61A Lecture 3 Similar in format to a homework assignment. If you - PDF document

Announcements • Homework 1 is due next Tuesday at 5pm (no email when you submit).  Homework is graded for effort. • Take-home quiz released next Wednesday 9/11 at 1pm, due Thursday 9/12 at 11:59pm.  3 points, graded for correctness. 61A Lecture 3  Similar in format to a homework assignment.  If you receive 0/3, you will need to talk to the course staff or be dropped. Friday, September 6  Open-computer : You can use the Python interpreter, watch course videos, and read the online text (http://composingprograms.com).  No external resources : Please don't search for answers, talk to your classmates, etc. • Project 1 posted this Friday, due Thursday 9/19 at 11:59pm.  Demo during next lecture 2 Life Cycle of a User-Defined Function What happens? Formal parameter R e t u r n e x p r e s s i o n N a m e Def statement: >>> def square ( x ): A new function is created! return mul(x, x) Name bound to that function D e f in the current frame s t a t e m e n t B o d y (return statement) Multiple Environments operand: 2+2 Operator & operands evaluated Call expression: square(2+2) argument: 4 Function (value of operator) called on arguments operator: square (values of operands) function: func square(x) A new frame is created! 4 square ( x ): Calling/Applying: Parameters bound to arguments 16 Argument Signature Body is executed in that new environment Return value 4 Multiple Environments in One Diagram! Multiple Environments in One Diagram! square(square(3)) square(square(3)) func square(x) func square(x) 9 square(3) square(3) func square(x) func square(x) 3 3 5 6 Example: http://goo.gl/XVtEms Example: http://goo.gl/XVtEms

Multiple Environments in One Diagram! Names Have No Meaning Without Environments 1 1 2 2 2 2 1 1 81 square(square(3)) Every expression is 1 1 func square(x) 9 evaluated in the context of an environment. square(3) A name evaluates to the func square(x) An environment is a sequence of frames . An environment is a sequence of frames . 3 value bound to that name in the earliest frame of • • The global frame alone The global frame alone the current environment in • • A local, then the global frame which that name is found. A local, then the global frame 7 8 Example: http://goo.gl/XVtEms Example: http://goo.gl/XVtEms Miscellaneous Python Features Conditional Statements Operators Multiple Return Values Docstrings Doctests Default Arguments (Demo) Statements Compound Statements Compound statements : A statement is executed by the interpreter to perform an action A suite is a sequence of <header>: statements <statement> Suite Compound statements : <statement> ... The first header determines a <separating header>: Statement To “execute” a suite means to statement’s type <statement> Clause execute its sequence of <statement> statements, in order ... <header>: ... <statement> The header of a clause <statement> Suite “controls” the suite that ... follows <separating header>: Execution Rule for a sequence of statements: <statement> <statement> def statements are compound ... • Execute the first statement statements ... • Unless directed otherwise, execute the rest 11 12

Conditional Statements Boolean Contexts (Demo) def absolute_value(x): def absolute_value(x): """Return the absolute value of x.""" """Return the absolute value of x.""" if x < 0: if x < 0: 1 statement, return -x return -x elif x == 0: elif x == 0: 3 clauses, 3 headers, return 0 return 0 else : else : 3 suites return x return x George Boole Execution rule for conditional statements: Syntax Tips Each clause is considered in order. 1. Always starts with "if" clause. 1. Evaluate the header's expression. 2. Zero or more "elif" clauses. 2. If it is a true value, 3. Zero or one "else" clause, execute the suite & skip the remaining clauses. always at the end. 13 14 Boolean Contexts def absolute_value(x): """Return the absolute value of x.""" if x < 0: return -x elif x == 0: Two boolean contexts Two boolean contexts return 0 Iteration else : return x George Boole False values in Python: False, 0, '', None (more to come) True values in Python: Anything else (True) Read Section 1.5.4! 15 Reading: http://composingprograms.com/pages/15-control.html#conditional-statements While Statements Discussion Question (Demo) Complete the following definition by placing an expression in ______________________ . def choose(total, selection): """Return the number of ways to choose SELECTION items from TOTAL. 1 2 3 1 3 6 choose(n, k) is typically defined in math as: n! / (n-k)! / k! >>> choose(5, 2) n · ( n − 1) · ( n − 2) · . . . · ( n − k + 1) 10 k · ( k − 1) · ( k − 2) · . . . · 2 · 1 >>> choose(20, 6) Execution rule for while statements: George Boole 38760 """ 1. Evaluate the header’s expression. ways = 1 2. If it is a true value, selected = 0 execute the ( whole ) suite, ... while selected < selection: then return to step 1. ... selected = selected + 1 total // selected ways, total = ways * ______________________ , total - 1 return ways 17 18 Example: http://goo.gl/0d2cjF Example: http://goo.gl/38ch3o