Programming Languages

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1 CSE 130 : Fall 2008 Programming Languages Lecture 12: What s in a Name? Ranjit Jhala UC San Diego A crash course in Python Interpreted, imperative, OO Language g Everything is an object Dynamic Typing Programs are made up of: Expressions Statements Assignment if/elif/else while-loops Functions Classes (still to come) Today: Revisit some objects Exploit features and build powerful expressions Base: int, float, complex Sequence: string, tuple, list What can sequences do? Select i-th element: s[i] subsequence ( slice ): s[i:j] Update -- For mutable sequences (e.g. Lists) Update i-th element: s[i] = e Update subsequence: s[i:j] = e

2 Update subsequence s[i:j]=e x in s Update subsequence s[i:j]=e >>> z = [1,2,3,4,5,6,7,8,9,10] >>> z[3:6] = [ a, b, c ] >>> z [1,2,3, a, b, c,7,8,9,10],,,7,8,9,10] >>> z[3:6] = [ a, b ] * 2 >>> z [1,2,3, a, b, a, b,7,8,9,10] >>> z[4:]=[] >>> z [1,2,3, a ] >>> z[:0] = [ al, be ] >>> z [ al, be,1,2,3, a, b, a, b,7,8,9,10] Update subsequence: s[i:j] = e Changes the object referred to by s May change the length of the sequence Increase: if RHS length > j-i Decrease: if RHS length < j-i What else can sequences do? Sequence contains x in s Q: Suppose you are given a sequence s How to find if the element x appears in s? x in s Works for any sequence type >>> a in cat True >>> a in entebbe False >>> a in ( c, a, t ) True >>> 2 in [1,2,3,4,5] True >>> 2 in [1,4, 92,2.4] 2 4] False

3 What can sequences do? Select i-th element: s[i] subsequence ( slice ): s[i:j] Update -- For mutable sequences (e.g. Lists) Update i-th element: s[i] = e Update subsequence: s[i:j] = e Member Is an element in a sequence: x in s Doesn t Python have For-Loops? Why haven t we seen For-loops yet? Because they are connected to sequences For-loops are used to iterate over sequences Unlike in C/Java/ Elegant, powerful mechanism use it! x=s[0] <BODY> for x in s: <BODY> x=s[1] <BODY>... x=s[len(s)-1] <BODY> Iteration for x in s: >>> for x in [ how, many, cannonballs ]: print x,len(x) how 3 many 4 cannonballs 11 Works for any sequence >>> for c in chimichanga : print c*3 ccc hhh iii mmm Iteration for x in s: >>> s=0 >>> z=(1,2,3,4.0, 5 ) #tuple >>> for i in z: s = s + i ERROR >>> s 10 >>> s=0 Can t add string to float >>> for i in z: Note that first 4 elts added! s=s+float(i) Dynamic Types! >>> s Run-time Type Error 15

4 Iteration + binding for x, in s: If s is a sequence of tuples/sequences, then we can Bind to individual elements of subsequences >>>mexmenu = [( burrito,3.50),( taco,1.90),( sopa,2. 50),( quesadilla,3.00),( frijole,0.75)] >>>for i,p in menu: print One,i, costs,p One burrito costs 3.50 One taco costs 1.90 One sopa costs 2.50 One quesadilla costs 3.00 One frijole costs 0.75 Old school For-loops There s a simple way to write good-old for-loops Built-in in function: range >>> range(10) [0,1,2,3,4,5,6,7,8,9] >> for i in range(10): print i for(i=0,i<10,i++){ print i; } >>> range(5,15) #fixed upper bound [5,6,7,8,9,10,11,12,13,14] >>> range(15,5,-1) #step [15, 14, 13, 12, 11, 10, 9, 8, 7, 6] range(len( )) to get position def printpos(s): for i in range(len(s)): print at position:,i, we have,s[i] >>> printpos((5, 15,[2,5])) at position 0 we have 5 at position 1 we have 15 at position 2 we have [2,5] >>> printpos( chimichanga ) at position 0 we have c at position 1 we have h at position 2 we have i Polymorphism via Dynamic Typing But lookout! For-loops are used to iterate over sequences for x in s: <BODY> What if object referred to by s is changed in BODY? Unpleasantness ensues: Try to ensure this never happens Iterate over a copy of the object s[:]

