Lists (& Sequences) Announcements for Today (Optional) Reading - PowerPoint PPT Presentation

Lecture 12 Lists (& Sequences)

Announcements for Today (Optional) Reading Assignments • Read 10.0-10.2, 10.4-10.6 • A2 is now graded • Read all of Chapter 8 for Thu § Access it in Gradescope § Graded out of 50 points • Prelim, 10/17 at 7:30 pm § Mean : 43.9, Median : 47 § Material up to TODAY § A : 46 (58%), B : 37 (29%) § Study guide is posted § Times/rooms by last name • A3 due this Friday • Conflict with Prelim time? § Thurs last day for help § Submit conflict to CMS § Will grade over break § Applies to SDS students too 10/2/18 Lists & Sequences 2

Sequences: Lists of Values String List • s = 'abc d' • x = [5, 6, 5, 9, 15, 23] 0 1 2 3 4 0 1 2 3 4 5 a b c d 5 6 5 9 15 23 • Put characters in quotes • Put values inside [ ] § Use \' for quote character § Separate by commas • Access characters with [] • Access values with [] § s[0] is 'a' § x[0] is 5 § s[5] causes an error § x[6] causes an error § x[0:2] is [5, 6] (excludes 2 nd 5) § s[0:2] is 'ab' (excludes c ) § s[2:] is 'c d' § x[3:] is [9, 15, 23] 10/2/18 Lists & Sequences 3

Sequences: Lists of Values String List • s = 'abc d' • x = [5, 6, 5, 9, 15, 23] 0 1 2 3 4 0 1 2 3 4 5 a b c d 5 6 5 9 15 23 • Put characters in quotes • Put values inside [ ] § Use \' for quote character § Separate by commas Sequence is name given to both • Access characters with [] • Access values with [] § s[0] is 'a' § x[0] is 5 § s[5] causes an error § x[6] causes an error § x[0:2] is [5, 6] (excludes 2 nd 5) § s[0:2] is 'ab' (excludes c ) § s[2:] is 'c d' § x[3:] is [9, 15, 23] 10/2/18 Lists & Sequences 4

Lists Have Methods Similar to String x = [5, 6, 5, 9, 15, 23] But you get length of • index(value) a list with a regular § Return position of the value function, not method: len(x) § ERROR if value is not there § x.index(9) evaluates to 3 • count(value) § Returns number of times value appears in list § x.count(5) evaluates to 2 10/2/18 Lists & Sequences 5

Representing Lists Wrong Correct x x 5, 6, 7, -2 id1 Unique tab identifier Variable id1 holds id 0 5 Box is “too small” 1 7 to hold the list 2 4 Put list in 3 -2 a “folder” x = [5, 7, 4,-2] 10/2/18 Lists & Sequences 6

Lists vs. Class Objects List RGB • Attributes are indexed • Attributes are named § Example: x[2] § Example: c.red c id3 x id2 id2 id3 list RGB 0 5 red 128 1 7 green 64 2 4 blue 255 3 -2 10/2/18 Lists & Sequences 7

When Do We Need to Draw a Folder? • When the value contains other values § This is essentially want we mean by ‘object’ • When the value is mutable Type Container? Mutable? int No No float No No str Yes* No Point3 Yes Yes RGB Yes Yes list Yes Yes 10/2/18 Lists & Sequences 8

Lists are Mutable • x = [5, 7,4,-2] • List assignment : 0 1 2 3 <var>[<index>] = <value> 5 7 4 -2 § Reassign at index § Affects folder contents • x[1] = 8 § Variable is unchanged id1 • Strings cannot do this 0 5 § s = 'Hello World!' 1 7 x id1 2 § s[0] = 'J' ERROR 4 3 -2 § String are immutable 10/2/18 Lists & Sequences 9

Lists are Mutable • x = [5, 7,4,-2] • List assignment : 0 1 2 3 <var>[<index>] = <value> x 5 7 4 -2 § Reassign at index 8 § Affects folder contents • x[1] = 8 § Variable is unchanged id1 • Strings cannot do this 0 5 x 8 § s = 'Hello World!' 1 7 x id1 2 § s[0] = 'J' ERROR 4 3 -2 § String are immutable 10/2/18 Lists & Sequences 10

Slice Assignment • Can embed a new list inside of a list § Syntax: <var>[<start>:<end>] = <list> § Replaces that range with content of list • Example: >>> a = [1,2,3] >>> b = [4,5] Replaces [1,2] >>> a[:2] = b with [4,5] >>> a [4, 5, 3]

Lists Share Methods with Strings x = [5, 6, 5, 9, 15, 23] • index(value) These are § Return position of the value immutable § ERROR if value is not there methods § x.index(9) evaluates to 3 • count(value) § Returns number of times value appears in list § x.count(5) evaluates to 2

List Methods Can Alter the List x = [5, 6, 5, 9] • append(value) § A procedure method , not a fruitful method § Adds a new value to the end of list § x.append(-1) changes the list to [5, 6, 5, 9, -1] • insert(index, value) § Put the value into list at index; shift rest of list right § x.insert(2,-1) changes the list to [5, 6, -1, 5, 9,] • sort() 10/2/18 Lists & Sequences 13

List Methods Can Alter the List x = [5, 6, 5, 9] • append(value) § A procedure method , not a fruitful method § Adds a new value to the end of list § x.append(-1) changes the list to [5, 6, 5, 9, -1] • insert(index, value) § Put the value into list at index; shift rest of list right § x.insert(2,-1) changes the list to [5, 6, -1, 5, 9,] • sort() What do you think this does? 10/2/18 Lists & Sequences 14

Where To Learn About List Methods? In the documentation!

