EECS 110: Lec 12: Mutable Data

Aleksandar Kuzmanovic

Northwestern University

http://networks.cs.northwestern.edu/EECS110-s15/

>>> diff([7, 0, 6, 4])

1

"Quiz"Print a list of strings with

the specified format

def diff( L ):

Return the min difference between any 2 elements in L

You need determine each element's index…

Example:

Example:

You can assume at least 2 elements in the list

Only consider unsigned differences. >>> items = [“coke”, “pepsi”, “sprite”]

>>> printItems(items)0 : coke1 : pepsi2 : sprite

def printItems( items ):

Name(s):_______________________________

Hint: use a NESTED loop

"Quiz" part 1: Print Items

>>> items = [“coke”, “pepsi”, “sprite”]>>> printItems(items)0 : coke1 : pepsi2 : sprite

"Quiz" part 1: Print Items

>>> items = [“coke”, “pepsi”, “sprite”]>>> printItems(items)0 : coke1 : pepsi2 : sprite

def printItems(items): for x in range(len(items)): print(x,”:”,items[x])

"Quiz" part 1: Print Items

>>> items = [“coke”, “pepsi”, “sprite”]>>> printItems(items)0:coke1:pepsi2:sprite

def printItems(items): for x in range(len(items)): print(str(x)+”:”+items[x])

>>> diff([7, 0, 6, 4])

1

"Quiz" Part 2

def diff( L ):

Return the min difference between any 2 elements in L

Example:

You can assume at least 2 elements in the list

Only consider unsigned differences.

Hint: use a NESTED loop

>>> diff([7, 0, 6, 4])

1

"Quiz" Part 2

def diff( L ):

mindif = abs(L[1] – L[0])

for i in range(len(L)-1):

for j in range(i+1,len(L)):

d = abs(L[i]-L[j])

if d < mindiff:

mindiff = d

return mindif

Return the min difference between any 2 elements in L

Example:

You can assume at least 2 elements in the list

Only consider unsigned differences.

Hint: use a NESTED loop

EECS 110 Today

infinitely nested structure… HW 5 Preview:

Pr 1 (Lab): The Game of Life

Pr 2: Markov Text Generation

required

optional

Pr 3: ASCII Art

Pr 4: A program that reads

Pr 5: Gaussian Elimination

Mutable vs. Immutable data

Changeable types:

dictionary

Unchangeable types:

list

tuple

string

int

float

bool

Functions and (immutable) Variablesdef fact(a): result = 1 while a > 0: result *= a a -= 1 return result

>>> x = 5>>> y = fact(x)>>> x??

Functions and (immutable) Variablesdef fact(a): result = 1 while a > 0: result *= a a -= 1 return result

>>> x = 5>>> y = fact(x)>>> x5

Functions and (immutable) Variables

def swap(a, b): temp = a a = b b = temp

>>> x = 5>>> y = 10>>> swap(x, y)>>> print(x, y)??

x

y

a

b

temp

Functions and (immutable) Variables

def swap(a, b): temp = a a = b b = temp

>>> x = 5>>> y = 10>>> swap(x, y)>>> print(x, y)5, 10

x

y

a

b

temp

Functions and Mutable Types

def swap(L, i1, i2): temp = L[i1] L[i1] = L[i2] L[i2] = temp

>>> MyL = [2, 3, 4, 1]>>> swap(myL, 0, 3) >>> print(myL)??

RAM

MyL

L

i1

i2

42

43

44

45

Functions and Mutable Types

def swap(L, i1, i2): temp = L[i1] L[i1] = L[i2] L[i2] = temp

>>> MyL = [2, 3, 4, 1]>>> swap(myL, 0, 3) >>> print(myL)[1,3,4,2]

RAM

MyL

L

i1

i2

42

43

44

45

Reference vs. Value

Mutable types:

dictionary

Unmutable types:

list

tuple

string

int

float

bool

LL[0] L[1] L[2]

Reference,Pointer,

id

L = [5,42,'hi']

L5 42 'hi'

