For-Loops Announcements for This Lecture Reading Assignments/Lab - - PowerPoint PPT Presentation

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For-Loops Announcements for This Lecture Reading Assignments/Lab - - PowerPoint PPT Presentation

Oct 03, 2022 266 likes •621 views

Lecture 13 For-Loops Announcements for This Lecture Reading Assignments/Lab Today: Chapters 8, 10 A2 has been graded Pick up in Gates 216 Thursday: Chapter 11 Grades generally good Prelim, Oct 12 th 7:30-9:00 A3 is

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SLIDE 1

For-Loops

Lecture 13

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SLIDE 2

Announcements for This Lecture Reading

  • Today: Chapters 8, 10
  • Thursday: Chapter 11

Assignments/Lab

  • A2 has been graded

§ Pick up in Gates 216 § Grades generally good

  • A3 is due on Thursday

§ Will post survey today § Survey due next week

  • Lab is on lists/for-loops

§ Due in two weeks § But fair game on exam

10/3/17 For Loops 2

  • Prelim, Oct 12th 7:30-9:00

§ Material up to TODAY § Study guide is posted

  • Review next Wednesday

§ Room/Time are TBA § Will cover what is on exam

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SLIDE 3

Example: Summing the Elements of a List

def sum(thelist): """Returns: the sum of all elements in thelist Precondition: thelist is a list of all numbers (either floats or ints)""" pass # Stub to be implemented

10/3/17 For Loops 3

Remember our approach: Outline first; then implement

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SLIDE 4

Example: Summing the Elements of a List

def sum(thelist): """Returns: the sum of all elements in thelist Precondition: thelist is a list of all numbers (either floats or ints)""" # Create a variable to hold result (start at 0) # Add each list element to variable # Return the variable

10/3/17 For Loops 4

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SLIDE 5

Example: Summing the Elements of a List

def sum(thelist): """Returns: the sum of all elements in thelist Precondition: thelist is a list of all numbers (either floats or ints)""" result = 0 result = result + thelist[0] result = result + thelist[1] … return result

10/3/17 For Loops 5

There is a problem here

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SLIDE 6

Working with Sequences

  • Sequences are potentially unbounded

§ Number of elements inside them is not fixed § Functions must handle sequences of different lengths § Example: sum([1,2,3]) vs. sum([4,5,6,7,8,9,10])

  • Cannot process with fixed number of lines

§ Each line of code can handle at most one element § What if # of elements > # of lines of code?

  • We need a new control structure

10/3/17 For Loops 6

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SLIDE 7

For Loops: Processing Sequences

# Print contents of seq x = seq[0] print(x) x = seq[1] print(x) … x = seq[len(seq)-1] print(x)

  • Remember:

§ Cannot program … The for-loop: for x in seq: print(x)

  • Key Concepts

§ loop sequence: seq § loop variable: x § body: print(x) § Also called repetend

10/3/17 For Loops 7

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SLIDE 8

For Loops: Processing Sequences

The for-loop:

for x in seq: print(x)

  • loop sequence: seq
  • loop variable: x
  • body: print(x)

To execute the for-loop:

1. Check if there is a “next” element of loop sequence 2. If not, terminate execution 3. Otherwise, put the element in the loop variable 4. Execute all of the body 5. Repeat as long as 1 is true

seq has more elts put next elt in x

True False

print(x)

10/3/17 For Loops 8

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SLIDE 9

Example: Summing the Elements of a List

def sum(thelist): """Returns: the sum of all elements in thelist Precondition: thelist is a list of all numbers (either floats or ints)""" # Create a variable to hold result (start at 0) # Add each list element to variable # Return the variable

10/3/17 For Loops 9

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SLIDE 10

Example: Summing the Elements of a List

def sum(thelist): """Returns: the sum of all elements in thelist Precondition: thelist is a list of all numbers (either floats or ints)""" result = 0 for x in thelist: result = result + x return result

10/3/17 For Loops 10

  • loop sequence: thelist
  • loop variable: x
  • body: result=result+x
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SLIDE 11

Example: Summing the Elements of a List

def sum(thelist): """Returns: the sum of all elements in thelist Precondition: thelist is a list of all numbers (either floats or ints)""" result = 0 for x in thelist: result = result + x return result

