Loops

# Loops

Week 10

---

## Review

![](https://media3.giphy.com/media/v1.Y2lkPTc5MGI3NjExajdkbnBwbWVnbGVjM2ZhNGd4MHVram9jYnI0Znlwem56NzRjMTR2ciZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/MnL9Mu66fWdp7FgAuR/giphy.gif)

What is an algorithm?

A step-by-step procedure

What is the first index of a list?

0

What is wrong with this code?

```py []
my_list = 5, 8, 13
```

Missing brackets

What is wrong with this code?

```py [2]
my_list = [5, 8, 13]
print(my_list[3])
```

No index 3

---

## `while` Loops

I want to say "Hello!" four times

```py
print("Hello!")
print("Hello!")
print("Hello!")
print("Hello!")
```

What about 400 times?

```py
print_count = 0
while print_count < 4:
    print("Hello!")
    print_count += 1
```

Each time through the loop is called an **iteration**

```py [3-4]
print_count = 0
while print_count < 4:
    print("Hello!")
    print_count += 1
```

When repeating code, how do I know when to stop?

A condition

```py [2]
print_count = 0
while print_count < 4:
    print("Hello!")
    print_count += 1
```

```py [2]
while True:
    print("Hello!")
```

Infinite loop

```py []
should_keep_going = True
while should_keep_going:
    response = input("Do you quit yet?")
    if response.lower() == "yes":
        should_keep_going = False
```

What is wrong with this code?

```py []
print_count = 0
while print_count < 4:
    print("Hello!")
```

Condition never changes

The "while" loop is the most basic

Any loop can be a while loop

---

## `for` Loops

Consider this loop:

```py
groceries = ["apple", "banana", "coffee"]
index = 0
while index < len(groceries):
    item = groceries[index]
    print(f"- {item}")
```

Is there a simpler way?

```py [2-3]
groceries = ["apple", "banana", "coffee"]
for item in groceries:
    print(f"- {item}")
```

The "for" loop uses a list

One loop iteration for each list item

The condition is "when the list is done"

How many times does this loop run?

```py
for _ in []:
    print("Hello, World!")
```

0

What if I want to run a loop x times?

You can use the `range()` function

```py
for _ in range(10):
    print("Hello, World!")
```

What does the `range()` function do?

It creates a sequence of numbers

```py
numbers = range(5)
for n in numbers:
    print(n)
```

```
0
1
2
3
4
```

Technically, range does not create a list

```py
type([0, 1, 2]) # <class 'list'>
type(range(3))  # <class 'range'>
```

But we can treat it like one for loops!

A for loop is _syntactic sugar_

Any for loop can be a while loop!

```py
for _ in range(3):
    print("Hello, World!")
```

```py
i = 0
while i < 3:
    print("Hello, World!")
    i += 1
```

```py
students = ["Levi", "Nolan", "Tim", "Skyler"]
for student in students:
    print(student, "is smart")
```

```py
students = ["Levi", "Nolan", "Tim", "Skyler"]
i = 0
while i < len(students):
    print(students[i], "is smart")
    i += 1
```

```py
word = "hello"
for index in range(len(word)):
    print(f'Letter "{word[index]}" is index {index}')
```

```py
word = "hello"
index = 0
while index < len(word):
    print(f'Letter "{word[index]}" is index {index}')
    index += 1
```

When should you use each one?

- For loops are good for lists 
- While loops are for everything else

When in doubt, use "while"

This requires a while loop:

```py
should_keep_going = True
while should_keep_going:
    response = input("Do you quit yet?")
    if response.lower() == "yes":
        should_keep_going = False
```

---

## Exercise

Create a loop that computes a factorial

3! = 3 * 2 * 1 = 6

```py
user_number = int( input("Enter a number: ") )
factorial = 1

# TODO compute factorial

print("Factorial of", user_number, "is", factorial)
```

```py
user_number = int( input("Enter a number: ") )
factorial = 1

n = user_number
while n > 1:
    factorial *= n
    n -= 1

print("Factorial of", user_number, "is", factorial)
```

Now convert to "for" loop!

---

## Modules

Modules contain pre-written code we can use

Modules are just other Python files

Import a module with the `import` keyword:

```py
import random
```

Now you can use it!

```py
random.random()
random.randint(0, 100)
random.choice(["heads", "tails"])
```

---

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