Re: Using a switch-like if/else construct versus a dictionary?
On Jun 19, 12:40 pm, asincero [EMAIL PROTECTED] wrote: Which is better: using an if/else construct to simulate a C switch or use a dictionary? Example: Whichever results in the clearest code that meets the performance requirements. FWIW, if you define the dictionary beforehand, the dict solution is O(1) while if/else is O(N), which can be important. -Mike -- http://mail.python.org/mailman/listinfo/python-list
Re: Using a switch-like if/else construct versus a dictionary?
asincero [EMAIL PROTECTED] wrote:
handle_case = {}
handle_case[1] = doCase1()
handle_case[2] = doCase2()
handle_case[3] = doCase3()
handle_case[4] = doCase4()
handle_case[5] = doCase5()
handle_case[c]()
If the switch values are simple integers then a list would be a more
obvious choice:
handle_case = [ doCase1, doCase2, doCase3, doCase4, doCase5 ]
handle_case[c-1]()
Note that in this situation using OO polymorphism instead of a switch-
like construct wouldn't be applicable, or at least I can't see how it
could be.
That is often the case when you reduce your question to a mickeymouse do
nothing example.
There are many ways to write code that could be written using a switch
statement. If you took a few real use-cases then you would probably each
one needs a different technique for best effect depending on the number
of different cases, the values which must be accessed within each case,
the side-effects (if any) expected from handling the case etc.
The obvious options include: if/elif chain; a list or dictionary
containing data values (not functions); a list or dict containing
functions; command pattern (i.e. a class with a dispatch method); a
class hierarchy and polymorphism; visitor pattern.
For an example of the last of these see
http://www.chris-lamb.co.uk/blog/2006/12/08/visitor-pattern-in-python/
which uses decorators to produce a very clean implementation using (I
think) PyProtocols dispatch.
--
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Re: Using a switch-like if/else construct versus a dictionary?
Ahh .. yes of course, you are right. I mis-typed. I like how you
defined the dictionary all in one statement, though. I didn't think
of doing it that way.
-- Arcadio
On Jun 19, 4:11 pm, heltena [EMAIL PROTECTED] wrote:
asincero ha escrit:
def foo():
def doCase1():
pass
def doCase2():
pass
def doCase3():
pass
def doCase4():
pass
def doCase5():
pass
handle_case = {}
handle_case[1] = doCase1()
handle_case[2] = doCase2()
handle_case[3] = doCase3()
handle_case[4] = doCase4()
handle_case[5] = doCase5()
Sorry, but I think this is not correct. Now, you put the result of the
function call into the dictionary, but you want the function address.
The correct code is:
handle_case = {}
handle_case[1] = doCase1
handle_case[2] = doCase2
handle_case[3] = doCase3
handle_case[4] = doCase4
handle_case[5] = doCase5
Or:
handle_case = { 1: doCase1, 2: doCase2, 3: doCase3, 4: doCase4, 5:
doCase 5 }
Or:
try:
{ 1: doCase1, 2: doCase2, 3: doCase3, 4: doCase4, 5: doCase 5 }
[c]()
except:
print Catch the correct exception
But this solutions only works fine if the params of the functions are
the same for all calls (in if/else construct you don't need it).
Bye!
--
Helio Tejedor
--
http://mail.python.org/mailman/listinfo/python-list
Using a switch-like if/else construct versus a dictionary?
Which is better: using an if/else construct to simulate a C switch or
use a dictionary? Example:
def foo():
if c == 1:
doCase1()
elif c == 2:
doCase2()
elif c == 3:
doCase3()
elif c == 4:
doCase4()
elif c == 5:
doCase5()
else:
raise shouldn't get here
or:
def foo():
def doCase1():
pass
def doCase2():
pass
def doCase3():
pass
def doCase4():
pass
def doCase5():
pass
handle_case = {}
handle_case[1] = doCase1()
handle_case[2] = doCase2()
handle_case[3] = doCase3()
handle_case[4] = doCase4()
handle_case[5] = doCase5()
handle_case[c]()
Note that in this situation using OO polymorphism instead of a switch-
like construct wouldn't be applicable, or at least I can't see how it
could be.
