Create Python276tutorial-FunctionalProgramming.py

Author: InonS

Python276tutorial-FunctionalProgramming.py

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+#! /usr/bin/python
+
+# In Python, and other languages, functions are "first class citizens":
+# They are objects which can be treated as you would a variable or any other object 
+# (passed as argument to another function, returned by a function, and so on). 
+#
+# A function which takes or returns a function is called a functional, or higher-order function 
+# (e.g. an integral takes a function as an argument). 
+#
+# Use of these properties is called the functional programming paradigm.
+#
+# The origin for (and simplest instance of) a functional object is the labmda expression.
+
+#http://docs.python.org/release/2.7.6/tutorial/
+
+#==============================================
+
+# 4.7.5 Lambda expressions (c.f. C function pointers)
+# "Lambda expression" here is a small, anonymous function;
+# used whenever a function is required to be represented as an object.
+# Syntactically restricted to a single expression...
+
+# Lambda expression returning unevaluated function object
+add = lambda a,b: a+b
+print add(1,2) # evaluate function object
+argsList=[2,3]
+print apply(add,argsList) # using non-essential 'apply()' function
+
+# Lambda expresssion referrencing value from containing scope (like nested function definitions)
+def make_incrementor(n):
+    return lambda x: x+n # given parameter 'n' and taking argument 'x', return the value 'x+n'
+f=make_incrementor(42) # 'f' is now a function object with n=42
+print "f(0)= ", f(0) # print the evaluated function for x=0
+print "f(1)= ", f(1) # print the evaluated function for x=1
+
+# Sort list according to a key, where the key is a lambda expression
+pairs=[(1,'one'),(2,'two'),(3,'three'),(4,'four')]
+print "Before sort: ", pairs
+pairKey=lambda pair: pair[1] # Lambda expression which, given an object 'pair', returns the [1] element (i.e. the second elemnt in the tuple)
+pairs.sort(key=pairKey) # Sort pairs according to the name of the number (i.e. alphabetically)
+print "After sort: ", pairs
+
+#==============================================
+print
+
+# 5.1.3 Functional programming (functional = higher-order function)
+
+# Filter (aka C++ <algorithm> remove_copy_if() , etc.)
+
+# Example: filter out Capital letters, punctuation marks, and spaces
+stringEx="Hello World!"
+print stringEx
+def someBoolFunction(x):
+    return (ord(x) > 97) # is the ASCII value of the character x greater than 97?
+print filter(someBoolFunction,stringEx) # returns only string, tuple or list (depending on input)
+
+# Example: compute a sequence of numbers not divisible by 2 or 3
+def notDiv2or3(x):
+    return (0 != x % 2) and (0 != x % 3)
+print filter(notDiv2or3, range(2,25)) # recall that range is inclusive of 'from' argument and exclusive of 'to' argument
+
+#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+print
+
+# Map (aka C++ <algorithm> transform()
+
+# Example: "do for each" return cube
+def cube(x):
+    return x*x*x
+print map(cube, range(1,10))
+
+# Example: map multiple sequences (e.g. lists)
+def add2(a,b):
+##    if int(a) and int(b): return a+b
+##    elif int(a): return a                 # i.e. ! int(b)
+##    else: return b                        #  int(b) but ! int(a)
+##    if (type(a) is IntType) and (type(b) is IntType): return a+b # requires 'from types import *'
+    return a+b if (type(a)== type(b)) else 'undefined'
+seq1=range(0,8)
+seq2=range(-6,4) # note that len(seq2) > len(seq1)
+print map(add2,seq1,seq2)
+
+#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+print
+
+# Reduce (aka Fold, C++ <numeric> accumulate()
+# see also python 3 'compose' and Haskell 'scan'
+
+# serial composition of binary function (takes two arguments at a time) from left to right:
+
+# leftFold(f;a,b,c,d)  = f( f( f(a,b) ,c) ,d)
+print "((2/3)/4)/5 = 1/30",
+def div(x,y): return float(x)/float(y)
+l=range(2,6)
+print reduce(div,l)
+
+# can take initial value argument:
+print "((((1)/2)/3)/4)/5 = 1/120",
+initVal=1
+print reduce(div,l,initVal)
+
+# rightFold(f;a,b,c,d) = f(a , f(b, f(c,d) ) )
+print "2/(3/(4/5)) = 8/15",
+def vid(x,y):
+    return div(y,x) # reverse function object
+lRev=reversed(range(2,6)) # reverse argument list (compose d,c, then b, then a ...)
+print reduce(vid,lRev)
+
+# Example: sum numbers from 1 through 10 (inclusive)
+def add(x,y): return x+y
+print reduce(add,range(1,11))
+
+
+#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+print
+
+# Example: Parralelization of search for all multiples of 'q' in the range [a:b]
+# 1) Split range [a:b] to n sub-ranges.
+# 2) MAP multiple-finding function to each sub-range, and execute in parallel.
+# 3) REDUCE result lists back to a single list.
+
+
+#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+print
+
+# Creating a list using functional programming
+# specifically, using map()...
+
+squaresLambda=map(lambda x: x**2, range(3))
+print squaresLambda
+
+
+#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+print
+# 5.1.4 listcomp: List comprehension
+# Concise way of creating a list in which each element is the result of some opperation, applied to each member of another sequence or iterable.
+
+squaresExplicit=[] # create empty list
+for x in range(3):
+    squaresExplicit.append(x**2)
+print squaresExplicit
+
+squaresListcomp=[x**2 for x in range(3)] #concise expression, using list compehension
+print squaresListcomp
+
+
+#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+print
+# More complex list comprehension (more than one 'for' and additional 'if' clauses)
+# For example, the sequence of subsequences of non-negative integers,
+# such that each subsequence is shorter than 6 integers long (including zero)
+seqOfsubSeq=[x for y in range(10) for x in range(y) if y<6]
+print seqOfsubSeq