Writing Solid Code Introduction to Python

Program 1 python -c "print 'Hello World' “ python -c "import time; print time.asctime()“ Start an interactive session: ◦ python –v  help(command/expression)

Unix Executable Scripts Most portable version: ◦ #!/usr/bin/env python $ cat first.py #!/usr/bin/env python # A comment print 'Hello World‘ print 2**100 $ chmod a+x first.py $ cat first.py #!/usr/bin/env python # A comment print 'Hello World‘ print 2**100 $ chmod a+x first.py $./first.py Hello World $./first.py Hello World

Basic Elements Keywords: and assert break class continue def del elif else except exec finally for from global if import in is lambda not or pass print raise return try while with yield Operators: + - * / % ** // > & | ^ ~ >= <> != == Delimiters: ( ) [ ] { }, :. ' = ; += -= *= /= //= %= &= |= ^= >>= <<= **=

Data Types All data values are objects ◦ type(obj) returns the type. Numbers ◦ Integer: 23, 027 (octal), 0xDA5 (hex) ◦ Floating points : 1.00, 1.0e2 ◦ Complex numbers: 5+6j

Data Types Sequences ◦ Iterables: All sequences are iterable. (for) ◦ Strings:  Can use “” or ‘’ to specify. “abc” ‘abc’ (Same thing.) Unmatched ones can occur within the string. “matt’s”  Use triple double-quotes for multi-line strings or strings than contain both ‘ and “ inside of them: “““a‘b“c”””  Many Methods built into the string, for example: “hello”.upper() gives ‘HELLO’ ◦ Tuples  (x,y)  (100,200,300)

Sequences Lists ◦ [42, 3.14, ‘hello’] ◦ list(‘wow’) gives [‘w’,’o’,’w’] Dictionaries ( key:value pairs ) uses Hash. ◦ D = { ‘x’ : 42, ‘y’:3.14, ‘z’:7} ◦ {1:2, 3:4} ◦ A single dictionary can store values of different types ◦ D[‘x’] is 42. ◦ del D[‘x’] removes the key from D.

Sequences Concatenation: ◦ S1 + S2 ◦ S1 * n gives n copies of S1 concatenated. Membership ◦ x in S : tests to check whether x is in S. ◦ x not in S : Guess? ◦ For strings: x in S means if x is a substring of S Indexing ◦ x = [1,2,3,4] then x[1] is 2 and x[-1] is 4

Sequences Slicing a sequence: ◦ S[i:j]: from item i (included) to item j (excluded) ◦ x = [1,2,3,4] ◦ x[1:3] # [2, 3] ◦ x[1:] # [2, 3, 4] ◦ x[:2] # [1, 2]

List Methods MethodDescription Nonmutating methods L.count(x) Returns the number of items of L that are equal to x. L.index(x) Returns the index of the first occurrence of an item in L that is equal to x, or raises an exception if L has no such item.

List Methods Mutating methods L.append(x) Appends item x to the end of L ; e.g., L[len(L):]=[x]. L.extend(s) Appends all the items of iterable s to the end of L; e.g., L[len(L):]=s. L.insert(i, x) Inserts item x in L before the item at index i, moving following items of L (if any) "rightward" to make space (increases len(L) by one, does not replace any item, does not raise exceptions: acts just like L[i:i]=[x]). L.remove(x) Removes from L the first occurrence of an item in L that is equal to x, or raises an exception if L has no such item.

List Methods MethodDescription L.pop([i]) Returns the value of the item at index i and removes it from L; if i is omitted, removes and returns the last item; raises an exception if L is empty or i is an invalid index in L. L.reverse( )Reverses, in place, the items of L. L.sort([f]) (2.3) Sorts, in place, the items of L, comparing items pairwise via function f; if f is omitted, comparison is via the built-in function cmp. L.sort(cmp=cmp, key=None, reverse=False)(2.4) Sorts, in-place, the items of L, comparing items pairwise via the function passed as cmp (by default, the built-in function cmp). When argument key is not None, what gets compared for each item x is key(x), not x itself.

List Methods  >>> a + [‘whites'] [‘blend','eggs‘,2,234,‘whites']  >>> a.append('!') [‘blend','eggs‘,2,234,'!']  >>> 2*a [‘blend','eggs',2,234,'!',‘blend','eggs',2,234,'!']

