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Lists and Sequences

This overview covers list syntax, creation, manipulation, slicing, concatenation, comparison with tuples and strings, and two-dimensional lists in Python.

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Lists and Sequences

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  1. Lists and Sequences Review 4

  2. Overview of List Syntax Create list of length 4 with all zeroes • x = [0, 0, 0, 0] • x.append(2) • 3 in x • x[2] = 5 • x[0] = –4 • k = 3 • x[k] = 2 * x[0] • x[k–2] = 6 x 4300112 Append 2 to end of list x (now length 5) 4300112 Evaluates to False(3 not in x) ✗ 0 -4 0 ✗ 1 6 0 Assign 5 to element 2and –4 to element 0 ✗ 2 5 0 ✗ 3 -8 0 4 2 Assign -8 to x[3] and 6 to x[1] k 3

  3. Lists vs. Tuples vs. Strings • Creation x = [a1, a2, a3, …] Can contain anything • len(x) is length • Supports slicing Example: x[1:2] x[i] is an element • Can concatenate y = x + [1, 2] Makes a new list • Is mutable x.append(5) • Creation x = 'Hello' Only contains chars • len(x) is length • Supports slicing Example: x[1:2] x[i] is a substring • Can concatenate y = x + ' World' Makes a new string • Is not mutable • Creation x = (a1, a2, a3, …) Can contain anything • len(x) is length • Supports slicing Example: x[1:2] x[i] is an element • Can concatenate y = x + (1, 2) Makes a new tuple • Is not mutable

  4. Modified Question 4 from Fall 2011 Each elements in the list scores contains the number of students who received score i on a test. For example, if 30 students got 85, then scores[85] is 30.Write the body of function histogram, which returns a histogram as a list of strings. (You need not write loop invariants.) For example, if scores = [7, 0, 4, 3, 2, 0, …] then the first elements of the resulting string list are: '00 *******' '01 ' '02 ****' '03 ***' '04 *' '05 '

  5. Modified Question 4 from Fall 2011 defhistogram(scores): """Return a list of Strings (call it s) in which each s[i] contains: (1) i, as a two-digit integer (with leading zeros if necessary) (2) a blank, (3) n asterisks '*', where n is scores[i]. Precondition: scores is a list of nonnegative integers, len(scores) < 100""" # IMPLEMENT ME

  6. Modified Question 4 from Fall 2011 defhistogram(scores): """Return a list of Strings (call it s) in which each s[i] contains: (1) i, as a two-digit integer (with leading zeros if necessary) (2) a blank, (3) n asterisks '*', where n is scores[i]. Precondition: scores is a list of nonnegative integers, len(scores) < 100""" s = [] # List to contain the result. foriin range(len(scores)): # Need the value i, not the elements of scores # Inv: `row` is the string for this printout line # Conditional expression version: # row = (str(i)+' ')if scores[0] > 10 else ('0'+str(i)+' ') if scores[i] > 10: row = str(i)+' ' else: row = '0'+str(i)+' ' for n in range(scores[i]): # Loop over number of elements in scores[i] row = row+'*' # Add another * to the row s.append(row) # Add row to the list return s

  7. 0 1 2 3 d 5 4 7 3 4 8 9 7 5 1 2 3 4 1 2 9 6 7 8 0 Overview of Two-Dimensional Lists 0 • Access value at row 3, col 2: d[3][2] • Assign value at row 3, col 2: d[3][2] = 8 • An odd symmetry • Number of rows of d: len(d) • Number of cols in row r of d: len(d[r]) 1 2 3 4

  8. How Multidimensional Lists are Stored • b = [[9, 6, 4], [5, 7, 7]] • b holds name of a one-dimensional list • Has len(b) elements • Its elements are (the names of) 1D lists • b[i] holds the name of a one-dimensional list (of ints) • Has len(b[i]) elements id2 id3 9 6 4 5 7 7 id1 9 5 id2 6 7 id3 id1 b 4 7

  9. Ragged Lists: Rows w/ Different Length • b = [[17,13,19],[28,95]] • To create a ragged list • Create b as an empty list (b = []) • Create each row as a list (r1 = [17,13,19]; r2 = [28,95]) • Append lists to b (b.append(r1); b.append(r2)) id2 id3 id1 0 id1 b 17 28 0 0 id2 1 13 95 id2 1 1 2 19

  10. Modified Question 4 from Fall 2011 Someone messed up a method to create certain arrays for us. For example (and this is only an example), they produced the array: 3 1 2 1 2 3 2 1 7 8 5 instead of 1 7 8 5 2 5 the array 5 6 8 8 6 Thus, they put the last value of each row at the beginning instead of the end. Write a procedure that fixes this by rotating each row one position to the left; each element is moved one position earlier, and the first element is placed in the last position. Do not use recursion. DO NOT RETURN A VALUE. defrotate(b): """Rotate each row one position to the left, as explained above. Precondition: b is a list, might be ragged, and each row has >= 1 value"""

  11. Modified Question 4 from Fall 2011 defrotate(b): """Rotate each row one position to the left, as explained on the previous slide. Precondition: b is a list, might be ragged, and each row has >= 1 value""" # Process each row for r in rang(len(b)): # Remember the first element so we can put it at the end first = b[r][0] # Start at second element and shift each to the left for c in range(1,len(b[r])): b[r][c-1]= b[r][c]; # Put the first element at the end b[r][len(b[r])–1]= first

  12. Call Frames Final Review Spring 2019 CS 1110

  13. The Big Issue • Cannot answer questions on this topic without • drawing variables • drawing frames for function calls • drawing objects when they are created • Learning to do this is useful in general • Helps you “think like a computer” • Easier to find errors in your programs.

