PL Recitation 9/21/2010

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Abner Caldwell
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1 PL Recitation 9/21/2010

2 Recitation Leader Joanna Gilberti Office: WWH, Room 328 Web Site: courses/pl/index.html (linked from main course Web site)

3 Homework Submission Guidelines Submit all homework to me via subject: PL firstname lastname homework # (EXAMPLE: PL Joanna Gilberti Homework 1 ) Use file naming convention on all attachments: firstname_lastname_hw_# (example: "joanna_gilberti_hw_1") Include your name on all files (including source code)

4 Homework Submission Procedure All homework assignments are to be submitted electronically on the day they are due. Homework assignments submitted one day late from the due date will be penalized by 50%. Homework assignments will not be accepted if two days late (solutions will be posted) I will provide an notification ( received or late ) upon receipt of homework.

5 What s Covered in Recitation Homework solutions. Questions on the assignments. For additional questions on assignments, it is best to contact me via . I will hold office hours (time will be posted on the recitation Web site). Run sample problems that demonstrate concepts covered in class.

6 Homework 1: Clarification Question 2 must provide actual statements. It is not sufficient to just provide a count. Question 5 provide a sample whenever possible to justify your answer (don t simply answer yes or no, for example).

7 Binding & Scoping Concepts

8 Binding Binding - the operation of associating two things, like a name and the entity it represents. Binding time - the moment when the binding is performed (compilation, execution, etc). Static binding - dynamic binding. Refers to compilation vs. runtime.

9 Binding Time Language design - fundamental aspects of the language, built-in functions, keywords. Language implementation - details such as the size of each type, runtime exceptions. Programming - algorithms, design of data structures. Compilation - mapping between higher-level constructs and machine code, static data. Linking - layout of program in memory. Load - virtual addresses, dynamic libraries. Runtime - virtual functions, values to variables, many more.

10 Scope & Rules Scope of a binding - the textual region of the program in which a binding is active. Scope - sometimes a region of a program of maximal size in which no binding changes scope.

11 Scope of a Binding Usually the scope of a binding is determined statically, meaning at compilation time. When a function is called that has a local variable, the binding between the variable name and the instance of the variable local to the call is created. Any previous bindings for that same variable name are deactivated in the process (or hidden). When the function call ends, the previous binding for the name is restored.

12 Static (Lexical) vs. Dynamic Scopes Static scope - when the scope of a binding is determined during compilation. Current binding - the matching declaration whose block most closely surrounds the point in the program where the name is mentioned. Dynamic scope - Bindings that are defined at runtime. They depend on the order in which functions are called. Current binding - the one encountered the most recently during the execution and that has not yet been destroyed.

13 Dynamic Scope - Lisp (defvar y 3) ; y is global (defun f (x) (+ x y)) ; we think that f uses global y (f 10) ; => 13 (defun g () (let ((y 4) (a 1)) (f a))) (g) ; => 5 ; actually, f does not use global y

14 Static Scope - Python Python appears to have dynamic scope, but, in fact, has static scope. Example: pi = def area(r): return pi*r*r area(10) # pi = 3 Area(10) # 300 * in the second call to Area(), pi is changed.

15 Static Scope - Python Another example: pi = pi_holder = 10 def create_area(): pi_holder = pi # local pi_holder def area(r): return pi_holder*r*r return area area = create_area() area(10) # pi_holder # Still 10 pi_holder = 3 area(10) # Still

16 Static Scope - Python Analysis: The first example works, but is problematic. However, in the second example, when we declare pi_holder=10, which: (1) pi_holder inside of create_area() is a local variable; (2) the local pi_holder is in area s scope, thus changing the value of pi_holder does not affect it.

17 Static Scope - Python A third example (re-write of the second python example): pi = def create_area(pi = pi): def area(r): return pi*r*r return area area = create_area() area(10) # pi = 3 area(10) # ** Python has static scoping! Python functions inherit the surrounding scope, and so to prevent this, you would need to wrap your function definition inside another function that copies in the values you need into its local variables.

Programming Languages

Programming Languages Recitation Summer 2014 Recitation Leader Joanna Gilberti Email: jlg204@cs.nyu.edu Office: WWH, Room 328 Web site: http://cims.nyu.edu/~jlg204/courses/pl/index.html Homework Submission