CS 1110: Introduction to Computing Using Python Lists and Sequences

Transcription

1 CS : Introduction to Computing Using Python Lecture Lists and Sequences [Andersen, Gries, Lee, Marschner, Van Loan, White]

2 Prelim Lecture Announcements Date: Tuesday, March 4th, 7:3 pm to 9: pm Submit conflicts immediately through CMS A: You must scan or take a picture of your work to submit it through CMS Since you have been warned to submit early, do not expect that we will accept work that does not make it onto CMS on time. Set CMS notifications to receive all s! 3//7 Lists & Sequences

3 Sequences: Lists of Values s = 'abc d' String 3 4 List x = [, 6,, 9,, 3] 3 4 a b c d Put characters in quotes Use \' for quote character Access characters with [] s[] is 'a' s[] causes an error s[:] is 'ab' (excludes c) s[:] is 'c d' Put values inside [ ] Separate by commas Access values with [] x[] is x[6] causes an error x[:] is [, 6] (excludes nd ) x[3:] is [9,, 3] 3//7 Lists & Sequences 3

4 Sequences: Lists of Values s = 'abc d' String 3 4 List x = [, 6,, 9,, 3] 3 4 a b c d Put characters in quotes Use \' for quote character Access characters with [] s[] is 'a' s[] causes an error s[:] is 'ab' (excludes c) s[:] is 'c d' Put values inside [ ] Separate by commas Access values with [] x[] is x[6] causes an error x[:] is [, 6] (excludes nd ) x[3:] is [9,, 3] 3//7 Lists & Sequences 4

5 Lists Have Methods Similar to String x = [, 6,, 9,, 3] <list>.index(<value>) Return position of the value ERROR if value is not there x.index(9) evaluates to 3 <list>.count(<value>) Returns number of times value appears in list x.count() evaluates to But you get length of a list with a regular function, not method: len(x) 3//7 Lists & Sequences

6 Things that Work for All Sequences s = slithy x = [, 6, 9, 6,, ] s.index( s ) s.count( t ) len(s) 6 s[4] h s[:3] li s[3:] thy s[ ] h s + toves slithy toves s * slithyslithy t in s True methods built-in fn. slicing operators x.index() x.count(6) len(x) 6 x[4] x[:3] [6, 9] x[3:] [6,, ] x[ ] x + [, ] [, 6, 9, 6,,,, ] x * [, 6, 9, 6,,,, 6, 9, 6,, ] in x True 3//7 Lists & Sequences 6

7 Difference: Lists Can Hold Any Type a list of integers H e l l o World a list of strings id id id 3 id4 4 id3 a list of objects of class Point a joy 4.3 id id3 id a list of values of various types id id id3 3//7 Lists & Sequences 7 Point Point id4 Point id Point Point

8 Representing Lists Wrong Correct x, 6, 7, - x id Unique tab identifier Variable holds id id x = [, 7, 4,-] Indices 3//7 Lists & Sequences 8

9 Lists vs. Class Objects List Attributes are indexed Example: x[] Objects Attributes are named Example: p.x x id p id3 id id3 list Point x. y. z 3. 3//7 Lists & Sequences 9

10 List Assignment Format: <var>[<index>] = <value> Reassign at index Affects folder contents Variable is unchanged x = [, 7,4,-] x[] = 8 id x id //7 Lists & Sequences

11 List Assignment Format: <var>[<index>] = <value> Reassign at index Affects folder contents Variable is unchanged Strings cannot do this s = 'Hello World!' s[] = 'J' ERROR String are immutable x = [, 7,4,-] 3 x8 7 x[] = 8 x id 4-3 id x 8 3//7 Lists & Sequences

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

13 List Methods Can Alter the List x = [, 6,, 9] <list>.append(<value>) Procedure, not a fruitful method Adds a new value to the end of list x.append(-) changes the list to [, 6,, 9, -] <list>.insert(<index>,<value>) Procedure, not a fruitful method Puts value into list at index; shifts rest of list right x.insert(,-) changes the list to [, 6, -,, 9] <list>.sort() See Python API for more What do you think this does? 3//7 Lists & Sequences 4

14 Execute the following: >>> x = [, 6,, 9, ] >>> x[3] = - >>> x.insert(, ) What is x[4]? A: B: 9 C: - D: ERROR E: I don t know Clicker Exercise 3//7 Lists & Sequences

15 From Before: Attribute Assignment import geom p = geom.point3(.,.,3.) q = geom.point3(3.,4.,.) swap_x(p, q) def swap_x(p, q): t = p.x p.x = q.x 3 q.x = t swaps p.x and q.x import geom p = geom.point3(.,.,3.) q = geom.point3(3.,4.,.) swap(p, q) def swap(p, q): t = p p = q 3 q = t DOES NOT swap global p and q 3//7 Lists & Sequences 6

16 def swap(b, h, k): Lists and Functions: Swap """Procedure swaps b[h] and b[k] in b Precondition: b is a mutable list, h and k are valid positions in the list"" temp= b[h] b[h]= b[k] b[k]= temp 3 x = [,4,7,6,] swap(x, 3, 4) print x[3] What gets printed? A: B: 6 C: Something else D: I don t know 3//7 Lists & Sequences 7 x 3 id4 id

17 Lists and Functions: Swap def swap(b, h, k): """Procedure swaps b[h] and b[k] in b Precondition: b is a mutable list, h and k are valid positions in the list"" temp= b[h] b[h]= b[k] b[k]= temp 3 swap x = [,4,7,6,] swap(x, 3, 4) print x[3] b id4 h 3 k 4 3 id x id4 3//7 Lists & Sequences 8

18 Lists and Functions: Swap def swap(b, h, k): """Procedure swaps b[h] and b[k] in b Precondition: b is a mutable list, h and k are valid positions in the list"" temp= b[h] b[h]= b[k] b[k]= temp 3 swap x = [,4,7,6,] swap(x, 3, 4) print x[3] b id4 h 3 temp 6 k 4 3 id x id4 3//7 Lists & Sequences 9

19 Lists and Functions: Swap def swap(b, h, k): """Procedure swaps b[h] and b[k] in b Precondition: b is a mutable list, h and k are valid positions in the list"" temp= b[h] b[h]= b[k] b[k]= temp 3 swap x = [,4,7,6,] swap(x, 3, 4) print x[3] b id4 h 3 temp 6 k id x id4 3//7 Lists & Sequences

20 Lists and Functions: Swap def swap(b, h, k): """Procedure swaps b[h] and b[k] in b Precondition: b is a mutable list, h and k are valid positions in the list"" temp= b[h] b[h]= b[k] b[k]= temp 3 swap x = [,4,7,6,] swap(x, 3, 4) print x[3] b id4 h 3 temp 6 k 4 3 id x id4 3//7 Lists & Sequences

21 Lists and Functions: Swap def swap(b, h, k): """Procedure swaps b[h] and b[k] in b Precondition: b is a mutable list, h and k are valid positions in the list"" temp= b[h] b[h]= b[k] b[k]= temp 3 What gets printed? x = [,4,7,6,] swap(x, 3, 4) print x[3] A: B: 6 C: Something else D: I don t know Swaps b[h] and b[k], because parameter b contains name of list. x 3 id4 id //7 Lists & Sequences

22 List Slices Make Copies x = [, 6,, 9] y = x[:3] x id y id6 id list id6 list copy = new folder 3//7 Lists & Sequences 3

23 Clicker Exercises Execute the following: >>> x = [, 6,, 9, ] >>> y = x[:] >>> y[] = 7 What is x[]? A: 7 B: C: 6 D: ERROR E: I don t know Execute the following: >>> x = [, 6,, 9, ] >>> y = x >>> y[] = 7 What is x[]? A: 7 B: C: 6 D: ERROR E: I don t know 3//7 Lists & Sequences 4

24 Lists of Objects List positions are variables Can store base types But cannot store folders Can store folder ids Folders linking to folders Top folder for the list Other folders for contents Example: >>> p = Point3(.,., 3.) >>> p = Point3(4.,., 6.) >>> p3 = Point3(7., 8., 9.) >>> x = [p,p,p3] p id p id p3 id x id3 id Point3 x. y. z 3. id Point3 x 4. y. z 6. id3 id id id id list Point3 x 7. y 8. z 9. 3//7 Lists & Sequences

25 Lists of Objects Example: >>> p = Point3(.,., 3.) p id id3 list >>> p = Point3(4.,., 6.) >>> p3 = Point3(7., 8., 9.) >>> x = [p,p,p3] p p3 x id id id3 id id id How do I get this y? >>> x[].y id Point3 x. y. z 3. id Point3 x 4. y. z 6. id Point3 x 7. y 8. z 9. 3//7 Lists & Sequences 6

26 Lists and Strings Go Hand in Hand text.split(<sep>): return a list of the words in text (separated by <sep>, or whitespace by default) <sep>.join(words): concatenate the items in the list of strings words, separated by <sep>. returns a list of two strings text = 'A sentence is just\na list of words' words = text.split() lines = text.split('\n') print '-'.join(words) print '-'.join(lines[].split() + lines[].split()) [ A, sentence, is, just, a, ] A-sentence-is-just-a A-sentence-is-just a-list-of-words 3//7 Lists & Sequences 7

27 Example: Poetry Can we read a poem and count the number of: characters words lines stanzas 3//7 Lists & Sequences 8

28 Iteration To process a list, you often want to do the same thing to each item in the list. One way to do this: The map function: map( function, list ) Call the function once for each item in the list, with the list item as the argument, and put the return values into a list. 3//7 Lists & Sequences 9

29 The Map Function map( function, list ) Function has to have exactly parameter Otherwise, get an error Returns a new list Does the same thing as def map(f,x): result = [] # empty list for y in x: result.append(f(y)) return result map(f, x) [f(x[]), f(x[]),, f(x[n ])] calls the function f once for each item map(len, ['a', 'bc', 'defg']) returns [,, 4] 3//7 Lists & Sequences 3