12.11. Exercises — How to Think like a Computer Scientist: Interactive Edition

Draw a reference diagram for a and b before and after the third line of the following python code is executed:
a = [1, 2, 3] b = a[:] b[0] = 5
Your diagram should show two variables referring to two different lists. a refers to the original list with 1,2, and 3. b refers to a list with 5,2, and 3 since the zero-eth element was replaced with 5.
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Create a list called myList with the following six items: 76, 92.3, “hello”, True, 4, 76. Do it with both append and with concatenation, one item at a time.

Starting with the list of the previous exercise, write Python statements to do the following: a. Append "apple" and 76 to the list. #. Insert the value "cat" at position 3. #. Insert the value 99 at the start of the list. #. Find the index of "hello". #. Count the number of 76s in the list. #. Remove the first occurrence of 76 from the list. #. Remove True from the list using ``pop`` and ``index``.

myList = [76, 92.3, 'hello', True, 4, 76] myList.append("apple") # a myList.append(76) # a myList.insert(3, "cat") # b myList.insert(0, 99) # c print(myList.index("hello")) # d print(myList.count(76)) # e myList.remove(76) # f myList.pop(myList.index(True)) # g print (myList)

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Create a list containing 100 random integers between 0 and 1000 (use iteration, append, and the random module). Write a function called average that will take the list as a parameter and return the average.

Write a Python function that will take a the list of 100 random integers between 0 and 1000 and return the maximum value. (Note: there is a builtin function named max but pretend you cannot use it.)

import random def max(lst): max = 0 for e in lst: if e > max: max = e return max lst = [] for i in range(100): lst.append(random.randint(0, 1000)) print(max(lst))

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Write a function sum_of_squares(xs) that computes the sum of the squares of the numbers in the list xs. For example, sum_of_squares([2, 3, 4]) should return 4+9+16 which is 29: def sum_of_squares(xs): # your code here ==== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): self.assertEqual(sum_of_squares([2,3,4]),29,"Tested sum_of_squares on input [2,3,4]") self.assertEqual(sum_of_squares([0,1,-1]),2,"Tested sum_of_squares on input [0,1,-1]") self.assertEqual(sum_of_squares([5,12,14]),365,"Tested sum_of_squares on input [5,12,14]") myTests().main()

Write a function to count how many odd numbers are in a list. def countOdd(lst): # your code here ==== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): self.assertEqual(countOdd([1,3,5,7,9]),5,"Tested countOdd on input [1,3,5,7,9]") self.assertEqual(countOdd([1,2,3,4,5]),3,"Tested countOdd on input [-1,-2,-3,-4,-5]") self.assertEqual(countOdd([2,4,6,8,10]),0,"Tested countOdd on input [2,4,6,8,10]") self.assertEqual(countOdd([0,-1,12,-33]),2,"Tested countOdd on input [0,-1,12,-33]") myTests().main()

import random def countOdd(lst): odd = 0 for e in lst: if e % 2 != 0: odd = odd + 1 return odd # make a random list to test the function lst = [] for i in range(100): lst.append(random.randint(0, 1000)) print(countOdd(lst))

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Sum up all the even numbers in a list. def sumEven(lst): # your code here ==== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): self.assertEqual(sumEven([1,3,5,7,9]),0,"Tested sumEven on input [1,3,5,7,9]") self.assertEqual(sumEven([-1,-2,-3,-4,-5]),-6,"Tested sumEven on input [-1,-2,-3,-4,-5]") self.assertEqual(sumEven([2,4,6,7,9]),12,"Tested sumEven on input [2,4,6,7,9]") self.assertEqual(sumEven([0,1,12,33]),12,"Tested sumEven on input [0,1,12,33]") myTests().main()

Sum up all the negative numbers in a list. def sumNegatives(lst): # your code here ==== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): self.assertEqual(sumNegatives([-1,-2,-3,-4,-5]),-15,"Tested sumNegatives on input [-1,-2,-3,-4,-5]") self.assertEqual(sumNegatives([1,-3,5,-7,9]),-10,"Tested sumNegatives on input [1,-3,5,-7,9]") self.assertEqual(sumNegatives([-2,-4,6,-7,9]),-13,"Tested sumNegatives on input [-2,-4,6,-7,9]") self.assertEqual(sumNegatives([0,1,2,3,4]),0,"Tested sumNegatives on input [0,1,2,3,4]") myTests().main()

import random def sumNegative(lst): sum = 0 for e in lst: if e < 0: sum = sum + e return sum lst = [] for i in range(100): lst.append(random.randrange(-1000, 1000)) print(sumNegative(lst))

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Count how many words in a list have length 5. def countWords(lst): # your code here

