More Loop Examples Functions and Parameters

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Homer Garrett
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1 More Loop Eamples Functions and Parameters

2 Eample: min difference Given two of numbers, compute the minimum difference among any pair of numbers, one from each. E.g., 1 = [1, 2, 3, 4], 2 = [-2, 10, 5, 0, 6], the function mindiff would return 1, which occurs twice, {1,0}, {4,5}. Remember the findmin function? def findmin( alist ): min = alist[0] for in alist: if min > : min = return min We can use similar ideas.

3 Eample: min difference def mindiff( 1, 2 ): The parameters 1, 2 are two int s. Find minimum difference among any pairs. mindiffsofar = # some outrageously big value for in 1: for y in 2: diff = abs( y) if diff < mindiffsofar: mindiffsofar = diff return mindiffsofar

4 import sys Eample: min difference Alternatively, you can use Python s maimum value as the prime. def mindiff( 1, 2 ): The parameters 1, 2 are two int s. Find minimum difference among any pairs. mindiffsofar = sys.masize # largest Python value for in 1: for y in 2: diff = abs( y) if diff < mindiffsofar: mindiffsofar = diff return mindiffsofar

5 Functions and Parameters We ve learned how to develop functions def findma( alist ): ma = alist[0] for i in range( len(alist) ): if alist[i] > ma: ma = alist[i] return ma def sumlist( alist ): sum = 0 for i in range( len(alist) ): sum += alist[i] return sum In both cases, alist is called a parameter for the function A function can have multiple parameters Two types of parameters, mutable and immutable Let s try out the eamples (25_passing.py)

6 Pass By Value def main() """ calls conform """ print(" Welcome to Conformity, Inc. ") = 7 conform() print(" My orite number is", ) 7 type: int def conform() """ sets input to 42 """ = 42 return 7 Pass by value" means that the data's value is copied when sent to a function type: int PASS BY VALUE 6

7 Passing content by reference def main() """ calls conform2 """ print " Welcome to Conformity, Inc. " = [ 7, 11 ] conform2() print(" My orite numbers are", ) type: 7 11 [0] [1] def conform2() """ sets all of to 42 """ [0] = 42 [1] = 42 type: What gets passed by value here? 7

8 Passing content by reference def main() """ calls conform2 """ print " Welcome to Conformity, Inc. " = [ 7, 11 ] conform2() print(" My orite numbers are", ) [0] [1] def conform2() """ sets all of to 42 """ [0] = 42 [1] = 42 The reference is copied, i.e., passed by value, but the contents arn t! and it can change data elsewhere! 8

9 Watch out! You can change the contents of s in functions that take those s as input. (actually, s or any mutable objects) Those changes will be visible everywhere. (immutable objects are safe, however) 9

10 But s are passing by value!!! def main() """ calls conform3 """ print " Welcome to Conformity, Inc. " = [ 7, 11 ] conform3() print(" My orite numbers are", ) type: 7 11 [0] [1] def conform3() """ creates a new!!! """ = [ 42, 42 ] type: 10

11 But s are passing by value!!! def main() """ calls conform3 """ print " Welcome to Conformity, Inc. " = [ 7, 11 ] conform3() print(" My orite numbers are", ) 7 11 [0] [1] def conform3() """ creates a new!!! """ = [ 42, 42 ] [0] [1] 11

12 Mutable vs. Immutable Pass by REFERENCE dictionary Reference Pointer id = [5,42,'hi'] Pass by VALUE tuple float string bool int s = 'hi' 5 42 'hi' [0] [1] [2] s 'hi' Lists and dictionaries are handled by reference (the variables really hold a memory address) Other types of data, incl. strings, are handled by value: they hold the actual data 12

13 Lists fleibility Lists can hold ANY type of data = [ 42, 75, 70 ] int int int We can equally well imagine them as vertical structures. 42 int int 75 int 70 13

14 Lists fleibility Lists can hold ANY type of data double double double int int int go red String String String 14

15 2d s or arrays Lists can hold ANY type of data -- including s! = [ [1,2,3,4], [5,6], [7,8,9,10,11] ] 15

16 2d arrays Lists can hold ANY type of data -- including s! = [ [1,2,3,4], [5,6], [7,8,9,10,11] ] [0] [1] [2] 16

17 Jagged arrays Lists can hold ANY type of data -- including s! = [ [1,2,3,4], [5,6], [7,8,9,10,11] ] [0] [1] [2] Rows within 2d arrays need not be the same length 17

18 Rectangular arrays [0] [0][0] [1] [2] [2][3] What does [1] refer to? What value is changed with [1][2]=42? How many rows does have, in general? How many columns does have, in general? 18

Introduction to Python

Introduction to Python May 25, 2010 Basic Operators Logicals Types Tuples, Lists, & Dictionaries and or Building Functions Labs From a non-lab computer visit: http://www.csuglab.cornell.edu/userinfo Running your own python setup,