10/3/17 For Loops 11

  • loop sequence: thelist
  • loop variable: x
  • body: result=result+x

Accumulator variable

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SLIDE 12

For Loops and Conditionals

def num_ints(thelist): """Returns: the number of ints in thelist Precondition: thelist is a list of any mix of types""" # Create a variable to hold result (start at 0) # for each element in the list… # check if it is an int # add 1 if it is # Return the variable

10/3/17 For Loops 12

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SLIDE 13

For Loops and Conditionals

def num_ints(thelist): """Returns: the number of ints in thelist Precondition: thelist is a list of any mix of types""" result = 0 for x in thelist: if type(x) == int: result = result+1 return result

10/3/17 For Loops 13

Body

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SLIDE 14

Modifying the Contents of a List

def add_one(thelist): """(Procedure) Adds 1 to every element in the list Precondition: thelist is a list of all numbers (either floats or ints)""" for x in thelist: x = x+1 # procedure; no return

10/3/17 For Loops 14

DOES NOT WORK!

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SLIDE 15

For Loops and Call Frames

def add_one(thelist): """Adds 1 to every elt Pre: thelist is all numb.""" for x in thelist: x = x+1 add_one(seq):

10/3/17 For Loops 15

1 2 id4 5 4 7 seq id4 add_one thelist 1 id4 1 2

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SLIDE 16

For Loops and Call Frames

def add_one(thelist): """Adds 1 to every elt Pre: thelist is all numb.""" for x in thelist: x = x+1 add_one(seq):

10/3/17 For Loops 16

1 2 id4 5 4 7 seq id4 add_one thelist 2 id4 1 2 x 5

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SLIDE 17

For Loops and Call Frames

def add_one(thelist): """Adds 1 to every elt Pre: thelist is all numb.""" for x in thelist: x = x+1 add_one(seq):

10/3/17 For Loops 17

1 2 id4 5 4 7 seq id4 add_one thelist 1 id4 1 2 x 6

Increments x in frame Does not affect folder Loop back to line 1

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SLIDE 18

For Loops and Call Frames

def add_one(thelist): """Adds 1 to every elt Pre: thelist is all numb.""" for x in thelist: x = x+1 add_one(seq):

10/3/17 For Loops 18

1 2 id4 5 4 7 seq id4 add_one thelist 2 id4 1 2 x 4

Next element stored in x. Previous calculation lost.

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SLIDE 19

For Loops and Call Frames

def add_one(thelist): """Adds 1 to every elt Pre: thelist is all numb.""" for x in thelist: x = x+1 add_one(seq):

10/3/17 For Loops 19

1 2 id4 5 4 7 seq id4 add_one thelist 1 id4 1 2 x 5

Loop back to line 1

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SLIDE 20

For Loops and Call Frames

def add_one(thelist): """Adds 1 to every elt Pre: thelist is all numb.""" for x in thelist: x = x+1 add_one(seq):

10/3/17 For Loops 20

1 2 id4 5 4 7 seq id4 add_one thelist 2 id4 1 2 x 7

Next element stored in x. Previous calculation lost.

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SLIDE 21

For Loops and Call Frames

def add_one(thelist): """Adds 1 to every elt Pre: thelist is all numb.""" for x in thelist: x = x+1 add_one(seq):

10/3/17 For Loops 21

1 2 id4 5 4 7 seq id4 add_one thelist 1 id4 1 2 x 8

Loop back to line 1

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SLIDE 22

For Loops and Call Frames

def add_one(thelist): """Adds 1 to every elt Pre: thelist is all numb.""" for x in thelist: x = x+1 add_one(seq):

10/3/17 For Loops 22

1 2 id4 5 4 7 seq id4 add_one thelist id4 1 2 x 8

Loop is completed. Nothing new put in x.

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SLIDE 23

For Loops and Call Frames

def add_one(thelist): """Adds 1 to every elt Pre: thelist is all numb.""" for x in thelist: x = x+1 add_one(seq):

10/3/17 For Loops 23

1 2 id4 5 4 7 seq id4 1 2

No changes to folder

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SLIDE 24

On The Other Hand

def copy_add_one(thelist): """Returns: copy with 1 added to every element Precondition: thelist is a list of all numbers (either floats or ints)""" mycopy = [] # accumulator for x in thelist: x = x+1 mycopy.append(x) # add to end of accumulator return mycopy

10/3/17 For Loops 24

Accumulator keeps result from being lost

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SLIDE 25

How Can We Modify A List?

