subhabangal...@gmail.com wrote:
> Dear Group,
> To improve my code writing I am trying to read good codes. Now, I have
> received a code,as given below,(apology for slight indentation errors) the
> code is running well. Now to comprehend the code, I am looking to
> understand it completely.
>
> class Calculate:
> def __init__(self):
> self.prior = {}
> self.total = {}
> self.count = 0
> def add(self, cls, obs):
> self.prior[cls] = self.prior.get(cls, 0) + 1
> for idx, val in enumerate(obs):
> key = cls, idx, val
> self.total[key] = self.total.get(key, 0) + 1
> self.count += 1
> def discr(self, cls, obs):
> result = self.prior[cls]/self.count
> for idx, val in enumerate(obs):
> freq = self.total.get((cls, idx, val), 0)
> result *= freq/self.prior[cls]
> return result
> def classify(self, obs):
> candidates = [(self.discr(c, obs), c) for c in self.prior]
> return max(candidates)[1]
>
> I am not understanding many parts of it, I am understanding many parts of
> it also.
>
> So I am looking for an exercise what are the things I should know to
> understand it, (please do not give answers I would get back with the
> answers in a week and would discuss even how to write better than this).
Start with running the code for the simplest piece of the class:
>>> c = Calculate()
>>> c.add("x", [1,2,3])
Then inspect the attributes:
>>> c.prior
{'x': 1}
>>> c.total
{('x', 2, 3): 1, ('x', 1, 2): 1, ('x', 0, 1): 1}
>>> c.count
Now read the code for Calculate.add(). Do you understand what
> self.prior[cls] = self.prior.get(cls, 0) + 1
does? Experiment with a dict and its get() method in the interactive
interpreter. Next to the loop.
> for idx, val in enumerate(obs):
> key = cls, idx, val
> self.total[key] = self.total.get(key, 0) + 1
> self.count += 1
Do you understand what enumerate() does? If not read its documentation with
>>> help(enumerate)
Do you understand what key looks like? If you don't add a print statement
> for idx, val in enumerate(obs):
> key = cls, idx, val
print key
> self.total[key] = self.total.get(key, 0) + 1
> self.count += 1
What does
> self.total[key] = self.total.get(key, 0) + 1
do? Note that this line is very similar to
> self.prior[cls] = self.prior.get(cls, 0) + 1
which you have studied before.
> self.count += 1
This like the rest of your class is left as an exercise. The routine is
always the same:
- break parts that you don't understand into smaller parts
- consult the documentation on unknown classes, functions, methods,
preferrably with help(some_obj) or dir(some_obj)
- run portions of the code or similar code in the interactive interpreter or
with a little throw-away script.
- add print statements to inspect variables at interesting points in your
script.
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