42

L = 42

“Pass By Value”

def main(): """ calls conform """ print(" Welcome to Conformity, Inc. ”)

fav = 7 conform(fav)

print(" My favorite number is", fav)

def conform(fav): """ sets input to 42 """ fav = 42 return fav

7

fav

fav

7

“Pass By Value”

def main(): """ calls conform """ print " Welcome to Conformity, Inc. "

fav = 7 conform(fav)

print " My favorite number is", fav

def conform(fav): """ sets input to 42 """ fav = 42 return fav

7

fav

fav

PASSBY

VALUE

“Pass by value” means that data is copied when sent to a method

42

Passing lists by value…

def main() """ calls conform2 """ print " Welcome to Conformity, Inc. " fav = [ 7, 11 ] conform2(fav) print " My favorite numbers are", fav

def conform2(fav) """ sets all of fav to 42 """ fav[0] = 42 fav[1] = 42

What gets passed by value here?

favL[0] L[1]

7 11

fav

Passing lists by value…

def main(): """ calls conform2 """ print " Welcome to Conformity, Inc. " fav = [ 7, 11 ] conform2(fav) print " My favorite numbers are", fav

def conform2(fav): """ sets all of fav to 42 """ fav[0] = 42 fav[1] = 42

favL[0] L[1]

7 11

fav

can change data elsewhere!

The reference is copied!

The conclusion

You can change the contents of lists in functions that take those lists as input.

Those changes will be visible everywhere.

(actually, lists or any mutable objects)

(immutable objects are safe, however)

More examples

def zeroOdd1( L ): for i in range(len(L)): if L[i] % 2 == 1: L[i] = 0

def zeroOdd2( L ): for i in L: if i % 2 == 1: i = 0

>>> L = [1, 2, 3, 4, 5]>>> zeroOdd1(L)>>> L??>>> L = [1, 2, 3, 4, 5]>>> zeroOdd2(L)>>> L

More examples

def zeroOdd1( L ): for i in range(len(L)): if L[i] % 2 == 1: L[i] = 0

def zeroOdd2( L ): for i in L: if i % 2 == 1: i = 0

>>> L = [1, 2, 3, 4, 5]>>> zeroOdd1(L)>>> L[0,2,0,4,0]>>> L = [1, 2, 3, 4, 5]>>> zeroOdd2(L)>>> L??

More examples

def zeroOdd1( L ): for i in range(len(L)): if L[i] % 2 == 1: L[i] = 0

def zeroOdd2( L ): for i in L: if i % 2 == 1: i = 0

>>> L = [1, 2, 3, 4, 5]>>> zeroOdd1(L)>>> L[0,2,0,4,0]>>> L = [1, 2, 3, 4, 5]>>> zeroOdd2(L)>>> L[1,2,3,4,5]

Lists’ flexibility

Lists can hold ANY type of data

A = [ 42., 75., 70. ] 42.0 75.0 70.0float float floatlist

A

they don’t have to be horizontal

lists!

42.0

75.0

70.0

double

double

double

listAthey don’t have

to be horizontal lists!

Lists’ flexibility

Lists can hold ANY type of data

A = [ 42., 75., 70. ] 42.0 75.0 70.0float float floatlist

A

Lists’ flexibility

Lists can hold ANY type of data

42.0 75.0 70.0double double doublelist

A

42 7 -11int int intlist

A

“go” “red” “sox!”

String String StringlistA

2d lists or arrays

Lists can hold ANY type of data -- including lists !

listA

A = [ [1,2,3,4], [5,6], [7,8,9,10,11] ]

listA

2d arrays

list

list

list

A[0]

A[1]

A[2]

Lists can hold ANY type of data -- including lists !

A = [ [1,2,3,4], [5,6], [7,8,9,10,11] ]

listA

Jagged arrays

list

list

list

A[0]

A[1]

A[2]

Lists can hold ANY type of data -- including lists !

A = [ [1,2,3,4], [5,6], [7,8,9,10,11] ]

Rows within 2d arrays need not be the same length…

listA

list

list

list

A[0]

A[1]

A[2]

Lists can hold ANY type of data -- including lists !