  14. What Do You Need to Know? • Major topics • local variables (in a function body) • function call (call frames, call stack) • class folders, inheritance, and constructor calls • Examples from previous exams • Question 3 on prelim 1 • Question 6 on prelim 2

  15. Important • Code execution is an important part of the final • You need to know how to • draw variables • draw call frames • draw objects The purpose of such questions on executing statements with constructs and function calls is to test your understanding of how Python programs are executed.

  16. The Frame (box) for a Function Call • Function Frame: Representation of function call • A conceptual model of Python • Number of statement in the function body to execute next• Starts with function’s first executable statement Draw parameters as variables (named boxes) parameters function name instruction counter local variables (when assigned)

  17. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1 word countVowels ‘hi’

  18. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2 word 0 countVowels ‘hi’ x

  19. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2, 3 word 0 count 0 countVowels ‘hi’ x

  20. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2, 3, 4 word 0 count 0 countVowels ‘hi’ x

  21. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2, 3, 4, 6 word 0 count 0 countVowels ‘hi’ x

  22. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2, 3, 4, 6, 3 word 0 count 0 1 countVowels ‘hi’ x

  23. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2, 3, 4, 6, 3, 4 word 0 count 0 1 countVowels ‘hi’ x

  24. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2, 3, 4, 6, 3, 4, 5 word 0 count 0 1 countVowels ‘hi’ x

  25. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2, 3, 4, 6, 3, 4, 5, 6 word 0 1 count 0 1 countVowels ‘hi’ x

  26. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2, 3, 4, 6, 3, 4, 5, 6, 3 word 0 1 count 01 2 countVowels ‘hi’ x

  27. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2, 3, 4, 6, 3, 4, 5, 6, 3, 7 word 0 1 count 01 2 countVowels ‘hi’ x

  28. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Global Space: Call Stack: 1, 2, 3, 4, 6, 3, 4, 5, 6, 3, 7 word 0 1 count 01 2 1 RETURN countVowels ‘hi’ x

  29. Call: e = count_vowels(‘hi’) Iterative Example defcountVowels(word): """Returns: The number of vowels in the string s. Precondition: s is a string""" 1 x = 0 2 count = 0 3 while x < len(word): 4 if word[x] in [‘a’,’e’,’i’,’o’,’u’]: 5 count += 1 6 x += 1 7 return count Call Stack: Global Space: 1, 2, 3, 4, 6, 3, 4, 5, 6, 3, 7 word 0 1 count 01 2 1 RETURN countVowels ‘hi’ x e 1

  30. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) 3456 12 Call Stack: Heap Space: a id1 1 wkList id1 list printWeather id1 0 ‘Rainy’ 1 ‘Sunny’

  31. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) 3456 12 Heap Space: Call Stack: a id1 1, 2 wkList ‘Rainy’ id1 list printWeather id1 0 ‘Rainy’ item 1 ‘Sunny’

  32. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) 12 Call Stack: Heap Space: 3456 a id1 3 1, 2 wkList day ‘Rainy’ id1 list dayWeather printWeather id1 ‘Rainy’ 0 ‘Rainy’ item 1 ‘Sunny’

  33. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) 12 Call Stack: Heap Space: 3456 a id1 3, 5 1, 2 wkList day ‘Rainy’ id1 list dayWeather printWeather id1 ‘Rainy’ 0 ‘Rainy’ item 1 ‘Sunny’

  34. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) 12 Call Stack: Heap Space: 3456 a id1 3, 5, 6 1, 2 wkList day ‘Rainy’ id1 list dayWeather printWeather id1 ‘Rainy’ 0 ‘Rainy’ item 1 ‘Sunny’

  35. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) Output: Grab your umbrella! 3456 12 Call Stack: Heap Space: a id1 3, 5, 6 1, 2 wkList day ‘Rainy’ None RETURN id1 list printWeather dayWeather ‘Rainy’ id1 0 ‘Rainy’ item 1 ‘Sunny’

  36. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) Output: Grab your umbrella! 3456 12 Call Stack: Heap Space: a id1 3, 5, 6 1, 2, 1 wkList day ‘Rainy’ None RETURN id1 list printWeather dayWeather ‘Rainy’ id1 0 ‘Rainy’ item 1 ‘Sunny’

  37. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) Output: Grab your umbrella! 3456 12 Call Stack: Heap Space: a id1 1, 2, 1, 2 3, 5, 6 wkList day ‘Rainy’, ‘Sunny’ None RETURN id1 list printWeather dayWeather ‘Rainy’ id1 0 ‘Rainy’ item 1 ‘Sunny’

