Lists, ranges and “for” loops

Lists

Lists are a type of a container - they contain a number of other objects: numbers, strings, even other lists. A list is an ordered sequence of such objects, identified by surrounding square brackets []. List elements can be of any type, and can be of different types within the same list. Lists are mutable, i.e. once they are created, elements can be added to them, replaced or deleted.

Here are some examples of operations on lists:

Use square brackets to create a list:

mylist = [1, 'hello', 3.14]
print(mylist)

[1, ‘hello’, 3.14]

len() returns the number of elements in a list:

print(len(mylist))

3

Adding two lists makes a new list by concatenation:

list1 = [1, 2, 3]
list2 = [4, 5, 6]
list3 = list1 + list2
print(list3)

[1, 2, 3, 4, 5, 6]

Multiplying a list by an integer repeats the list:

list1 = ['a', 'b', 'c']
list2 = 3*list1
print(list2)

[‘a’, ‘b’, ‘c’, ‘a’, ‘b’, ‘c’, ‘a’, ‘b’, ‘c’]

Use empty square brackets to create an empty list:

mylist = []
print(mylist)

[]

append() adds an element to the end of a list:

mylist = ['a','b']
mylist.append('c')
print(mylist)

[‘a’, ‘b’, ‘c’]

Testing if an element is in a list:

primes = [2, 3, 5, 7, 11, 13, 17, 19]
12 in primes

False

13 in primes

True

Indexing

Individual elements of a list can be accessed using their indexes enclosed in square brackets. Indexing of list elements starts with 0:

elements = ['H','He','Li','Be','B','C']
print(elements[0])
print(elements[1])

H He

Negative indexes can be used to access elements counting from the end of the list:

print(elements[-1])
print(elements[-2])

C B

List elements can be changed by assigning a new element at a given index:

elements[0] = 'new'
print(elements)

[‘new’, ‘He’, ‘Li’, ‘Be’, ‘B’, ‘C’]

del() deletes an element from a list:

del(elements[1])
print(elements)

[‘new’, ‘Li’, ‘Be’, ‘B’, ‘C’]

Lists can be nested, that is a list can have other lists as its elements:

list1 = [1, 2, 3]
list2 = [4, 5]
list_of_lists = [list1, list2]
print(list_of_lists)

[[1, 2, 3], [4, 5]]

Using iterated indexes we can access elements of nested lists:

print(list_of_lists[1][0])

4

List slicing

List slicing produces a new list, consisting of some elements of the original list. Here are some examples.

Start at position 2, end at position 4:

letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
print(letters[2:5])

[‘c’, ‘d’, ‘e’]

Start at the beginning, end at position 3:

print(letters[:4])

[‘a’, ‘b’, ‘c’, ‘d’]

Start at position 3, end at the end of the list:

print(letters[3:])

[‘d’, ‘e’, ‘f’, ‘g’, ‘h’]

Optionally slicing can be used with three arguments. The third argument specifies by what value the index should be increased.

List slicing: start from position 1, end at position 5, increasing index by 2:

print(letters[1:6:2])

[‘b’, ‘d’, ‘f’]

Select every third element of the list:

print(letters[::3])

[‘a’, ‘d’, ‘g’]

One way to revert a list:

print(letters[::-1])

[‘h’, ‘g’, ‘f’, ‘e’, ‘d’, ‘c’, ‘b’, ‘a’]

Strings vs Lists

Slicing and indexing operations can be also applied to strings:

s = 'New York City'
print(s[0])

N

print(s[4:8])

York

print(s[::2])

NwYr iy

A difference between strings and lists is that it is not possible to append a character to a string or change a character using its index:

s[0] = 'A'

TypeError Traceback (most recent call last) <ipython-input-19-418888caed05> in <module>() —-> 1 s[0] = ‘A’ TypeError: ‘str’ object does not support item assignment

“for” loops

Python for loops provide a way to iterate (loop) over the items in a list, string, or any other iterable object, executing a block of code on each pass through the loop. The syntax is:

for <iteration variable(s)> in <iterable>:
    <code to execute inside loop>

Note:

• The for statement must be followed by a colon :.

• The block of code to execute each time through the loop starts on the next line, and must be indented.

• The block of code of the loop is finished by de-indenting back to the previous level.

Below are some examples of for loops.

Iteration over elements of a list. The iteration variable i gets bound to each item of the lists in turn:

for i in [2, 4, 6]:
    print(10*i)

20 40 60

Iteration over characters in a string. The iteration variable letter gets bound to each string character in turn:

for letter in 'hello':
    if letter != 'l':
        print(letter)

h e o

Printing vowels in word:

vowels = ['a', 'e', 'i', 'o', 'u']
for letter in 'Buffalo':
    if letter in vowels:
        print(letter)

u a o

Computing the sum of squares of numbers from 0 to 4:

total = 0
mylist = [0,1,2,3,4]
for x in mylist:
    total += x**2
print(total)

30

break can be used to terminate a loop early:

for letter in 'Buffalo':
    if letter == 'a':
        break
    else:
        print(letter)

B u f f

Range

As the examples above illustrate in many situations we may need to iterate a for loop over a sequence of consecutive integers. It would be inconvenient having to manually type a list of integers to code such iteration. Instead we can use the range function that generates integers in a given range.

range(n) generates consecutive integers, starting at 0 and ending at n-1:

for x in range(5):
    print(x)

0 1 2 3 4

range(m,n) creates a list of consecutive integers, from m to n-1:

for x in range(5,10):
    print(x)

5 6 7 8 9

The third argument to range() specifies the increment from one integer to the next:

odds = range(1,10,2)
for x in odds:
    print(x)

1 3 5 7 9

range() can also count backwards using a negative increment:

countdown = range(5,0,-1)
for x in countdown:
    print(x)

5 4 3 2 1

Note. In Python 3 the range function does not produce a list of integers, but instead generates integers one by one as needed. This saves computer memory and can speed up programs. In cases when we want to get an actual list of integers we can do it by using the list function:

odds = range(1,10,2)
odds_list = list(odds)
print(odds_list)

[1, 3, 5, 7, 9]