1. EECS 110: Lec 10: Definite Loops and User Input
Aleksandar Kuzmanovic
Northwestern University

2. Loops! [ x*6 for x in range(8) ] [ 0, 6, 12, 18, 24, 30, 36, 42 ]
We've seen variables change in-place before:
[ x*6 for x in range(8) ]
[ 0, 6, 12, 18, 24, 30, 36, 42 ]
remember range?

3. fore! for x in range(8): print 'x is', x print 'Phew!' 1 3 2 4
x is assigned each value from this sequence 1
for x in range(8):
print 'x is', x
print 'Phew!' 3
LOOP back to step 1 for EACH value in the list 2
the BODY or BLOCK of the for loop runs with that x 4
Code AFTER the loop will not run until the loop is finished.

4. four on for for x in range(8): print 'x is', x factorial function?
sum the list?
construct the list?

5. Fact with for def fact( n ): answer = 1 for x in range(n):
answer = answer * x
return answer

6. Fact with for def fact( n ): answer = 1 for x in range(1,n+1):
answer = answer * x
return answer

7. Accumulating an answer…
Finding the sum of a list:
def sum( L ):
""" returns the sum of L's elements """
sum = 0
for x in L:
sum = sum + x
return sum
Accumulator!
shortcuts?
vs. recursion?
sum every OTHER element?

8. Shortcut Shortcuts for changing variables: age = 38 age = age + 1
age += 1 #shortcut for age = age + 1

9. Two kinds of for loops sum = 0 for x in L: sum += x L = [ 42, -10, 4 ]
Element-based Loops
sum = 0
for x in L:
sum += x
L = [ 42, -10, 4 ] x
"selfless"

10. Two kinds of for loops sum = 0 for x in L: sum += x sum = 0 for i in :
Element-based Loops
Index-based Loops
sum = 0
for x in L:
sum += x
sum = 0
for i in :
sum += i 1 2
L = [ 42, -10, 4 ]
L = [ 42, -10, 4 ] x

11. Two kinds of for loops sum = 0 for x in L: sum += x sum = 0
Element-based Loops
Index-based Loops
sum = 0
for x in L:
sum += x
sum = 0
for i in range(len(L)):
sum += L[i] i 1 2
L = [ 42, -10, 4 ]
L = [ 42, -10, 4 ] x
L[i]

12. Sum every other element
Finding the sum of a list:
def sum( L ):
""" returns the sum of L's elements """
sum = 0
for i in range(len(L)):
if ________:
sum += L[i]
return sum
Accumulator!
shortcuts?
vs. recursion?
sum every OTHER element?

13. Sum every other element
Finding the sum of a list:
def sum( L ):
""" returns the sum of L's elements """
sum = 0
for i in range(len(L)):
if i%2 == 0:
sum += L[i]
return sum
Accumulator!
shortcuts?
vs. recursion?
sum every OTHER element?

14. Extreme Looping What does this code do? print 'It keeps on'
while True:
print 'going and'
print 'Phew! I\'m done!'

15. Extreme Looping Anatomy of a while loop: print 'It keeps on'
while True:
print 'going and'
print 'Phew! I\'m done!'
the loop keeps on running as long as this test is True
“while” loop
This won't print until the while loop finishes - in this case, never!
alternative tests?

16. the loop keeps on running as long as this test is True
Extreme Looping
Slowing things down…
import time
print 'It keeps on'
while True:
print 'going and'
time.sleep(1)
print 'Phew! I\'m done!'
the loop keeps on running as long as this test is True
“while” loop

17. Making our escape! import random escape = 0 while escape != 42:
print 'Help! Let me out!'
escape = random.choice([41,42,43])
print 'At last!'
how could we count the number of loops we run?

18. Loops aren't just for lists…
for c in 'down with CS!':
print c

19. "Quiz" def min( L ): n = 0 for c in 'forty-two': if c not in 'aeiou':
Names:
Write a loop to find and return the min of a list, L
def min( L ):
L is a list of numbers.
What do these two loops print?
n = 0
for c in 'forty-two':
if c not in 'aeiou':
n += 1
print n
Write a loop so that this function returns True if its input is prime and False otherwise:
def isPrime( n ):
n is a positive integer
n = 3
while n > 1:
print n
if n%2 == 0:
n = n/2
else:
n = 3*n + 1

20. What do these two loops print?
while n > 1:
print n
if n%2 == 0:
n = n/2
else:
n = 3*n + 1
n = 0
for c in 'forty-two':
if c not in 'aeiou':
n += 1
print n ??

