Introduction to Python

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In-Class Exercise Solutions, 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']   # list


# access and print the 2nd item (use list subscript)
itemsecond = mylist[1]                     # str, 'there'
print(itemsecond)


# access and print the last item (use negative subscript)
itemlast = mylist[-1]                     # str, 'friends'
print(itemlast)


# slice and print the first 3 items from the list (use list slice)
first3 = mylist[0:3]                      # list, ['hello', 'there', 'fine']
print(first3)


# slice and print the last 3 items from the list (leave off the 'upper bound')
last3 = mylist[-3:]                       # list, ['fine', 'Python', 'friends']
print(last3)
 
Ex. 4.2 List with summary functions. 
x = [3.3, 1.1, 4.4, 2.2]                  # list, [3.3, 1.1, 4.4, 2.2]


# in one statement, show count of items in a list (use len())
count = len(x)                            # int, 4
print(count)


# sum up values in the list without looping (use sum())
itemsum = sum(x)                          # float, 10.1
print(itemsum)


# show maximum value in the list (use max())
maxval = max(x)                           # float, 4.4
print(maxval)


# show minimum value in the list (use min())
minval = min(x)                           # float, 1.1
print(minval)


# show list items sorted lowest to highest (use sorted())
slist = sorted(x)                         # list, [1.1, 2.2, 3.3, 4.4]
print(slist)


# show list items sorted highest to lowest (add reverse=True)
srevlist = sorted(x, reverse=True)        # list, [4.4, 3.3., 2.2, 1.1]
print(srevlist)

print()
 
Ex. 4.3 Show the first 3 values in this list. 
x = [8, 1, 0, 2, 23, 4, 5, 11]        # list, [8, 1, 0, 2, 23, 4, 5, 11 ]

first3 = x[0:3]                       # list, [8, 1, 0]
print(first3)
 
Ex. 4.4 Show the last 3 values in this list. Use a negative slice. 
x = [8, 1, 0, 2, 23, 4, 5, 11]        # list, [8, 1, 0, 2, 23, 4, 5, 11 ]

last3 = x[-3:]                        # list, [4, 5, 11]
print(last3)
 
Ex. 4.5 Show the average of the values in this list. 
x = [8, 1, 0, 2, 23, 4, 5, 11]        # list, [8, 1, 0, 2, 23, 4, 5, 11]

avg = sum(x) / len(x)                 # float, 6.75
print(avg)
 
Ex. 4.6 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]        # list, [8, 1, 0, 2, 23, 4, 5, 11]

maxval = max(x)                       # int, 23
minval = min(x)                       # int, 0

print(maxval)
print(minval)
 
Ex. 4.7 Show the top 3 values in this list, sorted high-to-low. 
x = [8, 1, 0, 2, 23, 4, 5, 11]        # list, [8, 1, 0, 2, 23, 4, 5, 11]

xs = sorted(x, reverse=True)          # list, [11, 5, 4, 23, 2, 0, 1, 8]
print(xs[0:3])
 
Ex. 4.8 Show the bottom 3 values in this list, sorted low-to-high. 
x = [8, 1, 0, 2, 23, 4, 5, 11]        # list, [8, 1, 0, 2, 23, 4, 5, 11]

sx = sorted(x)                        # list, [0, 1, 2, 4, 5, 8, 11, 23]
print(sx[0:3])                        # [0, 1, 2]
 
Ex. 4.9 Show the 3rd highest value in this list. 
x = [8, 1, 0, 2, 23, 4, 5, 11]        # list, [8, 1, 0, 2, 23, 4, 5, 11]

sx = sorted(x)                        # list, [0, 1, 2, 4, 5, 8, 11, 23]
print(sx[-3])                         # [8, 11, 23]


# alternative
sx = sorted(x, reverse=True)          # list, [23, 11, 8, 5, 4, 2, 1, 0]
print(sx[2])                          # int, 8
 
Ex. 4.10 Show the average of the top 50% of values in this list. 
x = [8, 1, 0, 2, 23, 4, 5, 11]        # list, [8, 1, 0, 2, 23, 4, 5, 11]

sx = sorted(x, reverse=True)          # list, [23, 11, 8, 5, 4, 2, 1, 0]

lensx = len(sx)                       # int, 8
mid_idx = int(lensx/2)                # int, 4

firsthalf = sx[0:mid_idx]             # list, [23, 11, 8, 5]

print(sum(firsthalf) / len(firsthalf))   # float, 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. 

