thanks Duncan and Arnaud.
I'm learning Python from the "How to Think Like a Python Programmer"
book by Allen Downey. My first try used the "inv[val] = [key]" and
then the next problem was to incorporate the "D.setdefault(...)" method.
Thank you for your help. I'm always amazed how kind people are in this group.
On 2008-05-03 14:57:29 -0600, Arnaud Delobelle <[EMAIL PROTECTED]> said:
dave <[EMAIL PROTECTED]> writes:
Hello,
here is a piece of code I wrote to check the frequency of values and
switch them around to keys in a new dictionary. Just to measure how
many times a certain key occurs:
def invert(d):
inv = {}
for key in d:
val = d[key]
if val not in inv:
inv.setdefault(val, [key])
You can simply write:
inv[val] = [key]
else:
inv[val].append(key)
return inv
Using the methods above (I'm just a beginner) could I have written it
more concisely? Any criticism/critique in the code would be greatly
appreciated.
Apart from the unnecessary use of setdefault, it looks good to me.
* You could change if 'val not in inv:' to 'if val in inv:' (and swap
the if and else clauses of course) in order to have a positive
condition rather than a negative one
* If you want to use setdefault, you can replace the if .. else
construct by:
inv.setdefault(val, []).append(key)
* You can also iterate over keys and values using the items() or
iteritems() method of dictionaries:
def invert(d):
inv = {}
for key, val in d.iteritems():
inv.setdefault(val, []).append(key)
return inv
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