Dictionary Methods Nonmutating Methods D.copy( ) Returns a shallow copy of the dictionary (a copy whose items are the same objects as D's, not copies thereof) D.has_key(k) Returns TRue if k is a key in D; otherwise, returns False, just like k in D D.items( ) Returns a new list with all items (key/value pairs) in D D.keys( )Returns a new list with all keys in D D.values( )Returns a new list with all values in D D.iteritems( ) Returns an iterator on all items (key/value pairs) in D D.iterkeys( )Returns an iterator on all keys in D D.itervalues( )Returns an iterator on all values in D D.get(k[, x]) Returns D[k] if k is a key in D; otherwise, returns x (or None, if x is not given)

Dictionary Methods Mutating Methods D.clear( ) Removes all items from D, leaving D empty D.update(D1)For each k in D1, sets D[k] equal to D1[k] D.setdefault(k[, x]) Returns D[k] if k is a key in D; otherwise, sets D[k] equal to x and returns x D.pop(k[, x]) Removes and returns D[k] if k is a key in D; otherwise, returns x (or raises an exception if x is not given) D.popitem( ) Removes and returns an arbitrary item (key/value pair)

Control Flow if expression: statement(s) elif expression: statement(s) elif expression: statement(s)... else: statement(s)

Control Flow if x < 0: print "x is negative" elif x % 2: print "x is positive and odd" else: print "x is even and non-negative"

Control Flow : while x = 64 count = 0 while x > 0: x = x // 2 # truncating division count += 1 print "The approximate log2 is", count # if count == 6: break

Control Flow : for for target in iterable: statement(s) for letter in "ciao": if letter == ‘c’: continue print "give me a", letter, "...“ for key, value in d.items( ): # cannot use iteritems if not key or not value: # keep only true keys and values del d[key] for x in range(1,5): print x # output:

Sample Code #!/usr/bin/env python import string, sys # If no arguments were given, print a helpful message if len(sys.argv)==1: print 'Usage: celsius temp1 temp2...' sys.exit(0) # Loop over the arguments for i in sys.argv[1:]: fahrenheit=float(string.atoi(i)) celsius=(fahrenheit-32)*5.0/9.0 print '%i\260F = %i\260C' % (int(fahrenheit), int(celsius+.5))

Functions def function-name(parameters): statement(s) def double(x): return x*2 Calling Functions in python function-object(arguments) print double(432) def f(x, y): x = 23 y.append(42) a = 77 b = [99] f(a, b) print a, b

Import statement A Typical python program is made up of several source files. Each source file corresponds to a module. “import” keyword allows to include other modules into a python program. Modules ◦ sys: stdin, stderr, argv ◦ os: system, path ◦ string: split ◦ re: match compile ◦ math: exp, sin, sqrt, pow

Calling External programs import os ◦ subprocess.Popen(["ls", "-la"]).wait()

Sample Program # average4.py # A program to average a set of numbers # Illustrates sentinel loop using empty string as sentinel def main(): sum = 0.0 count = 0 xStr = raw_input("Enter a number ( to quit) >> ") while xStr != "": x = eval(xStr) sum = sum + x count = count + 1 xStr = raw_input("Enter a number ( to quit) >> ") print "\nThe average of the numbers is", sum / count

Output Enter a number ( to quit) >> 34 Enter a number ( to quit) >> 23 Enter a number ( to quit) >> 0 Enter a number ( to quit) >> -25 Enter a number ( to quit) >> Enter a number ( to quit) >> 22.7 Enter a number ( to quit) >> The average of the numbers is

Sample Program # average5.py # Computes the average of numbers listed in a file. def main(): fileName = raw_input("What file are the numbers in? ") infile = open(fileName,'r') sum = 0.0 count = 0 for line in infile.readlines(): sum = sum + eval(line) count = count + 1 print "\nThe average of the numbers is", sum / count

Sample Program # average6.py # Computes the average of numbers listed in a file. def main(): fileName = raw_input("What file are the numbers in? ") infile = open(fileName,'r') sum = 0.0 count = 0 line = infile.readline() while line != "": sum = sum + eval(line) count = count + 1 line = infile.readline() print "\nThe average of the numbers is", sum / count

Assignment for today Implement: closest_pair([(0,0),(7,6),(2,20),(12,5),(16,16),(5,8),(19,7),(14,22),(8,19),(7,29),(10,11),(1,13)]) returns: (7,6),(5,8) Should run in O(nlogn)

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