Introduction to Python

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In-Class Exercises, Session 4



PLEASE REFER to pythonreference.com for syntax to follow in coding these Exercises
 

NOTE THAT the data files read from these exercises is located in the "parent" directory. Thus any filename in these exercises should be preceded with ../. (If you create a script in the same directory as the data file, this would not be necessary.)
 

LISTS
 
Ex. 4.1 List slicing and subscripting operations.

Initialize a list of 5 strings. Then perform the below operations. 

# initialize your list of 5 strings
mylist = ['hello', 'there', 'fine', 'Python', 'friends']



# access and print the 2nd item (use list subscript)



# access and print the last item (use negative subscript)



# slice and print the first 3 items from the list (use list slice)



# slice and print the last 3 items from the list (leave off the 'upper bound')
Expected Output: 
there
friends
['hello', 'there', 'fine']
['fine', 'Python', 'friends']
 
Ex. 4.2 List with summary functions. 
x = [3.3, 1.1, 4.4, 2.2]


# in one statement, show count of items in a list (use len())



# sum up values in the list without looping (use sum())



# show maximum value in the list (use max())



# show minimum value in the list (use min())



# show list items sorted lowest to highest (use sorted())



# show list items sorted highest to lowest (add reverse=True)
Expected Output: 
4
11.0
4.4
1.1
[1.1, 2.2, 3.3, 4.4]
[4.4, 3.3, 2.2, 1.1]
 

LAB 1
 
Ex. 4.3 Show the first 3 values in this list. 
x = [8, 1, 0, 2, 23, 4, 5, 11]
Expected Output: 
[8, 1, 0]
 
Ex. 4.4 Show the last 3 values in this list. Use a negative slice. 
x = [8, 1, 0, 2, 23, 4, 5, 11]
Expected Output: 
[4, 5, 11]
 
Ex. 4.5 Show the average of the values in this list. 
x = [8, 1, 0, 2, 23, 4, 5, 11]
Expected Output: 
6.75
 
Ex. 4.6 Find highest and lowest values in a list.

In two simple statements (not using subscripting or sorted()), find the highest value and lowest value in this list. Print each value. 

x = [8, 1, 0, 2, 23, 4, 5, 11]
Expected Output: 
23
0
 
Ex. 4.7 Show the highest 3 values in this list, sorted high-to-low. 
x = [8, 1, 0, 2, 23, 4, 5, 11]
Expected Output: 
[23, 11, 8]
 
Ex. 4.8 Show the lowest 3 values in this list, sorted low-to-high. 
x = [8, 1, 0, 2, 23, 4, 5, 11]
Expected Output: 
[0, 1, 2]
 
Ex. 4.9 Show the 3rd highest value in this list. 
x = [8, 1, 0, 2, 23, 4, 5, 11]
Expected Output: 
8
 
Ex. 4.10 Show the average of the highest 50% of values in this list. 
x = [8, 1, 0, 2, 23, 4, 5, 11]
Expected Output: 
11.75
 

LISTS - BUILDING FROM FILE
 
Ex. 4.11 Build a list with .append.

Initialize a list. Then in two statements, append two float values 5.5 and 5.6 to your list, then print the entire list to show that the two items have been added.

Expected Output: 
[5.5, 6.6]
 
Ex. 4.12 Review: loop through a list. 
x = [3.3, 1.1, 4.4, 2.2]

# loop through the list and print each item individually (use 'for')
Expected Output: 
3.3
1.1
4.4
2.2
 
Ex. 4.13 Build a list of items from a CSV file.

Begin by reviewing and running the below code. Taking a look at file revenue.csv, build up a list of the float values in that file. 

fh = open('../revenue.csv')

fvals = []

for line in fh:
    line = line.rstrip()
    items = line.split(',')
    print(items)


fh.close()
Expected Output: 
[239.5, 53.9, 211.5, 11.98, 5.98, 23.95, 115.2]
 

LAB 2
 
Ex. 4.14 Review: loop through a file.

Open the below file, then use the next() function with the file object to skip the first line of the file. Strip and print each line. 

filename = '../student_db.txt'

# open file


# skip the first line of the file (use next())


# loop through file line-by-line


    # strip each line


    # print each line
Expected Output: 
jk43:23 Marfield Lane:Plainview:NY:10024
axe99:315 W. 115th Street, Apt. 11B:New York:NY:10027
jab44:23 Rivington Street, Apt. 3R:New York:NY:10002
ak9:234 Main Street:Philadelphia:PA:08990
ap172:19 Boxer Rd.:New York:NY:10005
jb23:115 Karas Dr.:Jersey City:NJ:07127
jb29:119 Xylon Dr.:Jersey City:NJ:07127

Note that the header (id:address:city:state:zip) has been skipped with next().
 