A = [ [1,2,3,4], [5,6], [7,8,9,10,11] ]

Rows within 2d arrays need not be the same length…

We will not use jagged arraysat least in hw 5

Rectangular arrays

Handling rectangular arrays …

listA

list

list

list

A[0]

A[1]

A[2]

How many rows does A have, in general ?

How many columns does A have, in general ?

A[2][3]

A[0][0]

Rectangular arrays

Handling rectangular arrays …

listA

list

list

list

A[0]

A[1]

A[2]

How many rows does A have, in general ?

How many columns does A have, in general ?

A[2][3]

A[0][0]

len(A)

Rectangular arrays

Handling rectangular arrays …

listA

list

list

list

A[0]

A[1]

A[2]

How many rows does A have, in general ?

How many columns does A have, in general ?

A[2][3]

A[0][0]

len(A)

len(A[0])

Which one works?

How could we create a rectangular array (of default data, 0),given its height and width ?

or

A = [ [0]*width ]*height

A = [ [0]*height ]*width

Which one works?

How could we create a rectangular array (of default data, 0), given its height and width ?

A = [ [0]*width ]*height

A = [ [0]*height ]*width

What's really going on?

A = [ [0]*width ]*height

inner = [0]*widthA = [inner]*height

Creating a 2d array

def create2dArray( width, height ): """ does just that """

A = [] # start with nothing

for row in range( height ):

for col in range( width ):

Creating a 2d array

def create2dArray( width, height ): """ does just that """

A = [] # start with nothing

for row in range( height ): A = A + [[]] for col in range( width ): A[row] = A[row] + [0] return A

"Quiz"

What are the values of A, B, C and D at the indicated points?

def mystery1(L1, L2): S = L1 for i in range(len(L2)): S[i] += L2[i] return L2

def mystery2(L1, L2): S = [0]*len(L1) for i in range(len(L2)): S[i] = L1[i] + L2[i]

>>> A = [22, 10, 30]>>> B = [20, 32, 12]>>> C = []>>> D = []>>> C = mystery1(A, B)>>> D = mystery2(A, B)

123

1)

2)

3)

Name(s)__________________________________

"Quiz"

def mystery1(L1, L2): S = L1 for i in range(len(L2)): S[i] += L2[i] return L2

def mystery2(L1, L2): S = [0]*len(L1) for i in range(len(L2)): S[i] = L1[i] + L2[i]

>>> A = [22, 10, 30]>>> B = [20, 32, 12]>>> C = []>>> D = []>>> C = mystery1(A, B)>>> D = mystery2(A, B)

123

1)

2)

3)

Name(s)__________________________________

A: [22, 10, 30]B: [20, 32, 12]C: []D: []

What are the values of A, B, C and D at the indicated points?

"Quiz"

def mystery1(L1, L2): S = L1 for i in range(len(L2)): S[i] += L2[i] return L2

def mystery2(L1, L2): S = [0]*len(L1) for i in range(len(L2)): S[i] = L1[i] + L2[i]

>>> A = [22, 10, 30]>>> B = [20, 32, 12]>>> C = []>>> D = []>>> C = mystery1(A, B)>>> D = mystery2(A, B)

123

1)

2)

3)

Name(s)__________________________________

A: [22, 10, 30]B: [20, 32, 12]C: []D: []

A: [22, 10, 30]B: [20, 32, 12]C: [20, 32, 12]D: []

What are the values of A, B, C and D at the indicated points?

"Quiz"

def mystery1(L1, L2): S = L1 for i in range(len(L2)): S[i] += L2[i] return L2

def mystery2(L1, L2): S = [0]*len(L1) for i in range(len(L2)): S[i] = L1[i] + L2[i]

>>> A = [22, 10, 30]>>> B = [20, 32, 12]>>> C = []>>> D = []>>> C = mystery1(A, B)>>> D = mystery2(A, B)

123

1)

2)

3)

Name(s)__________________________________

A: [22, 10, 30]B: [20, 32, 12]C: []D: []

A: [22, 10, 30]B: [20, 32, 12]C: [20, 32, 12]D: []

A: [22, 10, 30]B: [20, 32, 12]C: [20, 32, 12]D: None

What are the values of A, B, C and D at the indicated points?

