9.12. Exercises¶

What is the result of each of the following:

‘Python’[1]

“Strings are sequences of characters.”[5]

len(“wonderful”)

‘Mystery’[:4]

‘p’ in ‘Pineapple’

‘apple’ in ‘Pineapple’

‘pear’ not in ‘Pineapple’

‘apple’ > ‘pineapple’

‘pineapple’ < ‘Peach’
‘Python’[1] = ‘y’

‘Strings are sequences of characters.’[5] = ‘g’

len(‘wonderful’) = 9

‘Mystery’[:4] = ‘Myst’

‘p’ in ‘Pineapple’ = True

‘apple’ in ‘Pineapple’ = True

‘pear’ not in ‘Pineapple’ = True

‘apple’ > ‘pineapple’ = False

‘pineapple’ < ‘Peach’ = False
Show Comments
In Robert McCloskey’s book Make Way for Ducklings, the names of the ducklings are Jack, Kack, Lack, Mack, Nack, Ouack, Pack, and Quack. This loop tries to output these names in order.
```
prefixes = "JKLMNOPQ"
suffix = "ack"

for p in prefixes:
    print(p + suffix)
```
Of course, that’s not quite right because Ouack and Quack are misspelled. Can you fix it?
1

2
(ex_8_2)
Assign to a variable in your program a triple-quoted string that contains your favorite paragraph of text - perhaps a poem, a speech, instructions to bake a cake, some inspirational verses, etc.

Write a function that counts the number of alphabetic characters (a through z, or A through Z) in your text and then keeps track of how many are the letter ‘e’. Your function should print an analysis of the text like this:

Your text contains 243 alphabetic characters, of which 109 (44.8%) are 'e'.

3

1
def count(p):
2
# your code here
3

(ex_8_3)
25

1
def count(p):
2
lows = "abcdefghijklmnopqrstuvwxyz"
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ups = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
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numberOfe = 0
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totalChars = 0
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for achar in p:
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if achar in lows or achar in ups:
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totalChars = totalChars + 1
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if achar == 'e':
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numberOfe = numberOfe + 1
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percent_with_e = (numberOfe / totalChars) * 100
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print("Your text contains", totalChars, "alphabetic characters of which", numberOfe, "(", percent_with_e, "%)", "are 'e'.")
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p = '''
18
"If the automobile had followed the same development cycle as the computer, a
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Rolls-Royce would today cost $100, get a million miles per gallon, and explode
20
once a year, killing everyone inside."
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-Robert Cringely
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'''
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count(p)
25

(str_q3_answer)
Show Comments
Print out a neatly formatted multiplication table, up to 12 x 12.
2

1

2
(ex_8_4)
Write a function that will return the number of digits in an integer.

4

1
def numDigits(n):
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# your code here
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4

(ex_7_10)
9

1
def numDigits(n):
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n_str = str(n)
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return len(n_str)
4

5

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print(numDigits(50))
7
print(numDigits(20000))
8
print(numDigits(1))
9

(str_q5_answer)
Show Comments
Write a function that reverses its string argument.
4

1
def reverse(astring):
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# your code here
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4
(ex_8_5)
Write a function that mirrors its string argument, generating a string containing the original string and the string backwards.

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1
def mirror(mystr):
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# your code here
3

4

(ex_8_6)
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1
from test import testEqual
2

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def reverse(mystr):
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reversed = ''
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for char in mystr:
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reversed = char + reversed
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return reversed
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def mirror(mystr):
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return mystr + reverse(mystr)
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testEqual(mirror('good'), 'gooddoog')
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testEqual(mirror('Python'), 'PythonnohtyP')
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testEqual(mirror(''), '')
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testEqual(mirror('a'), 'aa')
16

(str_q7_answer)
Show Comments
Write a function that removes all occurrences of a given letter from a string.
4

1
def remove_letter(theLetter, theString):
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# your code here
3

4
(ex_8_7)
Write a function that recognizes palindromes. (Hint: use your reverse function to make this easy!).

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1
def is_palindrome(myStr):
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# your code here
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4

(ex_8_8)
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1
from test import testEqual
2

3
def reverse(mystr):
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reversed = ''
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for char in mystr:
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reversed = char + reversed
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return reversed
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def is_palindrome(myStr):
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if myStr in reverse(myStr):
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return True
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else:
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return False
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testEqual(is_palindrome('abba'), True)
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testEqual(is_palindrome('abab'), False)
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testEqual(is_palindrome('straw warts'), True)
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testEqual(is_palindrome('a'), True)
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testEqual(is_palindrome(''), True)
20

(str_q9_answer)
Show Comments
Write a function that counts how many non-overlapping occurences of a substring appear in a string.
5

1
def count(substr,theStr):
2
# your code here
3

4

5
(ex_8_9)
Write a function that removes the first occurrence of a string from another string.