Ex. 4.15 Review: loop through a file and split out values.

Continuing the previous exercise, split each line and print the list split from each line. 

filename = '../student_db.txt'

fh = open(filename)

next(fh)  # skip the first line of the file (use next())

for line in fh:

    line = line.rstrip()

    # your code here -- remember that this file is colon-separated
Expected Output: 
['jk43', '23 Marfield Lane', 'Plainview', 'NY', '10024']
['axe99', '315 W. 115th Street, Apt. 11B', 'New York', 'NY', '10027']
['jab44', '23 Rivington Street, Apt. 3R', 'New York', 'NY', '10002']
['ak9', '234 Main Street', 'Philadelphia', 'PA', '08990']
['ap172', '19 Boxer Rd.', 'New York', 'NY', '10005']
['jb23', '115 Karas Dr.', 'Jersey City', 'NJ', '07127']
['jb29', '119 Xylon Dr.', 'Jersey City', 'NJ', '07127']
 
Ex. 4.16 Compile a list of values from file lines.

Build a list of student ids (first field) from student_db.txt. See 'next()' below, which shows how to skip the header 

filename = '../student_db.txt'

fh = open(filename)

next(fh)             # skip the first line of the file

for line in fh:

    line = line.rstrip()

    items = line.split(':')
Expected Output: 
['jk43', 'axe99', 'jab44', 'ak9', 'ap172', 'jb23', 'jb29']
 
Ex. 4.17 Calculate an average of values from a file.

Show the average value from a list of the float values in file revenue.csv.

Expected Output: 
94.57285714285716
 
Ex. 4.18 Show the highest 3 values from the last column in dated_file.csv, sorted from highest to lowest.
Expected Output: 
[28.3, 23.85, 19.09]
 
Ex. 4.19 Show the average of the highest 25% of values in dated_file.csv.
Expected Output: 
26.075000000000003
 

TUPLES - LIKE A LIST, BUT READ-ONLY
 
Ex. 4.20 Compare a tuple to a list.

Given the below code which shows several operations that are possible with a list, change the list to a tuple (replace the square brackets with parentheses in the first line) and re-run the code to see what operations may be possible with a tuple. (Note: spaces inside {} are included for clarity.) 

myseq = [3, 1, 5, 9, 7, 11]

print(f'the sequence:  {  myseq  }')

print(f'last item:     {  myseq[-1]  }')

print(f'first 3 items: {  myseq[0:3] }')

print(f'last 3 items:  {  myseq[-3:]  }')

print(f'# of items:    {  len(myseq) }')

print(f'maximum value: {  max(myseq) }')

print(f'minimum value: {  min(myseq) }')

print('each item in sequence:')
for item in myseq:
    print(item)
print()

print(f'sorted:        {  sorted(myseq)  }')

print('appending to sequence: ')
myseq.append(99)
print(myseq)
 

SETS - BUILDING AND READING
 
Ex. 4.21 Compare a set to a list.

Given the below code which shows several operations that are possible with a list, change the list to a set (replace the square brackets with curly braces in the first line) and re-run the code to see what operations may be possible with a tuple. (Note: spaces inside {} are included for clarity.) 

myseq = [3, 1, 5, 9, 7, 11]

print(f'the sequence:  {  myseq  }')

print(f'last item:     {  myseq[-1]  }')

print(f'first 3 items: {  myseq[0:3] }')

print(f'last 3 items:  {  myseq[-3:]  }')

print(f'# of items:    {  len(myseq) }')

print(f'maximum value: {  max(myseq) }')

print(f'minimum value: {  min(myseq) }')

print('each item in sequence:')
for item in myseq:
    print(item)
print()

print(f'sorted:        {  sorted(myseq)  }')

print('appending to sequence: ')
myseq.append(99)
 

SETS - BUILDING FROM FILE
 
Ex. 4.22 Initialize a set.

Initialize a new set of 7-10 string and number values (make sure to include both). Use .add() to add one new, and one repeat value to the set. Print the set to see its contents.

After executing, execute several more times, making note if the output changes. PLEASE NOTE that the order of the printed set is expected to change but that this apparently does not happen in Jupyter. Run this code in PyCharm or other IDE to see the effect - the set's order changes every time you run the program.
 
Ex. 4.23 Check for item membership in a set.

Check to see if the user's input value is in the set. If it is print 'found it', otherwise print 'not found'. 

# set of 4 items
x = {'a', 'b', 'c', 'd'}


item = input('check for an item:  ')

# your code here
 
Ex. 4.24 Build a set of items from a CSV file.

