CS 2120: Topic 8¶

Videos for this week:¶

We can concatenate lists with the + operator:

>>> a=[5,7,9,10]
>>> b=['also','a','list']

>>> a+b
[5, 7, 9, 10, 'also', 'a', 'list']

We can concatenate a list with itself, multiple times, using the * operator:
```
>>> a*3
[5, 7, 9, 10, 5, 7, 9, 10, 5, 7, 9, 10]
```
We can slice lists:
```
>>> a[0:2]
[5,7]
>>> a[1:3]
[7,9]
```
We can find a list’s length:
```
>>> len(a)
4
```

We can check if something is in a list:

>>> if 5 in a:
        print("yes, 5 is in a!!!")
yes, 5 is in a!!!

We can go through each list entry:

>>> for i in a:
      print(i)
5
7
9
10

We can create sequential lists with “range()”

>>> list(range(1,5))
      [1, 2, 3, 4]

>>> list(range(5,10))
      [5, 6, 7, 8, 9]

Strings are kind of like a “list of characters”.. but recall that strings are immutable. What about lists?

Let’s try:

>>> a=[5,7,9,10]
>>> print(a)
[5, 7, 9, 10]
>>> a[2]='I changed!'
>>> print(a)
[5, 7, 'I changed!', 10]

A cleaner way to delete an element from a list is with the del statement:

>>> a=[5,7,9,10]
>>> a
[5, 7, 9, 10]
>>> del a[2]
>>> a
[5, 7, 10]

What is this code doing?

>>> a=[1,2,3,4]
>>> print(a)
[1, 2, 3, 4]
>>> b=a
>>> b[2]='Z'
>>> print(a)
[1, 2, 'Z', 4]

You might imagine that b is a separate copy of a, so why does "Z" appear when you print a?
The code b=a creates an Alias for a called b; in other words, we are saying that "b" is another name for "a".

Warning

In Python, when you “assign” a list, you are not copying the list. You are saying ‘this is another name for the exact same list’.

If you want to copy your list, use slicing: b = a[:]. Slicing always creates a new list.

>>> a=[1,2,3,4]
>>> print(a)
[1, 2, 3, 4]
>>> b=a[:]
>>> b[2]='Z'
>>> print(a)
[1, 2, 3, 4]

Often we’ll want to print out the index with each list entry:

>>> list=['a','b','c','d']

>>> for i in range(len(list)):
      print(i, list[i])

0 a
1 b
2 c
3 d

This pattern is so common that Python has given us a built in function enumerate to do this:
```
for i in enumerate(list):
        print i[0], i[1]
```
i[0] is the index of the item in the list
i[1] is the actual item itself

When you pass a list as an argument in a function, that function is called a modifier and the changes it makes are called side effects
The function is not changing a copy of the list; rather, it is modifying the original (the list argument is an alias)

>>> def double(list):
      """ Double each element of list """
      for i in range(len(list)):
          list[i] = 2 * list[i]

>>> myList = [2, 5, 9]
>>> print(myList)
[2, 5, 9]
>>> double(myList)
>>> print(myList)
[4, 10, 18]

>>> listA = [a, b, c, d]
>>> listB = [1, 3, "horse"]
>>> listA.append(listB)
>>> print(listA)
[a, b, c, d, [1, 3, "horse"]]