Lists & Algorithms

# Lists & Algorithms

Week 9

---

## Review

![](https://media1.giphy.com/media/v1.Y2lkPTc5MGI3NjExNmwzbDMwb2ptdmd6cndwMGsxejl6OTNxZXh0ZmJvODU1MjQ5cWM3aCZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/3V33ssIjg0BUGafaU0/giphy.gif)

What happens to RAM when the power goes off?

It is forgotten

What does an operating system do?

It lets programs use the hardware

What are some roles of the operating system?

Users, files, multitasking, drivers, VRAM

Which of these is a Windows path?

`C:\Users\Rylan`

`/Users/Rylan`

`C:\Users\Rylan`

---

## Lists

A variable holds **one** value

```py
score = 95
```

What if you need to keep track of dozens of numbers?

Use a list

A **list** holds values in order

```py
scores = [85, 92, 78]
```

NOTES: Week 2 showed `[1, 2, 3]` and called it a compound type. Square brackets, commas between the values.

Each spot has an **index**

Indexes start at **0**

| Index | Score |
| --- | --- |
| 0 | 85 |
| 1 | 92 |
| 2 | 78 |

NOTES: The index is how far the item is from the start. The first item is zero steps away.

```py
scores = [85, 92, 78]

print(scores[0])
print(scores[2])
```

```
85
78
```

`len` is how many items are in the list

```py
print(len(scores))
```

```
3
```

NOTES: `len` is short for length.

The last index is one less than the length

0, 1, 2, 3, 4

That is 5 items

```py
print(scores[5])
```

`IndexError: list index out of range`

NOTES: There is no index 5. The last score is at index 4. You can also write `scores[-1]` for the last item. Counting backward is not needed for the project.

You can replace one item

```py
scores = [2, 3, 5, 8]
scores[0] = 100
print(scores)
```

```
[100, 3, 5, 8]
```

A list is a **data structure**

A way of organizing values

NOTES: An array is the same idea. Python calls it a list.

A string has similar operations

A string is **NOT** a list

```py [2]
message = "Hello, World!"
print("First character is", message[0])
```

```
H
```

```py [2]
message = "Hello, World!"
print("Length is", len(message))
```

```
5
```

```py [2]
message = "Hello, World!"
message[len(message) - 1] = "?"
print(message)
```

```
Hello, World?
```

---

## Loops

NOTE: we will cover loops in detail next week

Printing every score by hand

```py
print(scores[0])
print(scores[1])
print(scores[2])
print(scores[3])
print(scores[4])
```

That breaks when the list changes length

A **for loop** runs once for each item

```py
for score in scores:
    print(score)
```

NOTES: `score` is 85, then 92, then 78, then 95, then 88. Same colon and indent rules as `if` and functions.

What is wrong with this code?

```py
for score in scores
    print(score)
```

Missing `:`

What is wrong with this code?

```py
for score in scores:
print(score)
```

The loop body must be indented

What does this print?

```py
scores = [85, 92, 78]

for score in scores:
    print(score)
```

```
85
92
78
```

---

## Algorithms

An **algorithm** is a step-by-step procedure

The list **stores** the scores

The algorithm **finds** an answer

NOTES: A recipe. The data structure is the ingredients. The algorithm is the steps.

Given a stack of bills, how do you count the total?

```py
[1, 1, 2, 5, 10, 10, 50]
```

Find the sum

1. Keep track of total, starting at 0 
2. Look at every number 
3. Add number to total

```py [2-4]
bills = [1, 1, 2, 5, 10, 10, 50]
total = 0
for bill in bills:
    total += bill
print(total)
```

Find the highest number in a list

1. Start with the first number 
2. Look at every number 
3. If this one is higher, remember it instead

```py [2-5]
scores = [85, 92, 78, 95, 88]
highest = scores[0]
for score in scores:
    if score > highest:
        highest = score
print(highest)
```

`scores = [85, 92, 78, 95, 88]`

| Score | Highest so far |
| --- | --- |
| 85 | <span class="fragment">85</span> |
| 92 | <span class="fragment">92</span> |
| 78 | <span class="fragment">92</span> |
| 95 | <span class="fragment">95</span> |
| 88 | <span class="fragment">95</span> |

---

## Exercise

**Vowel Counter**

- Ask use for a message
- Print the number of vowels in that message

For example, `"mississippi"` would be 4

```py
def count_vowels(message):
    vowels = ['a', 'e', 'i', 'o', 'u']
    total = 0
    for letter in message:
        if letter in vowels:
            total += 1
    return total

message = input("Enter a message: ")
vowel_count = count_vowels(message.lower())
print(f'There are {vowel_count} vowels in "{message}"')
```

---

![Jimmy Fallon thats it](https://media4.giphy.com/media/v1.Y2lkPTc5MGI3NjExd2NmamM3NmR2c3RtenR3bGV5dnR1MnNyM3BzNnh3ZGNwYTFyZ2V6YyZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/uSZQ6WPIcpouUEpNTe/giphy.gif)