Lists and Functions: Swap 1. def swap(b, h, k): Swaps b[h] and b[k], 2. """ Swaps b[h] and b[k] in b because parameter b Precond : b is a mutable list, 3. contains name of list. 4. h, k are valid positions""" 5. temp= b[h] id4 6. b[h]= b[k] 0 5 swap 5 7. b[k]= temp 1 4 b id4 h 3 2 7 3 6 swap(x, 3, 4) k 4 4 5 x id4 10/2/18 Lists & Sequences 16

Lists and Functions: Swap 1. def swap(b, h, k): Swaps b[h] and b[k], 2. """ Swaps b[h] and b[k] in b because parameter b Precond : b is a mutable list, 3. contains name of list. 4. h, k are valid positions""" 5. temp= b[h] id4 6. b[h]= b[k] 0 5 swap 6 7. b[k]= temp 1 4 b id4 h 3 2 7 3 6 swap(x, 3, 4) temp 6 k 4 4 5 x id4 10/2/18 Lists & Sequences 17

Lists and Functions: Swap 1. def swap(b, h, k): Swaps b[h] and b[k], 2. """ Swaps b[h] and b[k] in b because parameter b Precond : b is a mutable list, 3. contains name of list. 4. h, k are valid positions""" 5. temp= b[h] id4 6. b[h]= b[k] 0 5 swap 7 7. b[k]= temp 1 4 b id4 h 3 2 7 ✗ 3 5 6 swap(x, 3, 4) temp 6 k 4 4 5 x id4 10/2/18 Lists & Sequences 18

Lists and Functions: Swap 1. def swap(b, h, k): Swaps b[h] and b[k], 2. """ Swaps b[h] and b[k] in b because parameter b Precond : b is a mutable list, 3. contains name of list. 4. h, k are valid positions""" 5. temp= b[h] id4 6. b[h]= b[k] 0 5 swap 7. b[k]= temp 1 4 b id4 h 3 2 7 ✗ 3 5 6 swap(x, 3, 4) temp 6 k 4 ✗ 4 6 5 x id4 10/2/18 Lists & Sequences 19

List Slices Make Copies x = [5, 6, 5, 9] y = x [1:3] x y id5 id6 id5 id6 list list 0 6 0 5 1 5 1 6 2 5 3 9 copy = new folder 10/2/18 Lists & Sequences 20

Exercise Time • Execute the following: >>> x = [5, 6, 5, 9, 10] >>> x[3] = -1 >>> x.insert(1,2) • What is x[4]? A: 10 B: 9 C: -1 D: ERROR E: I don’t know 10/2/18 Lists & Sequences 21

Exercise Time • Execute the following: • Execute the following: >>> x = [5, 6, 5, 9, 10] >>> x = [5, 6, 5, 9, 10] >>> x[3] = -1 >>> y = x[1:] >>> x.insert(1,2) >>> y[0] = 7 • What is x[4]? • What is x[1]? A: 7 -1 B: 5 C: 6 D: ERROR E: I don’t know 10/2/18 Lists & Sequences 22

Exercise Time • Execute the following: • Execute the following: >>> x = [5, 6, 5, 9, 10] >>> x = [5, 6, 5, 9, 10] >>> x[3] = -1 >>> y = x[1:] >>> x.insert(1,2) >>> y[0] = 7 • What is x[4]? • What is x[1]? -1 6 10/2/18 Lists & Sequences 23

Lists and Expressions • List brackets [] can • Execute the following: contain expressions >>> a = 5 >>> b = 7 • This is a list expression >>> x = [a, b, a+b] § Python must evaluate it § Evaluates each expression • What is x[2]? § Puts the value in the list A: 'a+b' • Example: B: 12 >>> a = [1+2,3+4,5+6] C: 57 >>> a D: ERROR [3, 7, 11] E: I don’t know 10/2/18 Lists & Sequences 24

Lists and Expressions • List brackets [] can • Execute the following: contain expressions >>> a = 5 >>> b = 7 • This is a list expression >>> x = [a, b, a+b] § Python must evaluate it § Evaluates each expression • What is x[2]? § Puts the value in the list • Example: 12 >>> a = [1+2,3+4,5+6] >>> a [3, 7, 11] 10/2/18 Lists & Sequences 25

Lists of Objects • List positions are variables id13 r id10 § Can store base types list b id11 § But cannot store folders 0 id10 § Can store folder identifiers g id12 1 id11 • Folders linking to folders 2 x id13 id12 § Top folder for the list id12 § Other folders for contents id10 id11 RGB RGB • Example: RGB red 0 red 255 >>> r = introcs.RGB(255,0,0) red 0 green 0 green 0 >>> g = introcs.RGB(0,255,0) green 255 >>> b = introcs.RGB(0,0,255) blue 255 blue 0 blue 0 >>> x = [r,g,b] 10/2/18 Lists & Sequences 26