Loops

For loops are very common in Python and are similar to for in other languages, but one nice twist with Python is that you can iterate over any collection, e.g., a list, a character string, etc.

for number in [2, 3, 5]:    # number is the loop variable; [...] is a collection
    print(number)          # Python uses indentation to show the body of the loop

This is equivalent to:

print(2)
print(3)
print(5)

What will this do:

for number in [2, 3, 5]:
    print(number)
print(number)
  • the loop variable could be called anything
  • the body of a loop can contain many statements
  • use range to iterate over a sequence of numbers
for i in 'hello':
    print(i)

for i in range(0,3):
    print(i)

Let’s sum numbers 1 to 10:

total = 0
for number in range(10):
    total += (number + 1)   # 1. what's the other way to sum numbers 1 to 10? how about range(1,11)?
	                        # 2. can you rewrite this entire code as a one-liner?
print(total)
Question 6.1 Write a Python code to revert a string, e.g. ‘computer’ should become ‘retupmoc’.
 
Question 6.2 Print a difference between two lists, e.g. [1, 2, 3, 4, 6, 10] and [1, 2, 5, 10].
 
Question 6.3 Write a script to get the frequency of the elements in the list a = [77, 9, 23, 67, 73, 21, 23, 9]. You can google this problem :)
 

While loops

Since we talk about loops, we should also briefly mention while loops, e.g.

x = 2
while x > 1.:
    x /= 1.1
    print(x)
Question 6.4

Remove all occurrences of a specific item in a list, e.g. in this case number 20 in the list [5, 20, 15, 20, 25, 50, 20], so it becomes [5, 15, 25, 50].

Hint: use while to check if 20 is still in the list.

 

More on lists in loops

You can also form a zip object of tuples from two lists of the same length:

for i, j in zip(a,b):
    print(i,j)

And you can create an enumerate object from a list:

for i, j in enumerate(b):    # creates a list of tuples with an iterator as the first element
    print(i,j)
Question 6.5 Write a program to add two lists index-wise, e.g. ['M', 'na', 'i', 'Stu'] and ['y', 'me', 's', 'art'] should produce a single list ['My', 'name', 'is', 'Stuart'].
 

List comprehensions

It’s a compact way to create new lists based on existing lists/collections. Let’s list squares of numbers from 1 to 10:

[x**2 for x in range(1,11)]

Of these, list only odd squares:

[x**2 for x in range(1,11) if x%2==1]

The first list in the previous section was also generated via a list comprehension:

events = [random.randint(0,2024) for i in range(10)]

You can also use list comprehensions to combine information from two or more lists:

week = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
weekend = ['Sat', 'Sun']
print([day for day in week])                         # the entire week
print([day for day in week if day not in weekend])   # only the weekdays
print([day for day in week if day in weekend])       # in both lists

The syntax is:

[something(i) for i in list1 if i [not] in list2 if i [not] in list3 ...]
Question 6.6 Remove empty strings from the list of strings, e.g. ["Mike", "", "Emma", "Kelly", "", "Brad"] should become ["Mike", "Emma", "Kelly", "Brad"].
 
Question 6.7 Write a one-line code to sum up the squares of numbers from 1 to 100.
 
Question 6.8 Write a script to build a list of words that are shorter than n characters from a given list of words ['red', 'green', 'white', 'black', 'pink', 'yellow'].
 
Question 6.9

Write a program to flatten a nested list, e.g. [[11, 21.0, 3.5], ['Mercury', 'Venus', 'Earth'], 'hello'] should become [11, 21.0, 3.5, 'Mercury', 'Venus', 'Earth', 'hello'].

Hint: try two nested loops, or maybe extend two nested loops to a nested list comprehension.

 
Question 6.10 Write a program to convert a list of multiple integers into a single integer with all their digits combined, e.g. a list [11, 33, 50] should become 113350.