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PARAMETERS, INDEFINITE

LOOPS, AND LOOP PATTERNS

CSSE 120—Rose Hulman Institute of Technology

Speed Reading HW Questions?

 It is in HW 10 folder on ANGEL

 You should have already started working on it

 Ask questions if you got stuck

 Finish before Session 12

Functions mutating parameters

 Can we write a function that exchanges the values of its two

parameters?

attempt to exchange two integers

def swapInts(x, y): x,y = y,x

attempt to exchange two elements of a list

def swapListElements(list, i, j): list[i], list[j] = list[j], list[i]

x,y = 2, 5 print 'Before "swapInts": x=%d, y=%d' %(x, y) swapInts (x, y) print 'After "swapInts": x=%d, y=%d' %(x, y)

aList = [ 3 , 4 , 5 , 6 ] print “State of list before swapListElements:",aList swapListElements(aList, 1 , 3 )

print “State of list after swapListItems:",aList

Modifying Parameters

 How do functions send information back?

 Return statements

 Mutating parameters

 Value of actual parameter must be a mutable object

 State of the mutable object is changed

 The actual parameter itself is NOT changed since it refers to the same object

 Parameter is still passed by value

Some indefinite loop patterns

 Interactive loops

 Sentinel loops

 File loops

 post-test loops

 "loop and a half"

Interactive: Make the user count

 Not the best idea! (Why not?)

# average1.py

# A program to average a set of numbers

# Illustrates counted loop with accumulator

from win_in import *

def main():

n = win_input( "How many numbers do you have? ")

sum = 0.

for i in range(n):

x = win_input( "Enter a number >> ")

sum = sum + x

print "\nThe average of the numbers is", sum / n

main()

Sentinel loop

For this program, a negative input number is the sentinel that signals "no more data"

 User signals end of data by a special "sentinel"value.

 Note that the sentinel value is not used in calculations.

# average3.py

# A program to average a set of numbers

# Illustrates sentinel loop using negative input as sentinel

from win_in import *

def main():

sum = 0.

count = 0

x = win_input( "Enter a number (negative to quit) >> ")

while x >= 0:

sum = sum + x

count = count + 1

x = win_input( "Enter a number (negative to quit) >> ")

print "\nThe average of the numbers is", sum / count

Non-numeric Sentinel

 What if negative numbers are legitimate values?

 Again note: sentinel value is not used in calculations.

# average4.py

# A program to average a set of numbers

# Illustrates sentinel loop using the empty string as sentinel

from win_in import *

def main():

sum = 0.

count = 0

xStr = win_raw_input( "Enter a number ( to quit) >> ")

while xStr != "":

x = eval(xStr)

sum = sum + x

count = count + 1

xStr = win_raw_input( "Enter a number ( to quit) >> ")

print "\nThe average of the numbers is", sum / count

File loop

 Use a for loop as we have seen before:

 Also note the conditional execution of main()

# average5.py

# Computes the average of numbers listed in a file.

from win_in import *

def main():

fileName = win_raw_input( "What file are the numbers in? ")

infile = open(fileName, 'r')

sum = 0.

count = 0

for line in infile:

sum = sum + eval(line)

count = count + 1

print "\nThe average of the numbers is", sum / count

if name == 'main':

main()

Individual Exercise on Using loops

 Define function listAndMax() in module listMax.py that

 Prompts the user to enter numbers, one at a time

 Uses a blank line () as sentinel to terminate input

 Accumulates the numbers in a list

 Returns two values:

 the list of numbers entered in the order they were entered

 the maximum value of the numbers entered

 Define function main() in module listMax.py that

 Calls listAndMax()

 Prints the list of numbers entered

 Prints the maximum value of the list of numbers