CPSC120B
Fundamentals of Computer Science

Day 20 Notes

Mutability

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Mutability

Ints, floats, strings, and booleans are immutable
Lists are mutable, they can be changed
For example, assigning to an index in a string produces an error
```
text = "spam"
text[0] = "S" # this is an error
print(text)
```

But assigning to an index of a list is allowed

integers = [0, 1, 2]
integers[0] = -1
print(integers) # this will print [-1, 1, 2]

This can lead to confusion with list methods
Some methods mutate the list, others do not
For example, with strings the upper method does not change the text
```
text = "spam"
text.upper()
print(text) # this will print "spam"
```
So the method returns a new string, which must be used to update the variable
```
text = "spam"
text = text.upper()
print(text) # this will print "SPAM"
```

But with lists, a method can modify the lists

list = [1, 2, 3]
list.reverse()
print(list) # this will print [3, 2, 1]

If the method is used like the string upper method

list = [1, 2, 3]
list = list.reverse()
print(list) # this will print None

Unexpected things may happen
The reverse method modifies the list and does returns None
So using assignment with None overwrites the list with None
Some list methods mutate, some return something

Some list method mutate and return

list = [1, 2, 3]
print(list.pop()) # prints 3
print(list)       # prints [1, 2]

Concatenate Vs Append

In the last lab we generated a list of random numbers by doing the following
```
numbers = []
for i in range(SIZE):
    numbers += [random.randrange(SIZE)]
```
This is just like what we did with strings, create a new list by repeatedly adding on to the end
One thing to note though is that it creates a new list every time
If SIZE is large, the program is noticably slow
With strings we had to do this, but strings are immutable

Lists, however, are mutable, we can change them

numbers = []
for i in range(SIZE):
    numbers.append(random.randrange(SIZE))

This adds a new element to the end of the list, without creating a new one
This code is functionally equivalent, but it is more effiecient

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