Sum all the elements in a list up to but not including the first even number.

def sumUntilEven(lst): # your code here ==== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): self.assertEqual(sumUntilEven([1,2,3,4,5]),1,"Tested sumUntilEven on input [1,2,3,4.5]") self.assertEqual(sumUntilEven([1,3,5,7,9]),25,"Tested sumUntilEven on input [1,3,5,7,9]") self.assertEqual(sumUntilEven([2,4,6,7,9]),0,"Tested sumUntilEven on input [2,4,6,7,9]") myTests().main()

import random def sum(lst): sum = 0 index = 0 while index < len(lst) and lst[index] % 2 != 0: sum = sum + lst[index] index = index + 1 return sum lst = [] for i in range(100): lst.append(random.randint(0,1000)) print(sum(lst))

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Count how many words occur in a list up to and including the first occurrence of the word “sam”. def count(lst): # your code here

Although Python provides us with many list methods, it is good practice and very instructive to think about how they are implemented. Implement a Python function that works like the following:
1. count
2. in
3. reverse
4. index
5. insert
def count(obj, lst): count = 0 for e in lst: if e == obj: count = count + 1 return count def is_in(obj, lst): # cannot be called in() because in is a reserved keyword for e in lst: if e == obj: return True return False def reverse(lst): reversed = [] for i in range(len(lst)-1, -1, -1): # step through the original list backwards reversed.append(lst[i]) return reversed def index(obj, lst): for i in range(len(lst)): if lst[i] == obj: return i return -1 def insert(obj, index, lst): newlst = [] for i in range(len(lst)): if i == index: newlst.append(obj) newlst.append(lst[i]) return newlst lst = [0, 1, 1, 2, 2, 3, 4, 5, 6, 7, 8, 9] print(count(1, lst)) print(is_in(4, lst)) print(reverse(lst)) print(index(2, lst)) print(insert('cat', 4, lst))

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Write a function replace(s, old, new) that replaces all occurences of old with new in a string s:
```
test(replace('Mississippi', 'i', 'I'), 'MIssIssIppI')

s = 'I love spom!  Spom is my favorite food.  Spom, spom, spom, yum!'
test(replace(s, 'om', 'am'),
       'I love spam!  Spam is my favorite food.  Spam, spam, spam, yum!')

test(replace(s, 'o', 'a'),
       'I lave spam!  Spam is my favarite faad.  Spam, spam, spam, yum!')
```
Hint: use the split and join methods.
def replace(s, old, new): # your code here ==== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): self.assertEqual(replace('Mississippi','i','I'),'MIssIssIppI',"Tested replace on input 'Mississippi','i','I'") self.assertEqual(replace('Bookkeeper','e','A'),'BookkAApAr',"Tested failed on input 'Bookkeeper','e','A'") self.assertEqual(replace('Deeded','e','q'),'Dqqdqd',"Tested failed on input 'Deeded','e','q'") myTests().main()

Here are the rules for an L-system that creates something that resembles a common garden herb. Implement the following rules and try it. Use an angle of 25.7 degrees.
H H --> HFX[+H][-H] X --> X[-FFF][+FFF]FX
import turtle def createLSystem(numIters, axiom): startString = axiom endString = "" for i in range(numIters): endString = processString(startString) startString = endString return endString def processString(oldStr): newstr = "" for ch in oldStr: newstr = newstr + applyRules(ch) return newstr def applyRules(ch): newstr = "" if ch == 'H': newstr = 'HFX[+H][-H]' # Rule 1 elif ch == 'X': newstr = 'X[-FFF][+FFF]FX' else: newstr = ch # no rules apply so keep the character return newstr def drawLsystem(aTurtle, instructions, angle, distance): savedInfoList = [] for cmd in instructions: if cmd == 'F': aTurtle.forward(distance) elif cmd == 'B': aTurtle.backward(distance) elif cmd == '+': aTurtle.right(angle) elif cmd == '-': aTurtle.left(angle) elif cmd == '[': savedInfoList.append([aTurtle.heading(), aTurtle.xcor(), aTurtle.ycor()]) #print(savedInfoList) elif cmd == ']': newInfo = savedInfoList.pop() aTurtle.setheading(newInfo[0]) aTurtle.setposition(newInfo[1], newInfo[2]) def main(): inst = createLSystem(4, "H") # create the string print(inst) t = turtle.Turtle() # create the turtle wn = turtle.Screen() t.up() t.back(200) t.down() t.speed(9) drawLsystem(t, inst, 27.5, 5) # draw the picture wn.exitonclick() main()

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Here is another L-System. Use an Angle of 25.
```
F
F --> F[-F]F[+F]F
```

12.11. Exercises¶