# Homework 1: Functions, Control

*Due by 11:59pm on Thursday, September 1*

## Instructions

Download hw01.zip.

**Submission:** When you are done, submit with ```
python3 ok
--submit
```

. You may submit more than once before the deadline; only the
final submission will be scored. Check that you have successfully submitted
your code on okpy.org. See Lab 0 for more instructions on
submitting assignments.

**Using Ok:** If you have any questions about using Ok, please
refer to this guide.

**Readings:** You might find the following references
useful:

Important:The lecture on Monday 8/29 will cover readings 1.3-1.5, which contain the material required for question 4. (Control)

**Grading:** Homework is graded based on
correctness. Each incorrect problem will decrease the total score by one point. There is a homework recovery policy as stated in the syllabus.
**This homework is out of 2 points.**

## Getting Started Videos

These videos may provide some helpful direction for tackling the problems on this assignment.

To see these videos, you should be logged into your berkeley.edu email.

# Required Questions

### Q1: A Plus Abs B

Python's `operator`

module defines *binary functions* for Python's intrinsic arithmetic operators. For example, calling `operator.add(2,3)`

is equivalent to calling the expression `2 + 3`

; both will return `5`

.

Note that when the

`operator`

module is imported into the namespace, like at the top of`hw01.py`

, you can just call`add(2,3)`

instead of`operator.add(2,3)`

.

Fill in the blanks in the following function for adding `a`

to the
absolute value of `b`

, without calling `abs`

. You may **not** modify any
of the provided code other than the two blanks.

```
def a_plus_abs_b(a, b):
"""Return a+abs(b), but without calling abs.
>>> a_plus_abs_b(2, 3)
5
>>> a_plus_abs_b(2, -3)
5
>>> a_plus_abs_b(-1, 4)
3
>>> a_plus_abs_b(-1, -4)
3
"""
if b < 0:
f = _____
else:
f = _____
return f(a, b)
```

Use Ok to test your code:

`python3 ok -q a_plus_abs_b`

### Q2: Two of Three

Write a function that takes three *positive* numbers as arguments and returns
the sum of the squares of the two smallest numbers. **Use only a single line
for the body of the function.**

```
def two_of_three(i, j, k):
"""Return m*m + n*n, where m and n are the two smallest members of the
positive numbers i, j, and k.
>>> two_of_three(1, 2, 3)
5
>>> two_of_three(5, 3, 1)
10
>>> two_of_three(10, 2, 8)
68
>>> two_of_three(5, 5, 5)
50
"""
return _____
```

Hint:Consider using the`max`

or`min`

function:`>>> max(1, 2, 3) 3 >>> min(-1, -2, -3) -3`

Use Ok to test your code:

`python3 ok -q two_of_three`

### Q3: Largest Factor

Write a function that takes an integer `n`

that is **greater than 1** and
returns the largest integer that is smaller than `n`

and evenly divides `n`

.

```
def largest_factor(n):
"""Return the largest factor of n that is smaller than n.
>>> largest_factor(15) # factors are 1, 3, 5
5
>>> largest_factor(80) # factors are 1, 2, 4, 5, 8, 10, 16, 20, 40
40
>>> largest_factor(13) # factor is 1 since 13 is prime
1
"""
"*** YOUR CODE HERE ***"
```

Hint:To check if`b`

evenly divides`a`

, you can use the expression`a % b == 0`

, which can be read as, "the remainder of dividing`a`

by`b`

is 0."

Use Ok to test your code:

`python3 ok -q largest_factor`

### Q4: Hailstone

Douglas Hofstadter's Pulitzer-prize-winning book, *Gödel, Escher, Bach*, poses
the following mathematical puzzle.

- Pick a positive integer
`n`

as the start. - If
`n`

is even, divide it by 2. - If
`n`

is odd, multiply it by 3 and add 1. - Continue this process until
`n`

is 1.

The number `n`

will travel up and down but eventually end at 1 (at least for
all numbers that have ever been tried -- nobody has ever proved that the
sequence will terminate). Analogously, a hailstone travels up and down in the
atmosphere before eventually landing on earth.

This sequence of values of `n`

is often called a Hailstone sequence. Write a
function that takes a single argument with formal parameter name `n`

, prints
out the hailstone sequence starting at `n`

, and returns the number of steps in
the sequence:

```
def hailstone(n):
"""Print the hailstone sequence starting at n and return its
length.
>>> a = hailstone(10)
10
5
16
8
4
2
1
>>> a
7
>>> b = hailstone(1)
1
>>> b
1
"""
"*** YOUR CODE HERE ***"
```

Hailstone sequences can get quite long! Try 27. What's the longest you can find?

Note that if

`n == 1`

initially, then the sequence is one step long.

Hint:Recall the different outputs from using regular division`/`

and floor division`//`

Use Ok to test your code:

`python3 ok -q hailstone`

**Curious about hailstones or hailstone sequences? Take a look at these articles:**

- Check out this article to learn more about how hailstones work!
- In 2019, there was a major development in understanding how the hailstone conjecture works for most numbers!

## Submit

Make sure to submit this assignment by running:

`python3 ok --submit`

If you completed all problems correctly, you should see that your score is 6.0 in terminal. Each homework assignment counts for 2 points, so in this case you will receive the full 2 points for homework. Remember that every incorrect question costs you 1 point, so a 5.0/6.0 on this assignment will translate to a 1.0/2.0 homework grade for this assignment.