#
Lab 6: Recursion

*Due at 11:59:59 pm on 10/12/2021.*

## Starter Files

Download lab06.zip. Inside the archive, you will find starter files for the questions in this lab, along with a copy of the OK autograder.

## Submission

By the end of this lab, you should have submitted the lab with `python3 ok --submit`

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

- To receive full credit for this lab, all questions must be attempted.

When you are ready to submit, run `ok`

with the `--submit`

option:

`python3 ok --submit`

After submitting, `ok`

will display a submission URL, with which you can view your submission on okpy.org.

## Recursion

### Question 1: Sum

Using recursion, write a function `sum`

that takes a single argument `n`

and computes the sum of all integers between 0 and `n`

inclusive. Do not write this function using a while
or for loop.
Assume `n`

is non-negative.

```
def sum(n):
"""Using recursion, computes the sum of all integers between 1 and n, inclusive.
Assume n is positive.
>>> sum(1)
1
>>> sum(5) # 1 + 2 + 3 + 4 + 5
15
"""
"*** YOUR CODE HERE ***"
if n == 1:
return 1
return n + sum(n - 1)
```

Use OK to test your code:

`python3 ok -q sum`

### Question 2: Has Seven

Write a function `has_seven`

that takes a positive integer `n`

and
returns whether `n`

contains the digit 7. *Do not use any assignment
statements - use recursion instead*:

```
def has_seven(k):
"""Returns True if at least one of the digits of k is a 7, False otherwise.
>>> has_seven(3)
False
>>> has_seven(7)
True
>>> has_seven(2734)
True
>>> has_seven(2634)
False
>>> has_seven(734)
True
>>> has_seven(7777)
True
"""
"*** YOUR CODE HERE ***"
if k % 10 == 7:
return True
elif k < 10:
return False
else:
return has_seven(k // 10)
```

Use OK to test your code:

`python3 ok -q has_seven`

### Question 3: Filter

Write the recursive version of the function `filter`

which takes

- filter - a one-argument function that returns True if the argument passed in should be included in the list, and False otherwise.
- s - a sequence of values

```
def filter(f, seq):
"""Filter a sequence to only contain values allowed by filter.
>>> def is_even(x):
... return x % 2 == 0
>>> def divisible_by5(x):
... return x % 5 == 0
>>> filter(is_even, [1,2,3,4])
[2, 4]
>>> filter(divisible_by5, [1, 4, 9, 16, 25, 100])
[25, 100]
"""
"*** YOUR CODE HERE ***"
if seq == []:
return seq
if f(seq[0]):
return [seq[0]] + filter(f, seq[1:])
return filter(f, seq[1:])
```

Use OK to test your code:

`python3 ok -q filter`

### Question 4: Decimal

Write the recursive version of the function `decimal`

which takes in `n`

, a number, and returns a list representing the decimal representation of the number.

```
def decimal(n):
"""Return a list representing the decimal representation of a number.
>>> decimal(55055)
[5, 5, 0, 5, 5]
>>> decimal(-136)
['-', 1, 3, 6]
"""
"*** YOUR CODE HERE ***"
if n < 0:
return ['-'] + decimal(-1 * n)
elif n < 10:
return [n % 10]
else:
return decimal(n // 10) + [n % 10]
```

Use OK to test your code:

`python3 ok -q decimal`

## Submit

Make sure to submit this assignment by running:

`python3 ok --submit`