Introduction to Python

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In-Class Exercise Solutions, Session 3



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.)
 

STRINGS
 
Ex. 3.1 .rstrip() a string of a charactrer or whitespace. Strip the sentence of the period; strip the csv line of the newline character. Print each. 
sentence = 'Hello  there   treasured  Python friends.'
csv_line = 'California,31,39.51,3200\n'

stripsen = sentence.rstrip('.')
stripcsv = csv_line.rstrip()

print(stripsen)
print(stripcsv)
 
Ex. 3.2 .split() a string on a character or on whitespace. Split the sentence into words; split the CSV line into fields. Print each. 
sentence = 'Hello  there   treasured  Python friends'
csv_line = 'California,31,39.51,3200'

words = sentence.split()
fields = csv_line.split(',')

print(words)
print(fields)
 
Ex. 3.3 Slice a string. Slice the 4-digit year, 2-digit month and 2-digit day of 'fw_line'. 
fw_line = 'DATE20150903'

year  = fw_line[4:8]
month = fw_line[8:10]
day   = fw_line[10:12]
 
Ex. 3.4 Use variables to slice the 4-digit year of 'fw_line'. 
fw_line = 'DATE20150903'

start_index = 4
end_index = 8

year = fw_line[start_index:end_index]
 

LISTS
 
Ex. 3.5 List operations.

Perform the noted operations on list 'x' 

x = ['hello', 'there', 'Python', 'friends']   # list, ['hello', ... ]


# print the type of the list (use type())
print(type(x))                                # <class 'list'>


# access and print the first item of the list (use list subscript)
print(x[0])                                   # hello


# access and print the last item of the list (use negative subscript)
print(x[-1])                                  # friends


# use a variable index to print the last item
last_idx = -1
print(x[last_idx])


# show the length of the list (use len())
print(len(x))                                 # 4


print()
 

LAB 1
 
Ex. 3.6 Print item from a delimited string.

Select and print the 3rd value. Do not use a slice.

 
delimited = 'New York,31,39.51,3200'  # str, 'New York,31...'

items = delimited.split(',')          # list, ['New York', '31' ... ]
fval = items[2]                       # str, '39.51'
print(fval)
 
Ex. 3.7 Select number from a delimited string, use as number.

Select the 2nd value, then double it to 62. Do not use a slice.

 
delimited = 'New York,31,39.51,3200'  # str, 'New York,31,39...'

items = delimited.split(',')          # list, ['New York', '31' ... ]
ival = items[1]                       # str, '31'
iival = int(ival)                     # int, 31
print(iival * 2)                      # 62
 
Ex. 3.8 Print a word from a space-delimited string.

Print the 4th word from dickens. Then print the last word with the trailing period stripped. Do not use a slice. 

dickens = 'It was  the best of times, it was the worst of times.'#

words = dickens.split()        # list, ['It', 'was', 'the', 'best', 'of', 'times,' ... ]

bw = words[3]                  # str, 'best'
tw = words[5]                  # str, 'times,'
stw = tw.rstrip(',')           # str, 'times'

print(bw)
print(stw)
 
Ex. 3.9 Determine number of words in a string.

In just two additional statements (not including printing), show the number of words in the string. 

swanns_way = 'For a long time I  used to go to bed early.'  # str, 'For a ...'

words = swanns_way.split()   # list, ['For', 'a', 'long', 'time' ...]
print(len(words))
 
Ex. 3.10 Show selected words from a string.

Print the 1st word in the setence, the 5th word in the sentence, and the last word in the sentence. 

gatsby = "In my  younger and more vulnerable years my father gave me some advice..."   #  str

words = gatsby.split()     # list, ['In', 'my', 'younger', 'and' ...]
print(words[0])            # str, 'In'
print(words[4])            # str, 'more'
print(words[-1])           # str, 'advice...'
 
Ex. 3.11 Slice part of a string.

Print the numeric SKU value (the 9 digits after SKU only, not the 8) from serial. 

serial = '#SKU000095327MRCOFFEE8CUPCAPUC'

sn = serial[4:13]        # str, '000095327'
print(sn)
 

LOOPING
 
Ex. 3.12 Loop through and print each individual item in the list (use 'for') 
x = ['What', 'do', 'you', 'think?']

for item in x:                        # str, 'hello' (initial value)
    print(item)
 
Ex. 3.13 List iteration and summary.

