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Notes/01_Introduction/05_Lists.md
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@@ -1,8 +1,12 @@
[Contents](../Contents) \| [Previous (1.4 Strings)](04_Strings) \| [Next (1.6 Files)](06_Files)
# 1.5 Lists
This section introduces lists, Python's primary type for holding an ordered collection of values.
### Creating a List
Use square brackets to define a list:
Use square brackets to define a list literal:
```python
names = [ 'Elwood', 'Jake', 'Curtis' ]
@@ -12,7 +16,7 @@ nums = [ 39, 38, 42, 65, 111]
Sometimes lists are created by other methods. For example, a string can be split into a
list using the `split()` method:
```pycon
```python
>>> line = 'GOOG,100,490.10'
>>> row = line.split(',')
>>> row
@@ -53,7 +57,7 @@ Negative indices count from the end.
names[-1] # 'Curtis'
```
You can change any item in the list.
You can change any item in a list.
```python
names[1] = 'Joliet Jake'
@@ -83,7 +87,7 @@ s * 3 # [1, 2, 3, 1, 2, 3, 1, 2, 3]
### List Iteration and Search
Iterating over the list contents.
Use `for` to iterate over the list contents.
```python
for name in names:
@@ -117,8 +121,9 @@ names.remove('Curtis')
del names[1]
```
Removing an item does not create a hole. Other items will move down to fill the space vacated.
If there are more than one occurrence of the element, `.remove()` will remove only the first occurrence.
Removing an item does not create a hole. Other items will move down
to fill the space vacated. If there are more than one occurrence of
the element, `remove()` will remove only the first occurrence.
### List Sorting
@@ -137,11 +142,17 @@ s = ['foo', 'bar', 'spam']
s.sort() # ['bar', 'foo', 'spam']
```
Use `sorted()` if you'd like to make a new list instead:
```python
t = sorted(s) # s unchanged, t holds sorted values
```
### Lists and Math
*Caution: Lists were not designed for math operations.*
```pycon
```python
>>> nums = [1, 2, 3, 4, 5]
>>> nums * 2
[1, 2, 3, 4, 5, 1, 2, 3, 4, 5]
@@ -149,7 +160,7 @@ s.sort() # ['bar', 'foo', 'spam']
[1, 2, 3, 4, 5, 10, 11, 12, 13, 14] >>>
```
Specifically, lists don't represent vectors/matrices as in MATLAB, Octave, IDL, etc.
Specifically, lists don't represent vectors/matrices as in MATLAB, Octave, R, etc.
However, there are some packages to help you with that (e.g. [numpy](https://numpy.org)).
## Exercises
@@ -157,13 +168,13 @@ However, there are some packages to help you with that (e.g. [numpy](https://num
In this exercise, we experiment with Python's list datatype. In the last section,
you worked with strings containing stock symbols.
```pycon
```python
>>> symbols = 'HPQ,AAPL,IBM,MSFT,YHOO,DOA,GOOG'
```
Split it into a list of names using the `split()` operation of strings:
```pycon
```python
>>> symlist = symbols.split(',')
```
@@ -228,7 +239,7 @@ For instance, in the above example, the last two items of `symlist` got replaced
The `for` loop works by looping over data in a sequence such as a list.
Check this out by typing the following loop and watching what happens:
```pycon
```python
>>> for s in symlist:
print('s =', s)
# Look at the output
@@ -238,7 +249,7 @@ Check this out by typing the following loop and watching what happens:
Use the `in` or `not in` operator to check if `'AIG'`,`'AA'`, and `'CAT'` are in the list of symbols.
```pycon
```python
>>> # Is 'AIG' IN the `symlist`?
True
>>> # Is 'AA' IN the `symlist`?
@@ -252,7 +263,7 @@ True
Use the `append()` method to add the symbol `'RHT'` to end of `symlist`.
```pycon
```python
>>> # append 'RHT'
>>> symlist
['HPQ', 'AAPL', 'AIG', 'MSFT', 'YHOO', 'GOOG', 'RHT']
@@ -261,7 +272,7 @@ Use the `append()` method to add the symbol `'RHT'` to end of `symlist`.
Use the `insert()` method to insert the symbol `'AA'` as the second item in the list.
```pycon
```python
>>> # Insert 'AA' as the second item in the list
>>> symlist
['HPQ', 'AA', 'AAPL', 'AIG', 'MSFT', 'YHOO', 'GOOG', 'RHT']
@@ -270,7 +281,7 @@ Use the `insert()` method to insert the symbol `'AA'` as the second item in the
Use the `remove()` method to remove `'MSFT'` from the list.
```pycon
```python
>>> # Remove 'MSFT'
>>> symlist
['HPQ', 'AA', 'AAPL', 'AIG', 'YHOO', 'GOOG', 'RHT']
@@ -281,7 +292,7 @@ Append a duplicate entry for `'YHOO'` at the end of the list.
*Note: it is perfectly fine for a list to have duplicate values.*
```pycon
```python
>>> # Append 'YHOO'
>>> symlist
['HPQ', 'AA', 'AAPL', 'AIG', 'YHOO', 'GOOG', 'RHT', 'YHOO']
@@ -290,7 +301,7 @@ Append a duplicate entry for `'YHOO'` at the end of the list.
Use the `index()` method to find the first position of `'YHOO'` in the list.
```pycon
```python
>>> # Find the first index of 'YHOO'
4
>>> symlist[4]
@@ -300,7 +311,7 @@ Use the `index()` method to find the first position of `'YHOO'` in the list.
Count how many times `'YHOO'` is in the list:
```pycon
```python
>>> symlist.count('YHOO')
2
>>>
@@ -308,7 +319,7 @@ Count how many times `'YHOO'` is in the list:
Remove the first occurrence of `'YHOO'`.
```pycon
```python
>>> # Remove first occurrence 'YHOO'
>>> symlist
['HPQ', 'AA', 'AAPL', 'AIG', 'GOOG', 'RHT', 'YHOO']
@@ -322,7 +333,7 @@ However, we'll see an elegant way to do this in section 2.
Want to sort a list? Use the `sort()` method. Try it out:
```pycon
```python
>>> symlist.sort()
>>> symlist
['AA', 'AAPL', 'AIG', 'GOOG', 'HPQ', 'RHT', 'YHOO']
@@ -331,7 +342,7 @@ Want to sort a list? Use the `sort()` method. Try it out:
Want to sort in reverse? Try this:
```pycon
```python
>>> symlist.sort(reverse=True)
>>> symlist
['YHOO', 'RHT', 'HPQ', 'GOOG', 'AIG', 'AAPL', 'AA']
@@ -345,7 +356,7 @@ Note: Sorting a list modifies its contents 'in-place'. That is, the elements of
Want to take a list of strings and join them together into one string?
Use the `join()` method of strings like this (note: this looks funny at first).
```pycon
```python
>>> a = ','.join(symlist)
>>> a
'YHOO,RHT,HPQ,GOOG,AIG,AAPL,AA'
@@ -363,7 +374,7 @@ Use the `join()` method of strings like this (note: this looks funny at first).
Lists can contain any kind of object, including other lists (e.g., nested lists).
Try this out:
```pycon
```python
>>> nums = [101, 102, 103]
>>> items = ['spam', symlist, nums]
>>> items
@@ -375,7 +386,7 @@ The first element is a string, but the other two elements are lists.
You can access items in the nested lists by using multiple indexing operations.
```pycon
```python
>>> items[0]
'spam'
>>> items[0][0]