Better Software - November 2007 - (Page 13)

Code Craft default value to return. To better under>>> set1 = 'abc' # A string is a special sequence holding ASCII code points stand reduce, suppose >>> set2 = [1,2,3] # A sequence of integers you want to summa>>> [(x,y) for x in set1 for y in set2 if x != 'b' if y >> map(len,["a", "fine", "mess"]) tion that checks to see if any value in a sequence of Booleans is [1, 4, 4] true: The map function also accommodates functions whose arity matches the number of subsequent lists: def any(bools): return reduce(operator.or_, bools, False) >>> map(operator.add, [1,2,3],[4,5,6]) [5, 7, 9] The reduce function is used to summarize a list, usually reducing it to a single value. For example, to sum the elements of derlined argument above is the initial value from the third a list, you could use the following expression: argument in the original call. >>> reduce(operator.add, [1,2,3]) 6 The function call any([False,True]), for example, computes or_(or_(False,False),True). The accumulated value at each call will be True if any value encountered so far is True. The un- Higher-Order Functions Now consider how to write a function that forms the union of two sets. The result will have everything from one set and In the case of addition, however, it is better to use Python’s everything in the other set that isn’t already in the first. Therefore the initial value can be one of the sets, and the accumulating built-in sum function: function inspects each element in the other set, discarding duplicates. Listing 4 is a first attempt. >>> sum([1,2,3]) 6 The reduce function is handy for other operations, though. The following expression forms the product of a list: >>> reduce(operator.mul, [1,2,3]) 6 def union(s,t): def form_union(sofar,x): return sofar + ([] if x in s else [x]) return reduce(form_union, t, s) Listing 4 An alternate form of reduce takes a third argument as an initializer. This comes in handy in case the list happens to be Since form_union is a one-liner, we can use a lambda expresempty: sion to create the function on-the-fly, instead, as shown in listing 5. The lambda keyword takes a parameter list followed by a colon and a single expression. >>> reduce(operator.mul, [], 1) As listing 6 shows, writing an intersection function is also 1 a one-liner. As shown in listing 3, without the third argument, there is no This time the default is the empty set, and elements of t are retained only if they are also >>> reduce(operator.mul, []) elements of s. Traceback (most recent call last): A function that takes other functions as paFile " ", line 1, in rameters or that returns a function as a result is TypeError: reduce() of empty sequence with no initial value known as a higher-order function. The map and reduce functions are examples that take a funcListing 3 www.StickyMinds.com NOVEMBER 2007 BETTER SOFTWARE 13 http://www.StickyMinds.com

Table of Contents Feed for the Digital Edition of Better Software - November 2007

Better Software - November 2007 Contents MarkYour Calendar Technically Speaking What’s Happening @StickyMinds.com Code Craft Test Connection Management Chronicles The Measure of a Management System Behind the Scenes A Story About User Stories and Test-Driven Development Product Announcements The Last Word Ad Index

Better Software - November 2007

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