x = []               # list, []

x.append(5.5)        # list, [5.5]
x.append(6.6)        # list, [5.5, 6.6]

print(x)
 
Ex. 4.12 Review: loop through a list. 
x = [3.3, 1.1, 4.4, 2.2]        # list, [3.3, 1.1, 4.4, 2.2]

for item in x:                  # float, 3.3 (initial value)
    print(item)
 
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')        # 'file' object

fvals = []                         # list, []

for line in fh:                    # str, "Haddad's,PA,239.50\n"
    line = line.rstrip()           # str, "Haddad's,PA,239.50"
    items = line.split(',')        # list, ["Haddad's", 'PA', '239.50\n']
    val = items[-1]                # str, '239.50\n'
    fval = float(val)              # float, 239.5
    fvals.append(fval)             # list, [239.5]

fh.close()
print(fvals)
 

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'     # str, '../student_db.txt'

# open file
fh = open(filename)                # 'file' object

# skip the first line of the file (use next())
next(fh)                           # str, 'id:address:city:state:zip\n'

# loop through file line-by-line
for line in fh:                    # str, 'jk43:23 Marfield Lane:Plainview:NY:10024\n'

    # strip each line
    line = line.rstrip()           # str, 'jk43:23 Marfield Lane:Plainview:NY:10024'

    # print each line
    print(line)
 
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'   # str, '../student_db.txt'

fh = open(filename)              # 'file' object

# skip the first line of the file (use next())
next(fh)                         # str, 'id:address:city:state:zip\n'

for line in fh:                  # str, 'jk43:23 Marfield Lane:Plainview:NY:10024\n'

    line = line.rstrip()         # str, 'jk43:23 Marfield Lane:Plainview:NY:10024'

    items = line.split(':')      # list, ['jk43', '23 Marfield Lane', ...]

    print(items)
 
Ex. 4.16 Build a list of student ids (first field) from student_db.txt.

(See below how to skip header.) 

filename = '../student_db.txt'        # str, '../student_db.txt'

fh = open(filenamne)                  # 'file' object

# skip header to first data row
next(fh)                              # str, 'id:address:city:state:zip\n'

stuids = []                           # list, []

for line in fh:                       # str, 'jk43:23 Marfield Lane:Plainview:NY:10024\n'

    line = line.rstrip()              # str, 'jk43:23 Marfield Lane:Plainview:NY:10024'

    items = line.split(':')           # list, ['jk43', '23 Marfield Lane', ...]

    stuids.append(items[0])           # list, ['jk43']

print(stuids)

Note: don't forget that student_db.txt is colon-separated.
 
Ex. 4.17 Show the average value from a list of the float values in file revenue.csv. 
fh = open('../revenue.csv')        # 'file' object

stuids = []                        # list, []

for line in fh:                    # str, "Haddad's,PA,239.50\n"
    items = line.split(',')        # list, ["Haddad's", 'PA', '239.50\n']
    fval = float(items[-1])        # float, 239.5
    stuids.append(fval)            # list, [239.5]

print(sum(stuids) / len(stuids))
 
Ex. 4.18 Show the top 3 values from the last column in dated_file.csv. 
fh = open('../dated_file.csv')         # 'file' object

values = []                            # list, []

for line in fh:                        # str, '10/15/2018,A,B,28.3\n'
    items = line.split(',')            # list, ['10/15/2018', 'A', 'B' ...]
    lval = float(items[-1])            # float, 28.3
    values.append(lval)                # list, [28.3]

svals = sorted(values, reverse=True)   # list, [28.3, 23.85, 19.09, 18.2, ...]

print(svals[0:3])                      # list, [28.3, 23.85, 19.09]
 
Ex. 4.19 Show the average of the top 25% of values in dated_file.csv. 
fh = open('../dated_file.csv')         # 'file' object

values = []                            # list, []

for line in fh:                        # str, '10/15/2018,A,B,28.3'
    items = line.split(',')            # list, ['10/15/2018', 'A', 'B', '28.3']
    lval = float(items[-1])            # float, 28.3
    values.append(lval)                # list, [28.3]

svals = sorted(values, reverse=True)   # list, [28.3, 23.85, 19.09, 18.2, ...]

