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 = total + (number + 1)   # what's the other way to sum numbers 1 to 10? how about range(1,11)?
print(total)

Quiz 3: revert a string

Exercise: Print a difference between two lists, e.g., [1, 2, 3, 4] and [1, 2, 5].

Exercise: write a script to get the frequency of the elements in a list. You are allowed to 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)

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)

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]

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 ...]

Quiz 4: sum up squares of numbers

Exercise: Write a script to build a list of words that are shorter than n from a given list of words [‘red’, ‘green’, ‘white’, ‘black’, ‘pink’, ‘yellow’].