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. for! 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: k = 38 k = k + 1
k += 1 #shortcut for k = k + 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.