in awk is much more elegant than

if i in a: a[i] += 1
else: a[i] = 1

I wonder how the latter is justified in Python.

You wrote it, so you have to "justify" it ;-)

While I agree that ++ and -- are handy abbreviations, and creating a key by default
makes for concise notation, a[i]++ means you have to make some narrow assumptions -- i.e.,
that you want to create a zero integer start value. You can certainly make a dict subclass
that behaves that way if you want it:

class D(dict): ... def __getitem__(sel f, i):
... if i not in self: self[i] = 0
... return dict.__getitem_ _(self, i)
... dink = D()
dink {} dink['a'] +=1
dink {'a': 1} dink['a'] +=1
dink {'a': 2} dink['b'] 0 dink['b'] 0 dink {'a': 2, 'b': 0}
Otherwise the usual ways are along the lines of
d = {}
d.setdefault('h ello',[0])[0] += 1
d {'hello': [1]} d.setdefault('h ello',[0])[0] += 1
d {'hello': [2]}

Or d['hi'] = d.get('hi', 0) + 1
d {'hi': 1, 'hello': [2]} d['hi'] = d.get('hi', 0) + 1
d {'hi': 2, 'hello': [2]} d['hi'] = d.get('hi', 0) + 1
d {'hi': 3, 'hello': [2]}

Or for x in xrange(3):

... try: d['yo'] += 1
... except KeyError: d['yo'] = 1
... print d
...
{'hi': 3, 'hello': [2], 'yo': 1}
{'hi': 3, 'hello': [2], 'yo': 2}
{'hi': 3, 'hello': [2], 'yo': 3}

Regards,
Bengt Richter

[Weiguang Shi]

Hi,

In article <op************ **@news.telkoms a.net>, Caleb Hattingh wrote:

...
Dict entries accessed with 'string' keys, Not necessarily. And doesn't make a difference in my question.
...

Which feature specifically do you want justification for?

Have it your way: string-indexed dictionaries.

a={}
a['1']+=1

Traceback (most recent call last):
File "<stdin>", line 1, in ?
KeyError: '1'

a['1'] when it referenced, is detected non-existent but not
automatically initialized so that it exists before adding 1 to its
value.

Weiguang

[Weiguang Shi]

Hi,

In article <41************ *****@news.oz.n et>, Bengt Richter wrote:

On Thu, 25 Nov 2004 18:38:17 +0000 (UTC), wg***@namao.cs. ualberta.ca
(Weiguang Shi) wrote:
You wrote it, so you have to "justify" it ;-) I guess :-)
While I agree that ++ and -- are handy abbreviations, and creating a
key by default makes for concise notation, a[i]++ means you have to
make some narrow assumptions ... Right, though generalization can be painful for the uninitiated/newbie.
You can certainly make a dict subclass that behaves that way if you
want it:
... This is nice even for someone hopelessly lazy as me.

Otherwise the usual ways are along the lines of
...

I would happily avoid them all.

Thanks a lot,
Weiguang

[Caleb Hattingh]

Hi

I apologise, but I don't actually know what the problem is? If you could
restate it a little, that would help.

I didn't check the code I posted earlier; This below is checked:
***
# Dont use a={}, just start as below
'>>> a['1']=0
'>>> a['1']+=1
'>>> a
{'1': 1}
***

Like I said, I am unsure of what your specific problem is?

Thanks
Caleb
On Thu, 25 Nov 2004 19:27:46 +0000 (UTC), Weiguang Shi
<wg***@namao.cs .ualberta.ca> wrote:

Hi,

In article <op************ **@news.telkoms a.net>, Caleb Hattingh wrote:

...
Dict entries accessed with 'string' keys,

Not necessarily. And doesn't make a difference in my question.

...

Which feature specifically do you want justification for?

Have it your way: string-indexed dictionaries.

>>> a={}
>>> a['1']+=1

Traceback (most recent call last):
File "<stdin>", line 1, in ?
KeyError: '1'

a['1'] when it referenced, is detected non-existent but not
automatically initialized so that it exists before adding 1 to its
value.

Weiguang

[Caleb Hattingh]

And I haven't even been drinking!

I apologise once more, this is better:

***
# You *must* use a={}, just start as below
'>>> a={}
'>>> a['1']=0
'>>> a['1']+=1
'>>> a
{'1': 1}
***

Like I said, I am unsure of what your specific problem is?

Thanks
Caleb

[Dan Perl]

I don't know awk, so I don't know how your awk statement works.

Even when it comes to the python statements, I'm not sure exactly what the
intentions of design intention were in this case, but I can see at least one
justification. Python being dynamically typed, b[1] can be of any type, so
you have to initialize b[1] to give it a type and only then adding something
to it makes sense. Otherwise, the 'add' operation not being implemented for
all types, 'b[1]+1' may not even be allowed.

You're saying that in awk a['hello'] is initialized to 0. That would not be
justified in python. The type of b[1] is undetermined until initialization
and I don't see why it should be an int by default.

Dan

"Weiguang Shi" <wg***@namao.cs .ualberta.ca> wrote in message
news:slrncqc9kq .hj3.wg***@nama o.cs.ualberta.c a...

Hi,

With awk, I can do something like
$ echo 'hello' |awk '{a[$1]++}END{for(i in a)print i, a[i]}'

That is, a['hello'] was not there but allocated and initialized to
zero upon reference.

With Python, I got

>>> b={}
>>> b[1] = b[1] +1

Traceback (most recent call last):
File "<stdin>", line 1, in ?
KeyError: 1

That is, I have to initialize b[1] explicitly in the first place.

Personally, I think

a[i]++

in awk is much more elegant than

if i in a: a[i] += 1
else: a[i] = 1

I wonder how the latter is justified in Python.

Thanks,
Weiguang

[Weiguang Shi]

In article <lI************ ********@rogers .com>, Dan Perl wrote:

I don't know awk, so I don't know how your awk statement works. It doesn't hurt to give it a try :-)

Even when it comes to the python statements, I'm not sure exactly what the
... I see your point.

You're saying that in awk a['hello'] is initialized to 0. More than that; I said awk recognizes a['hello']++ as an
arithmetic operation and initializes a['hello'] to 0 and add one to
it. (This is all guess. I didn't implement gawk. But you see my point.)
That would not be justified in python. The type of b[1] is
undetermined until initialization and I don't see why it should be
an int by default.

In my example, it was b[1]+=1. "+=1" should at least tell Python two
things: this is an add operation and one of the operands is an
integer. Based on these, shouldn't Python be able to insert the pair
"1:0" into a{} before doing the increment?

Weiguang

[Weiguang Shi]

Hi,

In article <op************ **@news.telkoms a.net>, Caleb Hattingh wrote:

...
***
# You *must* use a={}, just start as below
'>>> a={} Yeah I know. I can live with that.
'>>> a['1']=0
'>>> a['1']+=1

Right here. You have to say a['1'] = 0 before you can say a['1'] +=1
Python does not do the former for you. That's what I'm asking
justifications for.

Regards,
Weiguang