lensvals = len(svals)                  # int, 8
mid_idx = int(lensvals / 4)            # int, 2

firstquarter = svals[0:mid_idx]        # list, [28.3, 23.85]

print(sum(firstquarter) / len(firstquarter))
 

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)                # tuple, (3, 1, 5, 9, 7, 11)

print(f'the sequence:  {  myseq  }')       # tuple, (3, 1, 5, 9, 7, 11)

print(f'last item:     {  myseq[-1]  }')   # int, 11

print(f'first 3 items: {  myseq[0:3] }')   # tuple, (3, 1, 5)

print(f'last 3 items:  {  myseq[-3:]  }')  # tuplee, (9, 7, 11)

print(f'# of items:    {  len(myseq) }')   # int, 6

print(f'maximum value: {  max(myseq) }')   # int, 11

print(f'minimum value: {  min(myseq) }')   # int, 1

print('each item in sequence:')
for item in myseq:                         # int, 3 (initial value)
    print(item)
print()

print(f'sorted:        {  sorted(myseq)  }')  # list, [1, 3, 5, 7, 9, 11]

print('appending to sequence: ')
myseq.append(99)         # AttributeError:  'tuple' object has no attribute 'append'
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}               # set, {3, 1, 5, 9, 7, 11}

print(f'the sequence:  {  myseq  }')      # set, {3, 1, 5, 9, 7, 11}

print(f'last item:     {  myseq[-1]  }')  # TypeError:  'set' object is not subscriptable

print(f'first 3 items: {  myseq[0:3] }')  # TypeError:  'set' object is not subscriptable

print(f'last 3 items:  {  myseq[-3:]  }') # TypeError:  'set' object is not subscriptable

print(f'# of items:    {  len(myseq) }')  # int, 6

print(f'maximum value: {  max(myseq) }')  # int, 11

print(f'minimum value: {  min(myseq) }')  # int, 1

print('each item in sequence:')
for item in myseq:                 # int, 5 (sample value)
    print(item)
print()

print(f'sorted:        {  sorted(myseq)  }')  # list, [1, 3, 5, 7, 9, 11]

print('appending to sequence: ')
myseq.append(99)         # AttributeError:  'set' object has no attribute 'append'
 

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. 

myset = {'a', 55, '100', 100, 'hello', 'ABC', 99.03, 3, 7, 0.0003}  # set (any order, no duplicates)

myset.add('a')        # set (any order, no duplicates)
myset.add('b')        # set (any order, no duplicates)

print(myset)

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. 12
 
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'}                    # set, {'c', 'a', 'b', 'd'} (any order)


item = input('check for an item:  ')        # str, 'b' (sample value)

if item in x:                               # bool, True
    print('found it')
else:                                       #
    print('not found')
 
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.) 

fh = open('../revenue.csv')        # 'file' object

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

for line in fh:                    # str, "Haddad's,PA,239.50\n"
    items = line.split(',')        # list, ["Haddad's", 'PA', '239.50\n']
    val = items[1]                 # str, 'PA'
    states.add(val)                # set, {'PA'}

fh.close()

print(states)
 

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'        # str, '../student_db.txt'

fh = open(filename)                   # 'file' object
next(fh)                              # # str, 'id:address:city:state:zip\n'

states = set()                        # set

for line in fh:                       # str, 'jk43:23 Marfield Lane:Plainview:NY:10024\n'
    items = line.split(':')           # list, ['jk43', '23 Marfield Lane', ...]
    val = items[2]                    # str, 'NY'
    states.add(val)                   # set, {'NY'}

print(states)

