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CHAPTER 6 ABSTRACTION
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>>> search(seq, 100) 5 But why go to all this trouble For one thing, you could simply use the list method index, and if you wanted to implement this yourself, you could just make a loop starting at the beginning and iterating along until you found the number. Sure, using index is just fine. But using a simple loop may be a bit inefficient. Remember I said you needed seven questions to find one number (or position) among 100 And the loop obviously needs 100 questions in the worst-case scenario. Big deal, you say. But if the list has 100,000,000,000,000,000,000,000,000,000,000,000 elements, and the same number of questions with a loop (perhaps a somewhat unrealistic size for a Python list), this sort of thing starts to matter. Binary search would then need only 117 questions. Pretty efficient, huh 34
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Tip You can actually find a standard implementation of binary search in the bisect module.
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By now, you are probably used to using functions just like other objects (strings, number, sequences, and so on) by assigning them to variables, passing them as parameters, and returning them from other functions. Some programming languages (such as Scheme or Lisp) use functions in this way to accomplish almost everything. Even though you usually don t rely that heavily on functions in Python (you usually make your own kinds of objects more about that in the next chapter), you can. Python has a few functions that are useful for this sort of functional programming : map, filter, and reduce.4 (In Python 3.0, these are moved to the functools module.) The map and filter functions are not really all that useful in current versions of Python, and you should probably use list comprehensions instead. You can use map to pass all the elements of a sequence through a given function: >>> map(str, range(10)) # Equivalent to [str(i) for i in range(10)] ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'] You use filter to filter out items based on a Boolean function: >>> def func(x): ... return x.isalnum() ... >>> seq = ["foo", "x41", " !", "***"] >>> filter(func, seq) ['foo', 'x41']
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3. In fact, with the estimated number of particles in the observable universe at 1087, you would need only about 290 questions to discern between them! 4. There is also apply, but that was really only needed before we had the splicing discussed previously.
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CHAPTER 6 ABSTRACTION
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For this example, using a list comprehension would mean you didn t need to define the custom function: >>> [x for x in seq if x.isalnum()] ['foo', 'x41'] Actually, there is a feature called lambda expressions,5 which lets you define simple functions in-line (primarily used with map, filter, and reduce): >>> filter(lambda x: x.isalnum(), seq) ['foo', 'x41'] Isn t the list comprehension more readable, though The reduce function cannot easily be replaced by list comprehensions, but you probably won t need its functionality all that often (if ever). It combines the first two elements of a sequence with a given function, combines the result with the third element, and so on until the entire sequence has been processed and a single result remains. For example, if you wanted to sum all the numbers of a sequence, you could use reduce with lambda x, y: x+y (still using the same numbers):6 >>> numbers = [72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33] >>> reduce(lambda x, y: x+y, numbers) 1161 Of course, here you could just as well have used the built-in function sum.
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In this chapter, you ve learned several things about abstraction in general, and functions in particular:56 Abstraction: Abstraction is the art of hiding unnecessary details. You can make your program more abstract by defining functions that handle the details. Function definition: Functions are defined with the def statement. They are blocks of statements that receive values (parameters) from the outside world and may return one or more values as the result of their computation. Parameters: Functions receive what they need to know in the form of parameters variables that are set when the function is called. There are two types of parameters in Python: positional parameters and keyword parameters. Parameters can be made optional by giving them default values.
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5. The name lambda comes from the Greek letter, which is used in mathematics to indicate an anonymous function. 6. Actually, instead of this lambda function, you could import the function add from the operator module, which has a function for each of the built-in operators. Using functions from the operator module is always more efficient than using your own functions.
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