Thanks in advance for any tips!
-- Arcadio
--
http://mail.python.org/mailman/listinfo/python-list
Re: Using a switch-like if/else construct versus a dictionary?
asincero ha escrit:
def foo():
def doCase1():
pass
def doCase2():
pass
def doCase3():
pass
def doCase4():
pass
def doCase5():
pass
handle_case = {}
handle_case[1] = doCase1()
handle_case[2] = doCase2()
handle_case[3] = doCase3()
handle_case[4] = doCase4()
handle_case[5] = doCase5()
Sorry, but I think this is not correct. Now, you put the result of the
function call into the dictionary, but you want the function address.
The correct code is:
handle_case = {}
handle_case[1] = doCase1
handle_case[2] = doCase2
handle_case[3] = doCase3
handle_case[4] = doCase4
handle_case[5] = doCase5
Or:
handle_case = { 1: doCase1, 2: doCase2, 3: doCase3, 4: doCase4, 5:
doCase 5 }
Or:
try:
{ 1: doCase1, 2: doCase2, 3: doCase3, 4: doCase4, 5: doCase 5 }
[c]()
except:
print Catch the correct exception
But this solutions only works fine if the params of the functions are
the same for all calls (in if/else construct you don't need it).
Bye!
--
Helio Tejedor
--
http://mail.python.org/mailman/listinfo/python-list
Re: Using a switch-like if/else construct versus a dictionary?
asincero [EMAIL PROTECTED] wrote: Which is better: using an if/else construct to simulate a C switch or use a dictionary? Example: Here is a technique I've used a lot. I think I learnt it from Dive into Python class foo(object): def do_1(self): print I'm 1 def do_2(self): print I'm 2 def do_3(self): print I'm 3 def do_4(self): print I'm 4 def do_5(self): print I'm 5 def dispatch(self, x): getattr(self, do_+str(x))() f = foo() f.dispatch(1) f.dispatch(3) f.dispatch(17) It will blow up gracelessly with an AttributeError if you pass it something which it can't handle. That is easy to fix. You could name dispatch, __call__ if you wanted a slightly neater syntax. f = foo() f(1) f(3) f(17) You can do the same thing in a function using locals()[do_+str(x)] but I always find that if I've got to that point I should be using a class instead. Note that in this situation using OO polymorphism instead of a switch- like construct wouldn't be applicable, or at least I can't see how it could be. If there is a type or id in your objects and you are switching on it, then you should be using polymorphism. Make each seperate type a seperate class type and implement the methods for each one. All the switches will disappear from your code like magic! Post more details if you want more help! -- Nick Craig-Wood [EMAIL PROTECTED] -- http://www.craig-wood.com/nick -- http://mail.python.org/mailman/listinfo/python-list
Re: Using a switch-like if/else construct versus a dictionary?
asincero [EMAIL PROTECTED] writes:
def foo():
def doCase1():
pass
def doCase2():
pass
def doCase3():
pass
def doCase4():
pass
def doCase5():
pass
handle_case = {}
handle_case[1] = doCase1()
handle_case[2] = doCase2()
handle_case[3] = doCase3()
handle_case[4] = doCase4()
handle_case[5] = doCase5()
handle_case[c]()
(The above code is wrong. You want the dictionary to map to functions,
not to the result of calling those functions; the call to the function
comes later.)
Why not construct the dictionary in one step? ::
handle_case = {
1: do_case_1,
2: do_case_2,
3: do_case_3,
4: do_case_4,
5: do_case_5,
}
handle_case[c]()
You could even perform the lookup and function call in the same
statement as creating the dictionary, but I prefer the above form for
its readability.
Note that in this situation using OO polymorphism instead of a switch-
like construct wouldn't be applicable, or at least I can't see how it
could be.
I'm not sure what you mean. What would you apply polymorphism to, and
what would be the desired result?
--
\I used to work in a fire hydrant factory. You couldn't park |
`\ anywhere near the place. -- Steven Wright |
_o__) |
Ben Finney
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