Use 'for' to loop through list 'xval' and count the number of iterations, then extend it to also sum up the values. Print the sum and the count. 

xval = [3.9, 0.3, 2.1, 0.03]        # list, [3.9, 0.3, 2.1, 0.03]

lcount = 0                          # int, 0
lsum = 0.0                          # float, 0.0

for val in xval:                    # int, 3.9 (initial value)
    lcount = lcount + 1             # int, 1
    lsum = lsum + val               # float, 3.9

print(lcount)
print(lsum)
 

FILES
 
Ex. 3.14 File operations.

Perform the below operations: 

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


# open file 'pyku.txt' and assign to variable 'fh' (use open())
fh = open(filename)             # 'file' object


# print the type of variable 'fh'
print(type(fh))                 # <class '_io.TextIOWrapper'>


print()


# iterate over the variable 'fh' with 'for' and print each line in the file
for line in fh:         # str, "We're out of gouda.\n" (initial value)
    print(line)
 
Ex. 3.15 Using 'with' context to open files.

Again open pyku.txt for reading, and loop through and print each line, bu t use a 'with' block to open the file. 

filename = '../pyku.txt'

with open(filename) as fh:
    for line in fh:
        print(line)
 

LAB 2
 
Ex. 3.16 Loop through a list. Loop through and print each individual item in the list (use 'for') 
mylist = ['a', 'b', 'c', 'd']
Expected Output: 
a
b
c
d
 
Ex. 3.17 Loop through a file and print each line.

Loop through the file pyku.txt and print each line as you go. 

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

fh = open(filename)             # 'file' object

for line in fh:         # str, "We're out of gouda.\n" (initial value)
    print(line)
 
Ex. 3.18 Loop through a file and strip each line.

Loop through the file pyku.txt and strip and print each line as you go. You should see the lines printed without a blank line in between. 

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

fh = open(filename)             # 'file' object

for line in fh:                 # str, "We're out of gouda.\n" (initial value)
    line = line.rstrip()        # str, "We're out of gouda."
    print(line)
 
Ex. 3.19 Loop through file and count.

Loop through the file pyku.txt and keep a running count of each line. Print the count at the end. 

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

fh = open(filename)              # 'file' object

counter = 0                      # int, 0

for line in fh:                  # str, "We're out of gouda.\n" (initial value)
    counter = counter + 1        # int, 1

print(counter)

The call to .rstrip() is not necessary because we're not using the end of the line.
 
Ex. 3.20 Loop through file and print one field.

Loop through datafile.csv and print just the first field from each line. 

filename = '../datafile.csv'   # str, '../datafile.csv'

fh = open(filename)            # 'file' object

for line in fh:                # str, 'Alpha,this,that,3,3.3\n' (initial value)
    items = line.split(',')    # list, ['Alpha', 'this', 'that' ...]
    print(items[0])            # Alpha

The call to .rstrip() is not necessary because we're not using the end of the line.
 
Ex. 3.21 Loop through file and print one slice.

Print the month and day from each line in FF_tiny.txt 

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

fh = open(filename)           # 'file' object

for line in fh:               # str, '19260701    0.09    0.22 ... '
    print(line[4:8])          # 0701

The call to .rstrip() is not necessary because we're not using the end of the line.
 
Ex. 3.22 Loop through file, isolate and double numeric value.

Looping through datafile.csv, isolate the int value (4th field) and double it, printing each doubled value as you loop. 

fname = '../datafile.csv'      # str, '../datafile.csv'

fh = open(fname)               # 'file' object

for line in fh:                # str, 'Alpha,this,that,3,3.3' (sample value)
    items = line.split(',')    # list, ['Alpha', 'this', 'that', '3' ...]
    ival = int(items[3])       # int, 3
    print(ival * 2)            # 6

The call to .rstrip() is not necessary because we're not using the end of the line.
 
Ex. 3.23 Loop through file, isolate values and add together.

Looping through datafile.csv, isolate the int value and float value (4th and 5th fields) and add them together, printing each summed value as you loop. 

name = '../datafile.csv'             # str, '../datafile.csv'

fh = open(name)                      # 'file' object

for line in fh:                      # str, 'Alpha,this,that,3,3.3\n'
    line = line.rstrip()             # str, 'Alpha,this,that,3,3.3'
    items = line.split(',')          # list, ['Alpha', 'this' ... ]
    ival = items[3]                  # str, '3'
    fval = items[4]                  # str, '3.3'
    sval = int(ival) + float(fval)   # float, 6.3
    print(sval)
 
Ex. 3.24 Loop through file and sum up values in column.

Looping through datafile.csv, sum up the last column of float values. Print the sum at the end. 

fname = '../datafile.csv'          # str, '../datafile.csv'

fh = open(fname)                   # 'file' object

fsum = 0.0                         # float, 0.0

for line in fh:                    # str, 'Alpha,this,that,3,3.3\n'
    line = line.rstrip()           # str, 'Alpha,this,that,3,3.3'
    items = line.split(',')        # list, ['Alpha', 'this' ... ]
    fval = float(items[-1])        # str, '3.3'
    fsum = fsum + fval             # float, 3.3 (initial value)

print(fsum)
 
Ex. 3.25 Loop through file and selectively print using split().

Looping through revenue.csv, print the company names for rows with 'NY' as the state. 

filename = '../revenue.csv'    # str, '../revenue.csv'

fh = open(filename)            # 'file' object

for line in fh:                # str, "Haddad's,PA,239.50\n"
    items = line.split(',')    # list, ["Haddad's", 'PA', '239.50\n']
    company = items[0]         # str, "Haddad's"
    state = items[1]           # str, 'PA'
    if state == 'NY':          # bool, False (initial value)
        print(company)
 
Ex. 3.26 Loop through file and selectively print using a slice.

Looping through FF_tiny.txt, print the last float value on each line for the year 1927. 

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

fh = open(filename)            # 'file' object

for line in fh:                # str, '19260701    0.09    0.22    0.30   0.009\n'
    line = line.rstrip()       # str, '19260701    0.09    0.22    0.30   0.009'
    if line[0:4] == '1927':    # bool, False (initial value)
        items = line.split()   # list, ['19270103', '0.97', '0.21' ... ]
        print(items[-1])       # '0.015'

.rstrip() is required here because we're using the last value on the line.
 
Ex. 3.27 Loop through file and sum up selected value.

Looping through revenue.csv, sum up the float value (3rd field) only for those rows from NJ (2nd field). 

fname = '../revenue.csv'         #  str, '../revenue.csv'

fh = open(fname)                 # 'file' object

fsum = 0.0                       # float, 0.0

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'
    if state == 'NJ':            # bool, False (initial value)
        fval = float(items[2])   # float, 239.5
        fsum = fsum + fval       # float, 239.5

print(fsum)
 

SUMMARY ALGORITHM; BUILDING FROM FILE
 
Ex. 3.28 File line iteration and summary.

Use 'for' to loop through file 'revenue.csv' and count the number of iterations, then perform the operations noted below: test each step as you proceed. 

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

counter = 0                        # int, 0
summer = 0.0                       # float, 0.0

# 1. loop through file line-by-line; strip and print each line
for line in fh:                    # str, "Haddad's,PA,239.50\n" (initial value)
    line = line.rstrip()           # str, "Haddad's,PA,239.50"
    print(line)

    # 2. count each iteration; print count at end
    counter = counter + 1          # int, 1

    # 3. split each line on a comma; print each split line
    items = line.split(',')        # list, ["Haddad's", 'PA', '239.50']
    print(items)

    # 4. subscript float value from split list; print each subscript
    fval = items[2]                # str, '239.50'
    print(fval)

    # 5. convert float value (a string) to float
    ffval = float(fval)            # float, 239.5

    # 6. sum up float values; print sum after loop ends
    summer = summer + ffval        # float, 239.5

    # adding a horizontal bar to separate iterations
    print('------------------------------')


# close file
fh.close()


# print sum and count
print(counter)                      # 7
print(summer)                       # 662.00000000000001
 
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