5 But lookout! def dup1(s,k): for x in s: print x s[len(s):] = x*k Adds x*k to end object being iterated over! Loops forever def dup2(s,k): for x in s: print x s = s + x*k return s Creates new object w/ x*k added at end Iteration object is what x originally referred to Unchanged To make it more readable def dup3(s,k): def dup2(s,k): for x in s[:]: for x in s: print x print x s = s + x*k s = s + x*k return s return s Creates new object w/ x*k added at end Iteration object is what x originally referred to Unchanged What can sequences do? Select i-th element: s[i] subsequence ( slice ): s[i:j] Update -- For mutable sequences (e.g. Lists) Update i-th element: s[i] = e Update subsequence: s[i:j] = e Member: x in s Iteration: for x in s: <body> What else? Three useful functions for lists from ML? map filter fold (a.k.a. reduce) Built-in in Python:

6 map def dup(x): return 2*x >>> z = range(10) >>> z [0,1,2,3,4,5,6,7,8,9] >>> map(dup,z) [0,2,4,6,8,10,12,14,16,18] >>>map(dup, chimichanga ) [ cc, hh, ii, mm, ii, cc, hh, aa, nn, gg, aa] Works for all sequences, returns a list More flexible ways to call it, see documentation filter Works for all sequences, returns same kind of sequence >>> def even(x): return int(x)%2==0 >>> filter(even,range(10)) range(10)) [0,2,4,6,8] >>> filter(even, ) >>> filter(even,(1,2.0,3.2,4)) (2,4) Again, note the polymorphism p that we get from dynamic types and conversion reduce i.e. fold >>> def add(x,y): x+y >>> reduce(add,range(10),0) 45 >>> def fac(x): def mul(x,y): return x*y return reduce(prod,range(x+1),1) >>> fac(5) 120 What can sequences do? Select i-th element: s[i] subsequence ( slice ): s[i:j] Update -- For mutable sequences (e.g. Lists) Update i-th element: s[i] = e Update subsequence: s[i:j] = e Update subsequence: s[i:j] = e Member: x in s Iteration: for x in s: <body> Iteration: for x in s: body map,filter,reduce

7 List Comprehensions List Comprehensions A cleaner, nicer way to do map-like operations >>> [ x*x x for x in range(10)] [0,1,4,9,16,25,36,49,64,81] >>> [2*x for x in avra kedavra ] [ aa, vv, rr, aa,, kk, ] Syntax: [e x for x in s] A cleaner, nicer way to do map+filter-like operations >>> [ x*x for x in range(10) if even(x)] [0,4,16,36,64] >>> [ 2*x for x in if even(x)] [ 00, 22, 44, 66 ] >>> [z[0] for z in mexmenu if z[1]<3.0] [ taco, sopa, frijole ], Syntax: [e x for x in s if c x ] List Comprehensions Can nest the for to iterate over multiple sequences >>>[(x,y) for x in range(3) for y range(3)] [(0,0),(0,1),(0,2),(1,0),(1,1),(1,2),(2,0),(2, 1),(2,2)] >>>[(x,y) for x in range(3) for y in range(3) if x > y] [(1,0),(2,0),(2,1)] Syntax: >>> [e x for x in s if c x ] What can sequences do? Select i-th element: s[i] subsequence ( slice ): s[i:j] Update -- For mutable sequences (e.g. Lists) Update i-th element: s[i] = e Update subsequence: s[i:j] = e Member: x in s Iteration: for x in s: <body> map,filter,reduce Comprehensions: [e x for x in s if c x ]

8 Quicksort in Python def sort(l): if L==[]: return L else: l=sort([x for x in s[1:] if x < s[0]) r=sort([x for x in s[1:] if x >= s[0]) return(l+l[0:1]+r) Today: Revisit some objects Exploit features and build powerful expressions Base: int, float, complex Sequence: string, tuple, list Maps (Dictionary): key value Key data structure: Dictionaries Associative arrays, y, Hash tables A table storing a set of keys, And a value for each key. Any y( (immutable) object can be a key! int,float,string,tuples Very useful! Using Dictionaries Unsorted list of key,value pairs Empty Dictionary: {} Non-empty Dictionary: {k1:v1,k2:v2, } Membership: is k in dict: k in d Lookup value of key: d[k] Set value of key: d[k]=v

