[Tutor] how to delete some quasi-duplicated keys

[Christian Witts]

On 2011/11/25 11:15 AM, lina wrote:
> On Fri, Nov 25, 2011 at 5:05 PM, Christian Witts<cwitts at compuscan.co.za>  wrote:
>> On 2011/11/25 10:41 AM, lina wrote:
>>
>> pairs
>>
>> {('66', '69'): 217, ('69', '66'): 75, ('64', '71'): 25}
>>
>>
>> such as here ('66', '69') and ('69', '66') is one key,
>>
>> I wanna keep only one and add the value of those two keys, above is a
>> very simple example:
>>
>> here is the (failed) code:
>>
>>          for k, v in pairs.items():
>>              if str(k)[1]+str(k)[0] in pairs.keys():
>>                  print(pairs[str(k)[1]+str(k)[0]])
>>                  pairs[k]+=pairs[str(k)[1]+str(
>> k)[0]]
>>                  del pairs[str(k)[1]+str(k)[0]]
>>              print(v,k)
>>
>>
>> Thanks for any advice,
>> _______________________________________________
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>>
>>
>> pairs.items() is fine if it's a small dictionary but for larger ones it's
>> going to slow things down as it generates the entire list of items before
>> you continue, rather use .iteritems() as it creates an iterator which only
>> yields one item at a time making it more efficient.
>>
>> str(k)[1]+str(k)[0] is string concatenation so your first key tested is
>> '6669' and not ('66', '69') as you intended, you would have to create a new
>> tuple to get the key you wanted like `if (k[1], k[0]) in pairs.keys():`.
>> Also, your check to "in pairs.keys()" is wasteful as you generate a new list
>> of keys for every key and you will be better off using `in pairs:` directly
>> as it performs a dictionary lookup to test if the key exists.
>>
>> With that in mind, this is how I would re-write that code segment. YMMV
>>
>> for key in pairs.iterkeys():
>>      # The [::-1] creates a reverse of the iterable so ('66', '69') will be
>> ('69', '66')
> $ python3 dehydron_data_stastic.py | sort -nr
> Traceback (most recent call last):
>    File "dehydron_data_stastic.py", line 44, in<module>
>      for k in pairs.iterkeys():
> AttributeError: 'dict' object has no attribute 'iterkeys'
>
>>      if key[::-1] in pairs:
>>          try:
>>              # The first instance of the pair gets kept and the reversed gets
>> added
>>              pairs[key] += pairs[key[::-1]]
>>              del pairs[::-1]
>>          except KeyError:
>>              print "Key ('%s', '%s') already accumulated)" % key
>>
>> Hope that helps.
>>
>> --
>>
>> Christian Witts
>> Python Developer
>>
>
Ah, should have mentioned .iterkeys() / .itervalues() are in Python 2.x 
and .keys() and .values() have been changed in Py3 to match .iter* from 
Py2.x.

Just change that line to `for key in pairs.keys():` then.
-- 

Christian Witts
Python Developer
//
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