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# Iteration - PowerPoint PPT Presentation

Iteration. When statements are repeated, any simpler way ? A for statement with a range Tell Python that I have an iteration Tell Python how many times. def drawSquare ( myTurtle,sideLength ): myTurtle.forward ( sideLength ) myTurtle.right (90) # side 1 myTurtle.forward ( sideLength )

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## PowerPoint Slideshow about ' Iteration' - mina

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Presentation Transcript

• When statements are repeated, any simpler way ?

• A for statement with a range

• Tell Python that I have an iteration

• Tell Python how many times

defdrawSquare(myTurtle,sideLength):

myTurtle.forward(sideLength)

myTurtle.right(90) # side 1

myTurtle.forward(sideLength)

myTurtle.right(90) # side 2

myTurtle.forward(sideLength)

myTurtle.right(90) # side 3

myTurtle.forward(sideLength)

myTurtle.right(90) # side 4

defdrawS(myT,sideLength):

for i in range(0,4):

myT.forward(sideLength)

myTurtle.right(90)

defdrawSquare(myTurtle,sideLength):

myTurtle.forward(sideLength)

myTurtle.right(90) # side 1

myTurtle.forward(sideLength)

myTurtle.right(90) # side 2

myTurtle.forward(sideLength)

myTurtle.right(90) # side 3

myTurtle.forward(sideLength)

myTurtle.right(90) # side 4

• “for i in range(0,4):”

• “i” – a dummy variable keeping counts

• range(a, b):

• a, a+1, a+2, ….. , b-2, b-1

• Do not forget “:” at the end

• Statements to be repeated have to be indented

• Five numbers to get an average: 20, 30, 40, 5, 55

• A general way of getting an average of any five numbers ?

• A collection of five numbers – how to tell what it is ?

>>> avg2 = (20+30+40+5+55)/5

• Compute the class average

Initialize total to zero

Initialize counter to zero

while the user has not as yet entered the sentinel

input the next grade (possibly the sentinel)

if the counter is not equal to zero

set the average to the total divided by the counter

print the average

else

• Compute the class average

total = 0

counter = 0

counter = counter+1

if counter > 0:

average = total/counter

print average

else:

• List – an ordered set

• Index starting with 0 identifies each item in a list

• A general way of getting an average of any five numbers ?

• But, would be nice to ‘automate’ the summing process

• => Further abstraction

>>> myList = [20,30,40,5,55]

>>> avg1 = (myList[0]+myList[1]+myList[2]+myList[3]+myList[4])/5

sum = myList[0]+myList[1]+myList[2]+myList[3]+myList[4]

• How to automate the sum ?

sum = 0

sum = sum + myList[0]

sum = sum + myList[1]

sum = sum + myList[2]

sum = sum + myList[3]

sum = sum + myList[4]

sum = 0

sum = sum + myList[0]

sum = sum + myList[1]

sum = sum + myList[2]

sum = sum + myList[3]

sum = sum + myList[4]

• Haven’t really automate the process, but we have

sum = 0

sum = sum + myList[ ]

sum = sum + myList[ ]

sum = sum + myList[ ]

sum = sum + myList[ ]

sum = sum + myList[ ]

for” loop

sum = 0

sum = sum + myList[0]

sum = sum + myList[1]

sum = sum + myList[2]

sum = sum + myList[3]

sum = sum + myList[4]

• “for” steps through every element

sum = 0

forin range(0, 5):

sum = sum + myList[ ]

• Use any variable name

for” loop

for p in range(min, max):

block

p=min

true

p = max

false

Do block

p = p +1

Out of “for”

• Remember getting an average ?

• Can you write a command (in ???? below) to compute the average using ‘psum’

>>> myList = [20,30,40,5,55]

>>> avg1 = (myList[0]+myList[1]+myList[2]+myList[3]+myList[4])/5

>>> myList = [-20, 50, -4, 20, 7]

>>> n=5

>>> psum = 0

>>> for i in range(0,n):

… psum = psum + myList[i]

>>> ????

sum Function

• Hate typing an array of values, “for i in range(,):” each time you compute a sum

• Any easier way ? -- YES !!

• Create a GENERIC function to compute a sum regardless of arrays of values

• => Further ABSTRACTION

sum Function

def sum(thisList):

n = len(thisList)

psum = 0

for j in range(0,n):

psum = psum+thisList[j]

return psum

def drawSpiral(myTurtle,maxSide):

for sideLength in range(1,maxSide+1,5):

myTurtle.forward(sideLength)

myTurtle.right(90)