+= mutates Mutable Data (lists)

WARNING: For mutable data types, the following are NOTthe same

A = []B = AA += [42, 42]

A = []B = AA = A + [42, 42]

NOT THE SAME!

MUTATES A(B changes too)

COPIES A(B does not change)

The Game of Life: History

• Created by John Horton Conway,a British Mathematician

• Inspired by a problem presentedby John Von Neumann:– Build a hypothetical machine that

can build copies of itself

• First presented in 1970, Scientific American

Problem 1 -- “Life”

Evolutionary rules

Grid World

• Everything depends on a cell’s eight neighbors

red cells are alive

white cells are empty

• Exactly 3 neighbors give birth to a new, live cell!

• Exactly 2 or 3 neighbors keep anexisting cell alive

• Any other number of neighbors kill the central cell (or keep it dead)

Problem 1 -- Life

Evolutionary rules

Grid World

• Everything depends on a cell’s eight neighbors

red cells are alive

white cells are empty

• Exactly 3 neighbors give birth to a new, live cell!

• Exactly 2 or 3 neighbors keep anexisting cell alive

• Any other number of neighbors kill the central cell (or keep it dead)

Problem 1 -- Life

Evolutionary rules

Grid World

• Everything depends on a cell’s eight neighbors

red cells are alive

white cells are empty

• Exactly 3 neighbors give birth to a new, live cell!

• Exactly 2 or 3 neighbors keep anexisting cell alive

• Any other number of neighbors kill the central cell (or keep it dead)

Problem 1 -- Life

Evolutionary rules

Grid World

• Everything depends on a cell’s eight neighbors

red cells are alive

white cells are empty

• Exactly 3 neighbors give birth to a new, live cell!

• Exactly 2 or 3 neighbors keep anexisting cell alive

• Any other number of neighbors kill the central cell (or keep it dead)

life out there...

Keep going!

Problem 1 -- Creating Life

0 1 2 3 4 50 1 2 3 4 5

0

1

2

3

4

5

0

1

2

3

4

5

updateNextLife(oldB, newB)

old generation or "board" new generation or "board"

Problem 1 -- Creating Life

0 1 2 3 4 50 1 2 3 4 5

0

1

2

3

4

5

0

1

2

3

4

5

old generation or "board" new generation or "board"

updateNextLife(oldB, newB)

Problem 1 -- Details

For each generation… • 0 represents an empty cell

• 1 represents a living cell

• outermost edge should always be left empty (even if there are 3 neighbors)

• compute all cells based on their previous neighbors

http://www.math.com/students/wonders/life/life.html

old generation or "board" new generation or "board"

life out there...

updateNextLife(oldB, newB)

Problem 1 – Main Loop

def life( width, height ): """ will become John Conway's Game of Life... """ B = createBoard(width, height) csplot.showAndClickInIdle(B) while True: # run forever csplot.show(B) # show current B time.sleep(0.25) # pause a bit oldB = B B = createBoard( width, height ) updateNextLife( oldB, B ) # gets a new board

Problem 1 – Main Loop

def life( width, height ): """ will become John Conway's Game of Life... """ B = createBoard(width, height) csplot.showAndClickInIdle(B) while True: # run forever csplot.show(B) # show current B time.sleep(0.25) # pause a bit oldB = B B = createBoard( width, height ) updateNextLife( oldB, B ) # gets a new board

Why not just have update RETURN a new list? (i.e., why bother with mutation at all?)

Update MUTATES the list B

Problem 1 -- to and beyond!

• Are there stable life configurations?

• Are there oscillating life configurations?

• Are there self-propagating life configurations?

"rocks"

"plants"

"animals"

period 3

period 2

Problem 1 -- to and beyond!

• Are there life configurations that expand forever?

• What is the largest amount of the life universe that can be filled with cells?

• How sophisticated can the structures in the life universe be?

http://www.ibiblio.org/lifepatterns/

• Are all feasible configurations reachable?

See you in Lab!