  38. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) Output: Grab your umbrella! 3456 Heap Space: Call Stack: 12 a id1 1, 2, 1, 2 3, 5, 6 3 wkList day day ‘Rainy’, ‘Sunny’ None RETURN id1 list dayWeather printWeather dayWeather ‘Sunny’ ‘Rainy’ id1 0 ‘Rainy’ item 1 ‘Sunny’

  39. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) Output: Grab your umbrella! 3456 Heap Space: Call Stack: 12 a id1 1, 2, 1, 2 3, 5, 6 3, 4 wkList day day ‘Rainy’, ‘Sunny’ None RETURN id1 list dayWeather printWeather dayWeather ‘Sunny’ ‘Rainy’ id1 0 ‘Rainy’ item 1 ‘Sunny’

  40. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) Output: Grab your umbrella! 3456 Heap Space: Call Stack: 12 a id1 1, 2, 1, 2 3, 5, 6 3,4, 5 wkList day day ‘Rainy’, ‘Sunny’ None RETURN id1 list dayWeather printWeather dayWeather ‘Sunny’ ‘Rainy’ id1 0 ‘Rainy’ item 1 ‘Sunny’

  41. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) 3456 12 Call Stack: Output: Grab your umbrella! Time for a picnic! Heap Space: a id1 1, 2, 1, 2 3, 5, 6 3, 4, 5 day day wkList ‘Rainy’, ‘Sunny’ None None RETURN RETURN id1 list printWeather dayWeather dayWeather ‘Rainy’ ‘Sunny’ id1 0 ‘Rainy’ item 1 ‘Sunny’

  42. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) 3456 12 Call Stack: Output: Grab your umbrella! Time for a picnic! Heap Space: a id1 1, 2, 1, 2 3, 5, 6 3, 4, 5 day day wkList ‘Rainy’, ‘Sunny’ None None RETURN RETURN id1 list printWeather dayWeather dayWeather ‘Rainy’ ‘Sunny’ id1 0 ‘Rainy’ item 1 ‘Sunny’

  43. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) Output: Grab your umbrella! Time for a picnic! 3456 12 Heap Space: Call Stack: a id1 1, 2, 1, 2 3, 4, 5 3, 5, 6 day wkList day ‘Rainy’, ‘Sunny’ None None None RETURN RETURN RETURN id1 list dayWeather dayWeather printWeather id1 ‘Sunny’ ‘Rainy’ 0 ‘Rainy’ item 1 ‘Sunny’

  44. Call: printWeather(a) Subcall Example Global Space: def printWeather(wkList): for item in wkList: dayWeather(item) def dayWeather(day): if day == ‘Sunny’: print (‘Time for a picnic!’) if day == ‘Rainy’: print (‘Grab your umbrella!’) Output: Grab your umbrella! Time for a picnic! 3456 12 Heap Space: Call Stack: a id1 1, 2, 1, 2 3, 4, 5 3, 5, 6 day wkList day ‘Rainy’, ‘Sunny’ None None None RETURN RETURN RETURN id1 list dayWeather dayWeather printWeather id1 ‘Sunny’ ‘Rainy’ 0 ‘Rainy’ item 1 ‘Sunny’

  45. Diagramming Objects (Folders) Object Folder Class Folder Folder Name (arbitrary) No folder name id4 classname classname Class Attributes Method Names Instance Attributes Draw attributes as named box w/ value Parameters are required in methods

  46. Evaluation of a Constructor Call 3 steps to evaluating the call C(args) • Create a new folder (object) of class C • Give it with a unique name (any number will do) • Folder goes into heap space • Execute the method __init__(args) • Yield the name of the object as the value • A constructor call is an expression, not a command • Does not put name in a variable unless you assign it

  47. Diagramming Subclasses • Important Details: • Make sure you put the superclass-name in parentheses • Do not duplicate inherited methods and attributes • Include initializer and and other special methods (as applicable) • Method parameters are required • Class attributes are a box with (current) value superclass-name Declared in Superclass: Class Attributes Method Names subclass-name(super) Declared in Subclass: Class Attributes Method Names

  48. class B(A): y=4 z=10 def __init__(self,x,y): super().__init__(y) self.x = x def g(self): return self.x+self.z def h(self): return 42 Two Example Classes class A(object): x=3 y=5 def __init__(self,y): self.y = y def f(self): return self.g() def g(self): return self.x+self.y 

  49. Class Folders A A __init__(self,x) f(self) g(self) __init__(self,x) f(self) g(self) These folders will still exist in the following slides, but will not be redrawn; they exist in the heap space along with the object folders. x x 3 3 y y 5 5 B(A) __init__(self,x,y) g(self) h(self) x 4 y 10

  50. Call: a = A(1) b = B(7, 3) Constructor Examples Global Space: class A(object): x = 3 y = 5 def __init__(self,y): self.y = y class B(A): y = 4 z = 10 def __init__(self,x,y): super().__init__(y) self.x = x Heap Space: Call Stack: 23 1 1 1 y self id8 A A.__init__ id8

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