for loop is a way to do many operations
list a way to store many values
Unlike numpy, lits are built into the language so we don't need to load
[] creates a list



In [2]:

    
odds = [1,3,5,7]



In [3]:

    
print('odds are:',odds)









    



odds are: [1, 3, 5, 7]



In [4]:

    
print('first and last:', odds[0], odds[-1])









    



first and last: 1 7



In [5]:

    
for number in odds:
    print(number)

mutable and immutable objects

diff in strings and lists: we can change vales in a list, not string
cannot in char string



In [5]:

    
names = ['Newton', 'Darwing', 'Turing'] #typo in Darwins name
print('names is orignally: ', names)
names[1] = 'Darwin'
print('final value of names:', names)









    



names is orignally:  ['Newton', 'Darwing', 'Turing']
final value of names: ['Newton', 'Darwin', 'Turing']

works but how about with a string



In [6]:

    
name = 'Bell'
name[0] = 'b'









    



---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-220df48aeb2e> in <module>()
      1 name = 'Bell'
----> 2 name[0] = 'b'

TypeError: 'str' object does not support item assignment

mutable - data which can be changed in place
immutable - data which cannot be changed in place
strings and numbers are immutable, can only overwrite with new vals
lists and arrays are mutable: append, reorder, change elements
be careful - modifying data in place if 2 vars refer to same list and you modify list value, it will change both variables!
make copies to avoid



In [7]:

    
#nested lists
x = [['pepper', 'zucchini', 'onion'],
     ['cabbage', 'lettuce', 'garlic'],
     ['apple', 'pear', 'banana']]

visual representation of indexing nested lists



In [8]:

    
print([x[0]])









    



[['pepper', 'zucchini', 'onion']]



In [9]:

    
print(x[0])









    



['pepper', 'zucchini', 'onion']



In [10]:

    
print(x[0][1])









    



zucchini

Many ways to change the contents of lists besides adding values



In [12]:

    
odds.append(11)
print('odds after adding a value: ', odds)









    



odds after adding a value:  [1, 3, 5, 7, 11, 11]



In [15]:

    
del odds[0]
print('odds after removing the first element:', odds)









    



odds after removing the first element: [5, 7, 11, 11]



In [17]:

    
odds.reverse()
print('odds after reversing:', odds)









    



odds after reversing: [11, 11, 7, 5]

While modifying in places - good to remember python treats list in a counterintuitive way
if we make a list and copy it, then modify in place, trouble ensues



In [18]:

    
whos









    



Variable   Type    Data/Info
----------------------------
name       str     Bell
number     int     7
odds       list    n=4
x          list    n=3



In [19]:

    
odds = [1,3,5,7]
primes = odds
primes += [2]
print('primes:', primes)
print('odds:', odds)









    



primes: [1, 3, 5, 7, 2]
odds: [1, 3, 5, 7, 2]

they point to same list! Why?

python stores a list in memory and then can use multiple names to refer to the same list!
if we want a copy instead we can use the list function so we don't do this



In [20]:

    
odds = [1,3,5,7]
primes = list(odds)
primes += [2]
print('primes:', primes)
print('odds:', odds)









    



primes: [1, 3, 5, 7, 2]
odds: [1, 3, 5, 7]

List challenge:

Use a for-loop to convert the string “hello” into a list of letters:

["h", "e", "l", "l", "o"]

Hint: You can create an empty list like this:

my_list = []



In [1]:

    
my_list = []
for char in "hello":
    my_list.append(char)
print(my_list)









    



['h', 'e', 'l', 'l', 'o']



In [ ]: