Python Control Structures Exercises

Programming for Data Science Bootcamp

Exercise 1

What prints in the example below if val = 5?

val = -2

if -10 < val < -5:
    print('bucket 1')
elif -5 <= val < -2:
    print('bucket 2')
elif val == -2:
    print('bucket 3')

val = 5

if -10 < val < -5:
    print('bucket 1')
elif -5 <= val < -2:
    print('bucket   2')
elif val == -2:
    print('bucket 3')

How would you fix the problem?

val = 5

if -10 < val < -5:
    print('bucket 1')
elif -5 <= val < -2:
    print('bucket 2')
elif val == -2:
    print('bucket 3')
else:
    print('bucket 4')

bucket 4

Exercise 2

Write code that does the following:

• Defines a list of integers xx = [-2, 8, 0, 5, 6]
• Computes the maximum, storing in max_val
• Loops over each value in the list
• If the value is less than the maximum, print the value
• Otherwise, print the string "max_val"

Method 1

xx = [-2, 8, 0, 5, 6]

max_val = max(xx)

for x in xx:
    if x < max_val:
        print(x)
    else:
        print('max_val')

-2
max_val
0
5
6

Method 2

for x in xx:
    print(x) if x < max_val else print('max_val')

-2
max_val
0
5
6

Method 3

Jumping ahead to next week (Lambda λ functions)

_ = [(lambda x: print(x) if x < max_val else print('max_val'))(x) for x in xx]

-2
max_val
0
5
6

Exercise 3

Write code that does the following:

• Sets a variable it to $0$
• Sets a variable max_iter to $100$
• While it < max_iter, does the following:
  • If it equals $0$ or it is divisible by $10$, print it. Hint: use modulo operator %
• Increase it by $1$
• When it >= max_iter, the program should quit

it = 0
max_iter = 100
while it < max_iter:
    if (it == 0) or (it % 10 == 0):
        print(it)
    it += 1

0
10
20
30
40
50
60
70
80
90

Exercise 4

Create a list of four strings that are capitalized proper nouns.

Write a for loop to iterate over the strings and:

• lowercase the string
• print the string

hint: Use the string method .lower().

names = ['John', 'Paul', 'George', 'Ringo']

for name in names:
    print(name.lower())

john
paul
george
ringo

_ = [print(x.lower()) for x in names]

john
paul
george
ringo

Exercise 5

Using the list you made from the previous exercise, use iter() and next() to iterate over the list, printing each string.

The strings don't need to be lowercased.

beatles = iter(names)
print(next(beatles))
print(next(beatles))
print(next(beatles))
print(next(beatles))

John
Paul
George
Ringo

for name in iter(beatles):
    print(name)

beatles = iter(names)
_ = [print(next(beatles)) for i in range(len(names))]

John
Paul
George
Ringo

Exercise 6

Create a dictionary with two city names as keys and their zip codes as values.

Use a for loop with items() to print each key-value pair.

city_zip = {
    'Santa Barbara': 93103, 
    'Charlottesville': 22903
}

for city, zipcode in city_zip.items():
    print(city, zipcode)

Santa Barbara 93103
Charlottesville 22903

_ = [print(city, zipcode) for city, zipcode in city_zip.items()]

Santa Barbara 93103
Charlottesville 22903

print("\n".join([f"{city} {zipcode}" for city, zipcode in city_zip.items()]))

Santa Barbara 93103
Charlottesville 22903

Exercise 7

Using the dictionary from the previous exercise, use a for loop with keys() to print each key-value pair.

To extract the values, use the key to index into the dictionary value.

city_zip = {
    'Santa Barbara':93103, 
    'Charlottesville':22903
}

# for cit in city_zip.keys(): 
for city in city_zip:
    print(city, city_zip[city])

Santa Barbara 93103
Charlottesville 22903

Exercise 8

Write a list comprehension that takes a list of numeric values
and returns a list of their cubes.

vals = [1, 2, 3, 4]
cubes = [val**3 for val in vals]
cubes 

[1, 8, 27, 64]

Exercise 9

Create a list of seven $(7)$ strings that includes some numbers and some non-numbers.

The numbers should be quoted, like '12'.

Write a list comprehension that takes the list and returns a list of the strings that are numbers.

Hint: The function isdigit() will help.

strs = ['99', 'red balloons', 'floating', 'in the', 'summer sky.', '16', 'candles']
num_list = [st for st in strs if st.isdigit()]
num_list

['99', '16']

Without the function ...

numbers = list("1234567890")
num_list = []

for str in strs:
    
    # Innocent until proven guilty
    is_num = True 
    
    # Trial
    for char in str:
        if char not in numbers:
            is_num = False
            break
            
    # Verdict
    if is_num:
        num_list.append(str)

print(num_list)

['99', '16']

Exercise 10

Given the dictionary grid, use a comprehension to retain only key-value pairs where the key contains the substring 'max':

grid = {
    'max_depth': [5,10], 
    'ntrees': [100,200,300],
    'regularization': [11,12],
    'max_iter': [10,20]
}

grid = {
    'max_depth': [5,10], 
    'ntrees': [100,200,300],
    'regularization': [11, 12],
    'max_iter': [10,20]
}

grid_max = {key : grid[key] for key in grid.keys() if 'max' in key}

grid_max

{'max_depth': [5, 10], 'max_iter': [10, 20]}

Now create a dictionary with the same four keys but only the maximum values of each associated list.

new_grid = {}
for key in grid:
    new_grid[key] = max(grid[key]) 

new_grid

{'max_depth': 10, 'ntrees': 300, 'regularization': 12, 'max_iter': 20}

new_grid = {key:max(grid[key]) for key in grid}

new_grid

{'max_depth': 10, 'ntrees': 300, 'regularization': 12, 'max_iter': 20}

Exercise 11

Given the dictionary letter_to_idx which maps some letters to index values,
use a dictionary comprehension to create a reversed dictionary, idx_to_letter mapping the index values to the letters.

letter_to_idx = {
    'a': 0, 
    'b': 1, 
    'c': 2
}

letter_to_idx = {
    'a': 0, 
    'b': 1, 
    'c': 2
}

idx_to_letter = {letter_to_idx[k]:k for k in letter_to_idx.keys()}  

idx_to_letter

{0: 'a', 1: 'b', 2: 'c'}

Or try this:

idx_to_letter2 = {v:k for k, v in letter_to_idx.items()}

idx_to_letter2

{0: 'a', 1: 'b', 2: 'c'}