reveal.js

# Loops

---

CS 65 // 2021-03-04

## Extra Credit Activities
- MathCS Seminar (Dr. Enes Akbuga)
    + **Topic:** Mathematics Education
    + **When:** Friday, 2:00pm to 3:00pm
    + **Where:** Zoom (email me for the link)
- [Global Citizen Forum](https://www.drake.edu/diversity/initiatives/globalcitizenforum2021/)
    + Conference on equity and inclusion in higher ed
    + March 3-5

## Extra Credit Activities
- MathCS Special Interest Group
    + Microsoft Team to help foster community between the students and faculty interested in topics related to Mathematics and Computer Science

# Assignment 5
- Will be posted later today
- It is a **group** assignment
- Your partner today is your partner for the assignment

# Questions
## ...about anything?

# `fizzbuzz`

## `fizzbuzz`

- Write a program called `fizzbuzz.py` that asks the user to input a number `num` and outputs the following.
    + 
 Outputs `"fizz"` if `num` is divisible by 3,
    + 
 Outputs `"buzz"` if `num` is divisible by 5,
    + 
 Outputs `"fizzbuzz"` if `num` is divisible by both 3 and 5, and
    + 
 Outputs `num` otherwise.

## Is `num` divisible by 3?
- How can we check this?
- 
 `if num % 3 == 0:`

## Attempt 1

```py
num = float(input("Enter a number: "))
if num % 3 == 0:
    if num % 5 == 0:
        print("fizzbuzz")
    else:
        print("fizz")
elif num % 5 == 0:
    print("buzz")
else:
    print(num)
```
- 
 This works...but what about adding a case for if `num` is divisible by 7?

## It gets terribly messy...

```py
num = float(input("Enter a number: "))
if num % 3 == 0:
    if num % 5 == 0:
        if num % 7 == 0:
            print("fizzbuzzjazz")
        else:
            print("fizzbuzz")
    elif num % 7 == 0:
        print("fizzjazz")
    else:
        print("fizz")
elif num % 5 == 0:
    if num % 7:
        print("buzzjazz")
    else:
        print("buzz")
elif num % 7 == 0:
    print("jazz")
else:
    print(num)
```

## Attempt 2

```py
num = float(input("Enter a number: "))
output = ""

if num % 3 == 0:
    output = output + "fizz"

if num % 5 == 0:
    output = output + "buzz"

if num % 7 == 0:
    output = output + "jazz"

if output != "":
    print(output)
else:
    print(num)
```

# Repetition

## Repetition
- It is often the case when we need to perform a similar operation **repeatedly**
- 
 Before today, we've seen an approach to perform repetition. What is it?
    + 
 `for` loop

## For Loop
- We know we can use `for` loops to do something a many times
    <pre><code data-trim>
    # Calls the do_something function 10 times
    for i in range(10):
        do_something()
    </code></pre>
- 
 Next week we'll see that they can do even more

## While Loops
- A **while loop** in Python can be specified in the following way
    ```py
    while boolean_expression:
        statement1
        statement2
        ...
    ```
+ It executes its body of statements repeatedly as long as the condition remains **True**

## Example
- Consider the following program

```py
print("This program is designed to cheer you up!")

keep_going = "yes"
while keep_going == "yes":
    print("YOU ARE AWESOME! YOU ARE THE BEST!")
    keep_going = input("Do you need more cheers? (yes/no): ")

print("Hope you're cheered up!")
```

## Self Check 1
- Let's take a look at the following program
    ```py
    x = 5
    while x < 100:
        print(x)
        x = 2*x - 1

print(-1)
    ```
- What does this do? What will be printed?

## Hailstone with a While Loop

```py
def hailstone(n):
    if n % 2 == 0:
        return n//2
    else:
        return 3*n + 1
```

```py
def iterate_hailstone(n):
    while n > 1:
        n = hailstone(n)
        print(n)

print(n)   # prints 1 at the end
```

## `break`
- It is possible to immediately exit a `while` loop (even if the condition still holds) by using the `break` statement
- `break` can be convenient, but it can lead to code that is difficult to read
- Whenever you are tempted to use a `break` statement, consider changing the condition of your `while` loop instead

## Self Check 2
- Consider the following programs and determine whether they are functionally equivalent:

```py
while True:
    line = input('> ')
    if line == 'done':
        break
    print(line)

print('Done!')
```

```py
line = input('> ')
while line != "done":
    print(line)
    line = input('> ')

print('Done!')
```

# Lab Time