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'}              # set, {'NJ', 'PA'}

fh = open('../revenue.csv')        # 'file' object

for line in fh:                    # str, "Haddad's,PA,239.50\n"
    line = line.rstrip()           # str, "Haddad's,PA,239.50"
    items = line.split(',')        # list, ["Haddad's", 'PA', '239.50\n']
    state = items[1]               # str, 'PA'
    if state in wanted:            # bool, True
        print(line)

fh.close()                         #
 
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 = {'NJ', 'PA'}                    # set, {'NJ', 'PA'}
fvals = []                               # list, []

fh = open('../revenue.csv')              # 'file' object

for line in fh:                          # str, "Haddad's,PA,239.50\n"
    line = line.rstrip()                 # str, "Haddad's,PA,239.50"
    items = line.split(',')              # list, ["Haddad's", 'PA', '239.50']
    state = items[1]                     # str, 'PA'
    fval = items[2]                      # str, '239.50'
    if state in wanted:                  # bool, True
        fvals.append(float(fval))        # list, [239.5]

fh.close()

print(fvals)
 

"WHOLE FILE" PARSING AND RANGE
 
Ex. 4.28 Splitting 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"""          # str, 'this is a line\nthis is...'

lines = text.splitlines()        # list, ['this is a line', 'this is ...']
print(lines)


print()

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

fh = open(filename)              # 'file' object
text = fh.read()                 # str, "We're out of gouda.This parrot has ceased..."
lines = text.splitlines()        # list, ["We're out of gouda.", 'This parrot...']

print(lines)
 
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'        # str, '../pyku.txt'

fh = open(filename)             # 'file' object

text = fh.read()                # str, "We're out of gouda.\nThis parrot has ..."

words = text.split()            # list, ["We're", 'out', 'of', 'gouda.', 'This' ...]

print(words)
 
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'   # str, '../revenue.csv'

fh = open(filename)           # 'file' object

lines = fh.readlines()        # list, ["Haddad's,PA,239.50\n", "Westfield,NJ,53.90\n" ... ]
print(lines)
print()
print(lines[1:])              # list, ["Westfield,NJ,53.90\n", 'The Store,NJ,211.50\n' ...]
 

LAB 4
 
Ex. 4.31 Without looping, print the first and also the last line of pyku.txt. 
fh = open('../pyku.txt')   # str, '../pyku.txt'
lines = fh.readlines()     # list, ["We're out of gouda.", 'This parrot ...']

print(lines[0])            # str, "We're out of gouda.\n"
print(lines[-1])           # str, 'Spam, spam, spam, spam, spam.\n'
 
Ex. 4.32 Without looping, print the first word and also the last word of pyku.txt. 
fh = open('../pyku.txt')   # str, '../pyku.txt'

text = fh.read()           # str, "We're out of gouda.\nThis parrot ..."
words = text.split()       # list, ["We're", 'out', 'of', 'gouda.', 'This', 'parrot'...]
print(words[0])            # str, "We're"
print(words[-1])           # str, 'spam.'
 
Ex. 4.33 Without looping, print a slice of the 5th through 10th words of pyku.txt. 
fh = open('../pyku.txt')   # str, '../pyku.txt'

text = fh.read()           # str, "We're out of gouda.\nThis parrow ..."
words = text.split()       # list, ["We're", 'out', 'of', 'gouda.', 'This', 'parrot'...]

print(words[4:11])         # list, ['This', 'parrot', 'has', 'ceased', 'to', 'be.']
 
Ex. 4.34 Without looping, show how many words are in the first line of pyku.txt. 
fh = open('../pyku.txt')           # 'file' object

lines = fh.readlines()             # list, ["We're out of gouda.", 'This parrot ...']
first_line = lines[0]              # str, "We're out of gouda.\n"
first_words = first_line.split()   # list, ["We're", 'out', 'of', 'gouda.']

print(len(first_words))            # int, 4
 
Ex. 4.35 Without looping, show how many characters are in the first line of pyku.txt. 
fh = open('../pyku.txt')        # 'file' object

lines = fh.readlines()          # list, ["We're out of gouda.", 'This parrot ...']
first_line = lines[0]           # str, "We're out of gouda.\n"

print(len(first_line))          # int, 19
 
Ex. 4.36 Without looping, show how many lines are in pyku.txt. 
fh = open('../pyku.txt')        # 'file' object

lines = fh.readlines()          # list, ["We're out of gouda.", 'This parrot ...']

print(len(lines))               # 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. 

for num in range(0, 6):        # int, 1 (initial value)
    print(num)
 
Ex. 4.38 Use range() to create a list.

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

numlist = list(range(3, 6))       # list, [3, 4, 5, 6]

print(numlist)
 
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']

for idx, item in enumerate(days):
    print(idx, item)
 
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