Loop through the file revenue.csv and create a set of states drawn from the 2nd column of the file. (Note that your set order may be different than as shown below.) 

fh = open('../revenue.csv')

states = set()       # the only way to initialize an empty set

for line in fh:
    items = line.split(',')


fh.close()

print(states)
Expected Output: 
{'PA', 'NY', 'NJ'}           # your set order may be different

Note: your set may display in any order.
 

LAB 3
 
Ex. 4.25 Show a set of unique cities in student_db.txt.

You can begin by running the below code, which opens the file, advances one line, then loops through each line and splits the line.

 
filename = '../student_db.txt'

fh = open(filename)
next(fh)

for line in fh:
    items = line.split(':')
    print(items)
Expected Output: 
{'Plainview', 'Philadelphia', 'New York', 'Jersey City'}

Note: don't forget that student_db.txt is colon-separated, and that you must use next(fh) (where fh is the file object) to skip the header row of this file.
 
Ex. 4.26 Print only those lines of revenue.csv where the state can be found in the set of wanted states. 
wanted = {'NJ', 'PA'}
Expected Output: 
Haddad's,PA,239.50
Westfield,NJ,53.90
The Store,NJ,211.50
Awful's,PA,23.95
 
Ex. 4.27 Collect a list of float values from revenue.csv from only those rows where the state can be found in the set of wanted states. 
wanted = {'NY', 'PA'}
Expected Output: 
[239.5, 11.98, 5.98, 23.95, 115.20]
 

"WHOLE FILE" PARSING AND RANGE
 
Ex. 4.28 Split a string into lines with .splitlines().

Perform the operations below. 

# split the below multi-line string into a list of string
# lines (use .splitlines()), and print the list of lines

text = """this is a line
this is another line
this is another line"""




print()

# read file 'pyku.txt' as a single string using .read()
# split the string into a list of strings lines as in previous
filename = '../pyku.txt'

fh = open(filename)
Expected Output: 
['this is a line', 'this is another line',
 'this is another line']

["We're out of gouda.", 'This parrot has ceased to be.',
 'Spam, spam, spam, spam, spam.']
 
Ex. 4.29 Split a file as a list of words.

Read 'pyku.txt' as a single string; use .split() to split into words. Print the list of words. 

filename = '../pyku.txt'

fh = open(filename)
Expected Output: 
["We're", 'out', 'of', 'gouda.', 'This', 'parrot', 'has',
 'ceased', 'to', 'be.', 'Spam,', 'spam,', 'spam,', 'spam,',
 'spam.']
 
Ex. 4.30 Read a file as a list of string lines.

Read 'revenue.csv' as a list of strings (use .readlines()); print the list. Then use a list slice to select only the 2nd through last lines of the file - print these as well. 

filename = '../revenue.csv'

fh = open(filename)
Expected Output: 
["Haddad's,PA,239.50\n", 'Westfield,NJ,53.90\n',
 'The Store,NJ,211.50\n', "Hipster's,NY,11.98\n",
 'Dothraki Fashions,NY,5.98\n', "Awful's,PA,23.95\n",
 'The Clothiers,NY,115.20\n']

['Westfield,NJ,53.90\n', 'The Store,NJ,211.50\n',
 "Hipster's,NY,11.98\n", 'Dothraki Fashions,NY,5.98\n',
 "Awful's,PA,23.95\n", 'The Clothiers,NY,115.20\n']
 

LAB 4
 
Ex. 4.31 Without looping, print the first and also the last line of pyku.txt.
Expected Output: 
We're out of gouda.

Spam, spam, spam, spam, spam.
 
Ex. 4.32 Without looping, print the first word and also the last word of pyku.txt.
Expected Output: 
We're
spam.
 
Ex. 4.33 Without looping, print a slice of the 5th through 10th words of pyku.txt.
Expected Output: 
['This', 'parrot', 'has', 'ceased', 'to', 'be.']
 
Ex. 4.34 Without looping, show how many words are in the first line of pyku.txt.
Expected Output: 
4
 
Ex. 4.35 Without looping, show how many characters are in the first line of pyku.txt.
Expected Output: 
20
 
Ex. 4.36 Without looping, show how many lines are in pyku.txt.
Expected Output: 
3
 

range() AND enumerate()
 
Ex. 4.37 Use range() to count.

Without using a counter, create a for loop to iterate over each integer between 0 and 5.

Expected Output: 
0
1
2
3
4
5
 
Ex. 4.38 Use range() to create a list.

In a single statement, build a list of integers from 3 to 5.

Expected Output: 
[3, 4, 5]
 
Ex. 4.39 Use enumerate() to count as you loop through list items.

As you loop through this list of days, count each item starting at 0 using enumerate() (do not user a incrementing counter; you may put a comma between the number and item in your print statement.) 

days = ['Sunday', 'Monday', 'Tuesday']
Expected Output: 
0 Sunday
1 Monday
2 Tuesday
 
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