Lists.md

Lists

Lists contain a collection of things that are indexed by numbers starting at zero

Instantiation

empty = []
empty = list()

numbers = [1, 2, 3, 4, 5]
string_to_list = list('something')
print(string_to_list) # ['s', 'o', 'm', 'e', ...]
''.join(['a', 's', 'd', 'f']) # returns a string joined by ''

strings = ['this', 'can', 'also', 'contain', 'strings']

my_crazy_list = ['or', 1, 'a', 2.0, 'mixture', numbers, 'of things']
print(my_crazy_list)
# ['or', 1, 'a', 2.0, 'mixture', [1, 2, 3, 4, 5], 'of things']

Accessing

Items in the list are accessed by their index. The first item in the list has an index of 0, the second has an index of 1, etc.

my_list = ['a', 'b', 'c', 'd']
my_list[0] # 'a'
my_list[2] # 'c'
len(my_list) # 4

Fun note: Strings can be accessed like lists

a = 'sample string'
len(a) # 13
a[2] # 'm'

Excercise - Palindromes

Write a program that asks the user for a string, and tells them whether or not it is a palindrome (a word, phrase or sequence that is the same backwards as forwards).

Sample output:

What string would you like me to check? abba
Your string, "abba", is a palinedrome!

What string would you like me to check? crazy kids
Your string, "crazy kids", is not a palinedrome!

Contains

How can we tell if an item is in a list? Use the in check

my_list = [1, 2, 3]
if 2 in my_list:
    print('my_list contains 2')

if 5 not in my_list:
    print('my_list does not contain 5')

Modification

numbers = []
numbers.append(1)
numbers.append(2)
numbers.append(3)
print(numbers) # [1, 2, 3]

n = numbers.pop()
print(n) # 3
print(numbers) # [1, 2]

a = [1, 2]
b = [3, 4]
c = a + b
print(c) # [1, 2, 3, 4]

Excercise - Sieve of Eratosthenes

Let's find all the prime numbers up to a given limit.

The Sieve works by using a list of numbers, and then iteratively marks out multiples until all that are left are primes.

For instance, imagine we have the list

2, 3, 4, 5, 6, 7, 8, 9, 10

We begin with the first number, 2, which we know is a prime. We then mark out all the multiples of 2 from the list.

2, 3, 4, 5, 6, 7, 8, 9, 10

Once we're finished, we move on to the next number, 3. We now know this is a prime. We now mark out multiples of 3 from the list.

2, 3, 4, 5, 6, 7, 8, 9, 10

We continue this process until we've worked through the entire list. All of the unmarked numbers are primes.

2, 3, 5, 7

The excercise is to write a program that asks the user to what number they would like to find all the primes. The program then finds them utilizing the Sieve of Eratosthenes and prints out a list for the user.

Sample output:

To what number would you like to know primes? 10
The primes through 10 are:
2, 3, 5, 7