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1
def remove(substr,theStr):
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# your code here
3

4

(ex_8_10)
14

1
from test import testEqual
2

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def remove(substr,theStr):
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index = theStr.find(substr)
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if index < 0: # substr doesn't exist in theStr
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return theStr
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return_str = theStr[:index] + theStr[index+len(substr):]
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return return_str
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testEqual(remove('an', 'banana'), 'bana')
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testEqual(remove('cyc', 'bicycle'), 'bile')
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testEqual(remove('iss', 'Mississippi'), 'Missippi')
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testEqual(remove('egg', 'bicycle'), 'bicycle')
14

(str_q11_answer)
Show Comments
Write a function that removes all occurrences of a string from another string.
6

1
def remove_all(substr,theStr):
2
# your code here
3

4

5

6
(ex_8_11)
Here is another interesting L-System called a Hilbert curve. Use 90 degrees:

L L -> +RF-LFL-FR+ R -> -LF+RFR+FL-

2

1

2

(ex_8_12)
56

1
import turtle
2

3
def createLSystem(numIters, axiom):
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startString = axiom
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endString = ""
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for i in range(numIters):
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endString = processString(startString)
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startString = endString
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return endString
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def processString(oldStr):
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newstr = ""
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for ch in oldStr:
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newstr = newstr + applyRules(ch)
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return newstr
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def applyRules(ch):
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newstr = ""
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if ch == 'L':
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newstr = '+RF-LFL-FR+' # Rule 1
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elif ch == 'R':
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newstr = '-LF+RFR+FL-'
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else:
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newstr = ch # no rules apply so keep the character
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return newstr
29

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def drawLsystem(aTurtle, instructions, angle, distance):
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for cmd in instructions:
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if cmd == 'F':
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aTurtle.forward(distance)
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elif cmd == 'B':
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aTurtle.backward(distance)
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elif cmd == '+':
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aTurtle.right(angle)
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elif cmd == '-':
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aTurtle.left(angle)
40

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def main():
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inst = createLSystem(4, "L") # create the string
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print(inst)
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t = turtle.Turtle() # create the turtle
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wn = turtle.Screen()
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t.up()
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t.back(200)
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t.down()
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t.speed(9)
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drawLsystem(t, inst, 90, 5) # draw the picture
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# angle 90, segment length 5
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wn.exitonclick()
54

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main()
56

(str_q13_answer)
Show Comments
Here is a dragon curve. Use 90 degrees.:
```
FX
X -> X+YF+
Y -> -FX-Y
```
2

1

2
(ex_8_13)
Here is something called an arrowhead curve. Use 60 degrees.:

YF X -> YF+XF+Y Y -> XF-YF-X

2

1

2

(ex_8_14)
53

1
import turtle
2

3
def createLSystem(numIters, axiom):
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startString = axiom
5
endString = ""
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for i in range(numIters):
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endString = processString(startString)
8
startString = endString
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return endString
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def processString(oldStr):
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newstr = ""
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for ch in oldStr:
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newstr = newstr + applyRules(ch)
16

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return newstr
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def applyRules(ch):
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newstr = ""
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if ch == 'X':
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newstr = 'YF+XF+Y' # Rule 1
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elif ch == 'Y':
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newstr = 'XF-YF-X'
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else:
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newstr = ch # no rules apply so keep the character
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return newstr
29

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def drawLsystem(aTurtle, instructions, angle, distance):
31
for cmd in instructions:
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if cmd == 'F':
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aTurtle.forward(distance)
34
elif cmd == 'B':
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aTurtle.backward(distance)
36
elif cmd == '+':
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aTurtle.right(angle)
38
elif cmd == '-':
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aTurtle.left(angle)
40

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def main():
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inst = createLSystem(5, "YF") # create the string
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print(inst)
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t = turtle.Turtle() # create the turtle
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wn = turtle.Screen()
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t.speed(9)
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drawLsystem(t, inst, 60, 5) # draw the picture
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# angle 90, segment length 5
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wn.exitonclick()
51

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main()
53

(str_q15_answer)
Show Comments
Try the Peano-Gosper curve. Use 60 degrees.:
```
FX
X -> X+YF++YF-FX--FXFX-YF+
Y -> -FX+YFYF++YF+FX--FX-Y
```
2

1

2
(ex_8_15)
The Sierpinski Triangle. Use 60 degrees.:

FXF--FF--FF F -> FF X -> --FXF++FXF++FXF--

2

1

2

(ex_8_16)
59

1
import turtle
2

3
def createLSystem(numIters, axiom):
4
startString = axiom
5
endString = ""
6
for i in range(numIters):
7
endString = processString(startString)
8
startString = endString
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return endString
11