9 Dictionaries >>> d={} >>> d=dict(mexmenu) >>> d[ ceviche ] = 3.95 >>> d { } >>> d[ burrito ] 3.50 >>> d.keys() >>> d.values() Dictionaries def freq(s): d={} for c in s: if c in d: d[c]+=1 else: d[c]=1 return d def plotfreq(s): d=freq(s) for k in d.keys(): print k, * *d[k] >>> d=plotfreq([1,1,3.0, A,3.0, A, A,1,2,3.0,1, A ]) >>> d >>> d = plotfreq( avrakedavra ) >>> d.keys() >>> d Recap: Python Tutorial >>> f = open( foo.txt, read ) >>> f.readlines() >>> for l in f.readlines(): <BODY> >>> f.close You now know enough to do PA6 Python Tutorial: How to open files, read lines Use the help command Document every function: What does it do? Base Types Sequence Types Loops Functions List comprehensions map, filter, reduce enough to do PA 6

10 Next: What s in a name? More precisely: How should programmer think of data What does a variable x really mean? What s in a name? ML (or Functional Languages) Namerefers to a Value Binding maps Names to Values Environment list of bindings Environment can be extended d Environment can t be changed Data model in functional PL Vars = names in phonebook Evaluation = Most recent Environment frozen in function value behavior of function cannot be changed easier reasoning

11 Data model in OO langs Variables point to objects Objects = boxes with data inside X pumpkin Y Namespaces Manage variable names in Python Similar to, but different from Environments Core PL concept, unifies many ideas We will see very important differences Z [1,2,3] Ok, but what IS a namespace? A mapping from names to objects Namespaces vs. Environments Both are maps from variables to something X pumpkin Namespace Environment Y Z [1,2,3] What s the difference? 1. Assignment 2. Updates/Mutation t ti

12 1. Assignment Basic operation in Imperative PL: x = e Simple example i,s = 0,0 while (i <= 3): i,s = i+1, s+i 1. Compute object corresponding to e 2. Change the name x to refer to object Same name s - points to different objects -namespace is not extended 1. Assignment Basic operation in Imperative PL: x = e 1. Compute object corresponding to e 2. Change the name x to refer to object Assignment: changes box that name refers to 2. Update/Mutation Change what s inside the box (object) - Not with immutable objects -eg. integers -But with mutable objects - eg. tuples, arrays, lists, dictionaries >>> x = [100,200] >>> x [100, 200] >>> x[0] = "gobble gobble" >>> x ['gobble gobble', 200] >>> x How is it different from build a new box with updated value inside?

Aliasing Two or more names refer to same object Peter Parker Aliasing Two or more names

Update Two or more names refer to same object >> x = [100,200] >> y = x >> y[0] = gobble

object >> x = [100,200] >> y = x >> y[0] = gobble gobble" >> x [ gobble gobble,200] If

13 Aliasing Two or more names refer to same object Peter Parker Aliasing Two or more names refer to same object >> x = [100,200] >> y = x X Y [100,200] Spider Man Aliasing and Update Two or more names refer to same object >> x = [100,200] >> y = x >> y[0] = gobble gobble" >> x X Y [ gobble gobble [100,200],200] Aliasing Two or more names refer to same object >> x = [100,200] >> y = x >> y[0] = gobble gobble" >> x [ gobble gobble,200] If multiple names refer to same object, update affects values of all names X Y [ gobble gobble [100,200],200]

14 Aliasing Does not happen in Ocaml/Functional PLs actually it does happen (where?) but not exposed to the (130) programmer Aliasing Good because? Does happen in every imperative PL Java, Python: names point to objects C: names point to memory cells Bad because?

Programming Languages

CSE 130 : Spring 2010 Programming Languages News PA 4 is up (Due Fri, 5pm) Lecture 12: Crash course in Python Ranjit Jhala UC San Diego MidtermGeneral Statistics Total of 80 points < 20 9 people Mean: