Lecture 4: Defining Functions (Ch ) CS 1110 Introduction to Computing Using Python

Transcription

1 Lecture 4: Defining Functions (Ch..4-.) CS 0 Introduction to Computing Using Python [E. Andersen, A. Bracy, D. Gries, L. Lee, S. Marschner, C. Van Loan, W. White]

2 From last time: Function Calls Function expressions have the form fun(x,y, ) Examples (math functions that work in Python): round(2.4) max(a+,24) function name argument Let s define our own functions! 2

3 Anatomy of a Function Definition name parameters def increment(n): Function Header """Returns: the value of n+""" return n+ Docstring Specification Statements to execute when called also called Function Body The vertical line indicates indentation Use vertical lines when you write Python on exams so we can see indentation

4 The return Statement Passes a value from the function to the caller Format: return <expression> Any statements after return are ignored Optional (if absent, special value None will be sent back) 4

5 Function Definitions vs. Calls def increment(n): return n+ increment(2) Function definition Defines what function does Declaration of parameter n Parameter: the variable that is listed within the parentheses of a function header. Function call Command to do the function Argument to assign to n Argument: a value to assign to the function parameter when it is called simple_math.py 5

6 Executing the script simple_math.py # simple_math.py """script that defines and calls one simple math function"" Python skips Python skips Show in python tutor (put an error inside fn) def increment(n): """Returns: n+""" return n+ Python learns about the function Python skips everything inside the function increment(2) Python executes this statement C:/> python simple_math.py Now Python executes the function body 6

7 Understanding How Functions Work We will draw pictures to show what is in memory Function Frame: Representation of function call Draw parameters as variables (named boxes) Number of statement in the function body to execute next Starts with function name instruction counter parameters local variables (later in lecture) Note: slightly different than in the book (.9) Please do it this way. 7

8 Example: get_feet in height.py module >>> import height >>> height.get_feet(68) def get_feet(ht_in_inches): return ht_in_inches // 2 8

9 Example: get_feet(68) PHASE : Set up call frame next line to execute. Draw a frame for the call 2. Assign the argument value to the parameter (in frame). Indicate next line to execute get_feet ht_in_inches 68 def get_feet(ht_in_inches): return ht_in_inches // 2 9

10 Example: get_feet(68) PHASE 2: Execute function body Return statement creates a special variable for result get_feet ht_in_inches 68 RETURN 5 def get_feet(ht_in_inches): return ht_in_inches // 2 0

11 Example: get_feet(68) PHASE 2: Execute function body The return terminates; no next line to execute get_feet ht_in_inches 68 RETURN 5 def get_feet(ht_in_inches): return ht_in_inches // 2

12 Example: get_feet(68) PHASE : Erase call frame get_feet ht_in_inches 68 RETURN 5 def get_feet(ht_in_inches): return ht_in_inches // 2 2

13 Example: get_feet(68) PHASE : Erase call frame But don t actually erase on an exam def get_feet(ht_in_inches): return ht_in_inches // 2 ERASE WHOLE FRAME

14 Local Variables () Call frames can make local variables >>> import height >>> height.get_feet(68) get_feet def get_feet(ht_in_inches): ht_in_inches 68 2 feet = ht_in_inches // 2 return feet 4

15 Local Variables (2) Call frames can make local variables >>> import height >>> height.get_feet(68) get_feet 2 def get_feet(ht_in_inches): ht_in_inches 68 2 feet = ht_in_inches // 2 return feet feet 5 5

16 Local Variables () Call frames can make local variables >>> import height >>> height.get_feet(68) get_feet 2 def get_feet(ht_in_inches): ht_in_inches 68 2 feet = ht_in_inches // 2 return feet feet 5 RETURN 5 6

17 Local Variables (4) Call frames can make local variables >>> import height >>> height.get_feet(68) 2 def get_feet(ht_in_inches): feet = ht_in_inches // 2 return feet ERASE WHOLE FRAME Variables are gone! This function is over. 7

18 Exercise Time 2 Function Definition def foo(a,b): x = a y = b return x*y+y >>> foo(,4) Function Call What does the frame look like at the start? 8

19 Which One is Closest to Your Answer? A: B: foo a b 4 foo a b 4 x a C: D: foo a b 4 x foo a b 4 x y 9

20 And the answer is A: B: foo a b 4 foo a b 4 x a C: D: foo a b 4 foo a b 4 x x y 20

21 Exercise Time 2 Function Definition def foo(a,b): x = a y = b return x*y+y >>> foo(,4) B: Function Call foo a b 4 What is the next step? 2

22 Which One is Closest to Your Answer? A: B: foo 2 a b 4 foo a b 4 x C: D: foo 2 a b 4 x foo 2 a b 4 x y 22

23 And the answer is A: B: foo 2 a b 4 foo a b 4 x C: D: foo 2 a b 4 x foo 2 a b 4 x y 2

24 Exercise Time 2 Function Definition def foo(a,b): x = a y = b return x*y+y >>> foo(,4) Function Call foo 2 a b 4 x What is the next step? 24

25 Exercise Time 2 Function Definition def foo(a,b): x = a y = b return x*y+y >>> foo(,4) Function Call foo a b 4 x y 4 What is the next step? 25

26 Which One is Closest to Your Answer? A: B: foo foo RETURN 6 a b 4 x y 4 RETURN 6 C: D: foo a b 4 x y 4 RETURN 6 ERASE THE FRAME 26

27 And the answer is A: B: foo foo RETURN 6 a b 4 x y 4 RETURN 6 C: D: foo a b 4 x y 4 RETURN 6 ERASE THE FRAME 27

28 Exercise Time 2 Function Definition def foo(a,b): x = a y = b return x*y+y >>> foo(,4) foo Function Call a b 4 x y 4 RETURN 6 What is the next step? 28

29 Exercise Time 2 Function Definition def foo(a,b): x = a y = b return x*y+y >>> foo(,4) >>> 6 Function Call ERASE THE FRAME 29

30 Function Access to Global Space Top-most location in memory called global space Functions can access anything in that global space Global Space INCHES_PER_FT get_feet 2 2 INCHES_PER_FT = 2 def get_feet(ht_in_inches): feet = ht_in_inches // INCHES_PER_FT return feet get_feet 2 ht_in_inches 68 feet 5 get_feet(68) 0

31 What about this?? 2 What if you choose a local variable inside a function that happens to also be a global variable? INCHES_PER_FT = 2 feet = plural of foot def get_feet(ht_in_inches): feet = ht_in_inches // INCHES_PER_FT return feet get_feet(68) Global Space INCHES_PER_FT 2 feet plural of foot get_feet get_feet ht_in_inches 68

32 Look, but don t touch! Can t change global variables Assignment to a global makes a new local variable! INCHES_PER_FT = 2 feet = plural of foot Global Space INCHES_PER_FT 2 feet plural of foot get_feet get_feet 2 ht_in_inches 68 def get_feet(ht_in_inches): feet 5 2 feet = ht_in_inches // INCHES_PER_FT return feet get_feet(68) 2