21. What do these two loops print?
while n > 1:
print n
if n%2 == 0:
n = n/2
else:
n = 3*n + 1 ??
n = 0
for c in 'forty-two':
if c not in 'aeiou':
n += 1
print n 7

22. What do these two loops print?
while n > 1:
print n
if n%2 == 0:
n = n/2
else:
n = 3*n + 1 3
n = 0
for c in 'forty-two':
if c not in 'aeiou':
n += 1
print n 7

23. What do these two loops print?
while n > 1:
print n
if n%2 == 0:
n = n/2
else:
n = 3*n + 1 3 10
n = 0
for c in 'forty-two':
if c not in 'aeiou':
n += 1
print n 7

24. What do these two loops print?
while n > 1:
print n
if n%2 == 0:
n = n/2
else:
n = 3*n + 1 3 10 5
n = 0
for c in 'forty-two':
if c not in 'aeiou':
n += 1
print n 7

25. What do these two loops print?
while n > 1:
print n
if n%2 == 0:
n = n/2
else:
n = 3*n + 1 3 10 5 16
n = 0
for c in 'forty-two':
if c not in 'aeiou':
n += 1
print n 7

26. What do these two loops print?
while n > 1:
print n
if n%2 == 0:
n = n/2
else:
n = 3*n + 1 3 10 5 16 8
n = 0
for c in 'forty-two':
if c not in 'aeiou':
n += 1
print n 7

27. What do these two loops print?
while n > 1:
print n
if n%2 == 0:
n = n/2
else:
n = 3*n + 1 3 10 5 16 8 4
n = 0
for c in 'forty-two':
if c not in 'aeiou':
n += 1
print n 7

28. What do these two loops print?
while n > 1:
print n
if n%2 == 0:
n = n/2
else:
n = 3*n + 1 3 10 5 16 8 4 2
n = 0
for c in 'forty-two':
if c not in 'aeiou':
n += 1
print n 7

29. def min( L ): def isPrime( n ): L is a list of numbers.
n is a positive integer

30. for i in range(1,len(L)): if L[i] < mn: mn = L[i] return mn
def min( L ):
mn = L[0]
for i in range(1,len(L)):
if L[i] < mn:
mn = L[i]
return mn
L is a list of numbers.
def isPrime( n ):
n is a positive integer

31. for i in range(1,len(L)): if L[i] < mn: mn = L[i] return mn
def min( L ):
mn = L[0]
for i in range(1,len(L)):
if L[i] < mn:
mn = L[i]
return mn
mn=L[0]
for s in L:
if s < mn:
mn = s
L is a list of numbers.
def isPrime( n ):
n is a positive integer

32. for i in range(1,len(L)): if L[i] < mn: mn = L[i] return mn
def min( L ):
mn = L[0]
for i in range(1,len(L)):
if L[i] < mn:
mn = L[i]
return mn
mn=L[0]
for s in L:
if s < mn:
mn = s
L is a list of numbers.
def isPrime( n ):
for i in range (n):
if i not in [0,1]:
if n%i == 0:
return False
return True
n is a positive integer

33. Lab 8: the Mandelbrot Set
Consider the following update rule for all complex numbers c:
z0 = 0
zn+1 = zn2 + c
If z does not diverge, c is in the M. Set.
Benoit M.
Imaginary axis z3 c z1 z2 z4
Real axis z0

34. Lab 8: the Mandelbrot Set
Consider the following update rule for all complex numbers c:
z0 = 0
zn+1 = zn2 + c
If c does not diverge, it's in the M. Set.
Benoit M.
Imaginary axis z3 c z1 z2 z4 z0
Real axis
example of a non-diverging cycle

35. Lab 8: the Mandelbrot Set
Consider the following update rule for all complex numbers c:
z0 = 0
zn+1 = zn2 + c
The shaded area are points that do not diverge.

36. for creating and saving images
Python and images
import bmp.py
for creating and saving images
image = BitMap( 300, 200, Color.GREEN )
creates a bitmap object
and names it image

37. Python and images import bmp.py for creating and saving images
image = BitMap( 300, 200, Color.GREEN )
creates a bitmap object
and names it image
objects are software abstractions containing structured information

38. Python and images and objects
import bmp.py
image = BitMap( 300, 200, Color.GREEN )
here, a bitmap object named image
is calling an internal method named saveFile
image.saveFile( "test.bmp" )
objects are variables that can contain their own functions, often called methods

39. Python and images and objects
import bmp.py
image = BitMap( 300, 200, Color.GREEN )
image.setPenColor( Color.Red )
two more internal methods
image.plotPixel( 150, 100 )
image.saveFile( "test.bmp" )
objects are variables that can contain their own functions, often called methods

40. Q: What does this plot? from bmp import * def test():
""" image demonstration """
width = 200
height = 200
image = BitMap( width, height )
# a 2d loop over each pixel
for col in range(width):
for row in range(height):
if col == row:
image.plotPoint( col, row )
image.saveFile( "test.bmp" )

41. A: A diagonal in the SW -> NE direction
Q: What does this plot?
A: A diagonal in the SW -> NE direction
from bmp import *
def test():
""" image demonstration """
width = 200
height = 200
image = BitMap( width, height )
# a 2d loop over each pixel
for col in range(width):
for row in range(height):
if col == row:
image.plotPoint( col, row )
image.saveFile( "test.bmp" )

42. How could you change this code so that it plots a diagonal from NW to SE?
def test():
""" demonstrating images """
width = 200
height = 200
image = BitMap( width, height )
# a 2d loop over each pixel
for col in range(width):
for row in range(height):
if col == row:
image.plotPoint( col, row )
image.saveFile( "test.bmp" )

43. How could you change this code so that it plots a diagonal from NW to SE?
def test():
""" demonstrating images """
width = 200
height = 200
image = BitMap( width, height )
# a 2d loop over each pixel
for col in range(width):
for row in range(height):
if col == height – row -1:
image.plotPoint( col, row )
image.saveFile( "test.bmp" )

44. See you in Lab!