  • Never modify loop var!
  • This is an infinite loop:

for x in thelist: thelist.append(1)

  • Need a second sequence
  • How about the positions?

thelist = [5, 2, 7, 1] thepos = [0, 1, 2, 3] for x in thepos: thelist[x] = thelist[x]+1

10/3/17 For Loops 25

Try this in Python Tutor to see what happens

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SLIDE 26

How Can We Modify A List?

  • Never modify loop var!
  • This is an infinite loop:

for x in thelist: thelist.append(1)

  • Need a second sequence
  • How about the positions?

thelist = [5, 2, 7, 1] thepos = [0, 1, 2, 3] for x in thepos: thelist[x] = thelist[x]+1

10/3/17 For Loops 26

Try this in Python Tutor to see what happens

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SLIDE 27

This is the Motivation for Iterators

  • Iterators are objects

§ Contain data like a list § But cannot slice them

  • Access data with next()

§ Function to get next value § Keeps going until end § Get an error if go to far

  • Can convert back & forth

§ myiter = iter(mylist) § mylist = list(myiter)

10/3/17 For Loops 27

1 2 id1 5 4 7 seq id1 id2 alt id2

?

Type/Class conversion

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SLIDE 28

Iterators and Lists

>>> seq = [5, 4, 7] >>> alt = iter(seq) >>> next(alt) 5 >>> next(alt) 4 >>> next(alt) 7 >>> next(alt) Traceback…

10/3/17 For Loops 28

1 2 id1 5 4 7 seq id1 id2 alt id2

?

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SLIDE 29

Iterators and For Loops

>>> seq = [5, 4, 7] >>> alt = iter(seq) >>> for x in alt: print(x) 5 4 7

10/3/17 For Loops 29

1 2 id1 5 4 7 seq id1 id2 alt id2

?

Just like looping

  • ver the list
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SLIDE 30

Iterators and For Loops

>>> seq = [5, 4, 7] >>> alt = iter(seq) >>> for x in alt: print(x) 5 4 7

10/3/17 For Loops 30

1 2 id1 5 4 7 seq id1 id2 alt id2

?

But still not safe to modify iterator’s list

Just like looping

  • ver the list
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SLIDE 31

The Range Iterator

  • range(x)

§ Creates an iterator § Stores [0,1,…,x-1] § But not a list! § But try list(range(x))

  • range(a,b)

§ Stores [a,…,b-1]

  • range(a,b,n)

§ Stores [a,a+n,…,b-1]

  • Very versatile tool
  • Great for processing ints

total = 0 # add the squares of ints # in range 2..200 to total for x in range(2,201): total = total + x*x

10/3/17 For Loops 31

Accumulator

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SLIDE 32

Modifying the Contents of a List

def add_one(thelist): """(Procedure) Adds 1 to every element in the list Precondition: thelist is a list of all numbers (either floats or ints)""" size = len(thelist) for k in range(size): thelist[k] = thelist[k]+1 # procedure; no return

10/3/17 For Loops 32

WORKS!

Iterator of list positions (safe)

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SLIDE 33

Important Concept in CS: Doing Things Repeatedly

  • 1. Process each item in a sequence

§ Compute aggregate statistics for a dataset, such as the mean, median, standard deviation, etc. § Send everyone in a Facebook group an appointment time

  • 2. Perform n trials or get n samples.

§ A4: draw a triangle six times to make a hexagon § Run a protein-folding simulation for 106 time steps

  • 3. Do something an unknown

number of times

§ CUAUV team, vehicle keeps moving until reached its goal

10/3/17 For Loops 33

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SLIDE 34

Important Concept in CS: Doing Things Repeatedly

  • 1. Process each item in a sequence

§ Compute aggregate statistics for a dataset, such as the mean, median, standard deviation, etc. § Send everyone in a Facebook group an appointment time

  • 2. Perform n trials or get n samples.

§ A4: draw a triangle six times to make a hexagon § Run a protein-folding simulation for 106 time steps

  • 3. Do something an unknown

number of times

§ CUAUV team, vehicle keeps moving until reached its goal

10/3/17 For Loops 34

for x in sequence: process x for x in range(n): do next thing Cannot do this yet Impossible w/ Python for