12
def processString(oldStr):
13
newstr = ""
14
for ch in oldStr:
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newstr = newstr + applyRules(ch)
16

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return newstr
18

19
def applyRules(ch):
20
newstr = ""
21
if ch == 'F':
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newstr = 'FF' # Rule 1
23
elif ch == 'X':
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newstr = '--FXF++FXF++FXF--'
25
else:
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newstr = ch # no rules apply so keep the character
27

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return newstr
29

30
def drawLsystem(aTurtle, instructions, angle, distance):
31
for cmd in instructions:
32
if cmd == 'F':
33
aTurtle.forward(distance)
34
elif cmd == 'B':
35
aTurtle.backward(distance)
36
elif cmd == '+':
37
aTurtle.right(angle)
38
elif cmd == '-':
39
aTurtle.left(angle)
40

41
def main():
42
inst = createLSystem(5, "FXF--FF--FF") # create the string
43
print(inst)
44
t = turtle.Turtle() # create the turtle
45
wn = turtle.Screen()
46
t.up()
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t.back(200)
48
t.right(90)
49
t.forward(100)
50
t.left(90)
51
t.down()
52
t.speed(9)
53

54
drawLsystem(t, inst, 60, 5) # draw the picture
55
# angle 90, segment length 5
56
wn.exitonclick()
57

58
main()
59

(str_q17_answer)
Show Comments
Write a function that implements a substitution cipher. In a substitution cipher one letter is substituted for another to garble the message. For example A -> Q, B -> T, C -> G etc. your function should take two parameters, the message you want to encrypt, and a string that represents the mapping of the 26 letters in the alphabet. Your function should return a string that is the encrypted version of the message.
2

1

2
(ex_8_17)
Write a function that decrypts the message from the previous exercise. It should also take two parameters. The encrypted message, and the mixed up alphabet. The function should return a string that is the same as the original unencrypted message.

2

1

2

(ex_8_18)
31

1
def encrypt(message, cipher):
2
alphabet = "abcdefghijklmnopqrstuvwxyz"
3
encrypted = ''
4
for char in message:
5
if char == ' ':
6
encrypted = encrypted + ' '
7
else:
8
pos = alphabet.index(char)
9
encrypted = encrypted + cipher[pos]
10
return encrypted
11

12
def decrypt(encrypted, cipher):
13
alphabet = "abcdefghijklmnopqrstuvwxyz"
14
decrypted = ''
15
for char in encrypted:
16
if char == ' ':
17
decrypted = decrypted + ' '
18
else:
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pos = cipher.index(char)
20
decrypted = decrypted + alphabet[pos]
21
return decrypted
22

23

24
cipher = "badcfehgjilknmporqtsvuxwzy"
25

26
encrypted = encrypt('hello world', cipher)
27
print encrypted
28

29
decrypted = decrypt(encrypted, cipher)
30
print(decrypted)
31

(str_q19_answer)
Show Comments
Write a function called remove_dups that takes a string and creates a new string by only adding those characters that are not already present. In other words, there will never be a duplicate letter added to the new string.
7

1
def remove_dups(astring):
2
# your code here
3

4

5
print(remove_dups("mississippi")) #should print misp
6

7
(ex_8_19)
Write a function called rot13 that uses the Caesar cipher to encrypt a message. The Caesar cipher works like a substitution cipher but each character is replaced by the character 13 characters to ‘its right’ in the alphabet. So for example the letter a becomes the letter n. If a letter is past the middle of the alphabet then the counting wraps around to the letter a again, so n becomes a, o becomes b and so on. Hint: Whenever you talk about things wrapping around its a good idea to think of modulo arithmetic.

7

1
def rot13(mess):
2
# Your code here
3

4
print(rot13('abcde'))
5
print(rot13('nopqr'))
6
print(rot13(rot13('Since rot13 is symmetric you should see this message')))
7

(ex_8_20)
18

1
def rot13(mess):
2
alphabet = 'abcdefghijklmnopqrstuvwxyz'
3
encrypted = ''
4
for char in mess:
5
if char == ' ':
6
encrypted = encrypted + ' '
7
else:
8
rotated_index = alphabet.index(char) + 13
9
if rotated_index < 26:
10
encrypted = encrypted + alphabet[rotated_index]
11
else:
12
encrypted = encrypted + alphabet[rotated_index % 26]
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return encrypted
14

15
print(rot13('abcde'))
16
print(rot13('nopqr'))
17
print(rot13(rot13('since rot thirteen is symmetric you should see this message')))
18

(str_q21_answer)
Show Comments

Modify this code so it prints each subtotal, the total cost, and average price to exactly two decimal places.

(ex_8_22)

Next Section - 10. Lists

9.12. Exercises¶

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