CS 360
Winter 2018
Programming Language Concepts
CS 360-001 Tuesday/Thursday 15:30-16:50 (Rush 209)
CS 360-002 Tuesday/Thursday 14:00-15:20 (Rush 209)
CS 360-003 Tuesday 18:30-21:20 (UCross 151)

 Instructor: Geoffrey Mainland mainland@drexel.edu Office: University Crossings 106 Office hours: Mondays 4pm–7pm; Thursdays 5pm–6pm. Teaching Assistant: Xiao Han CLC office hours: Tuesday 12pm–2pm; Thursday 6pm–8pm Allen Yang CLC office hours: Wednesday 6pm–8pm

Due Monday, January 15, 11:59:59PM EST.

Accept this assignment on GitHub Classroom here. The standard homework instructions apply.

This assignment provides an introduction to list processing, functional programming, and Scheme. Implement, document (i.e., write a specification for), and test the following functions in scheme. Scheme includes some of the constructs below, but you may not use these implementations. Instead, you must implement them yourself. Make sure all functions are thoroughly tested.

You must implement the functions as specified. You may write other helper functions and define test data in your file, but you may not change the functions’ names or the number or order of arguments. You must also write a short description of what the function does in a comment, as we did for the examples in class. If we ask you to implement a recursive solution and your solution is not recursive, you will receive no credit.

Also note:

• No error-checking is required; you may assume input is valid.

• You may not use Racket’s built-in log or exp functions.

• You may not use set! or any form of mutable state in your solutions.

• We expect you to use proper Scheme style. We use proper Scheme style in class. You may also refer to this Scheme style guide.

You should complete the homework by modifying the file hw1.rkt that we provide as part of the assignment.

This assignment is worth 50 points. There are 50+1 possible points.

## Problem 1: (sigma m n)(10 points)

Define a recursive function sigma that takes two arguments, $m$ and $n$, and returns $\sum_{i=m}^{n} i$.

## Problem 2: (log m n)(10 points total)

Define a recursive function log that takes two arguments, $m$ and $n$, and returns the least integer $l$ such that $m^{l+1} > n$.

## Problem 3: (choose n k)(10 points total)

Define a recursive function choose that takes two arguments, $n$ and $k$, and calculates $n \choose k$, the binomial coefficient, which is also the number of ways of selecting $k$ items from a collection of $n$ items. Your choose should call itself recursively. An implementation that uses the closed-form formula for choose involving factorial will receive zero credit.

If you have forgotten the definition of the binomial coefficient, you may want to read the Wikipedia article. The following recurrence relation will be useful:

$${n \choose k} = {n-1 \choose k - 1} + {n-1 \choose k}$$

Make sure you handle the base cases:

$${n \choose 0} = {n \choose 1} = 1$$

## Problem 4: (binary n)(10 points)

Define a function binary that takes a single argument $n$ and returns the number whose decimal representation is the binary representation $n$. For example, (binary 2) should evaluate to 10.

Your function should be recursive. Your solution should not need to use strings or Racket’s binary number type. Your function must return a decimal number.

## Problem 5: (scan f z l)(10 points)

The scan function scan takes a binary function f, a value z, and a list l, and returns the list $z, f(z, x_1), f(f(z, x_1), x_2), \ldots, f(f(…f(f(z, x_1), x_2)…, x_{n-1}), x_n)$ where $x_1, x_2, …, x_n$ are the elements of the list l.

Examples:

This may remind you of the reduce function. You can think of scan as a version of reduce that returns a list of all the intermediate results of the reduction.

## Problem 6: Homework Statistics (1 point)

How long did spend on each problem? Please tell us in your README.md. You may enter time using any format described here.