## Spelling Numbers

### July 3, 2020

## Shuffle An Array

### June 30, 2020

Today’s exercise comes to us from Leetcode via Reddit:

Given an array consisting of 2

nelements in the form

[x1,x2,…,xn,y1,y2,…,yn], return the array in the form [x1,y1,x2,y2,…,xn,yn].

The Reddit poster claims to be new to Scheme and functional programming, and was thinking of a solution using length and list-ref, but couldn’t solve the problem.

Your task is to show the student how to solve the problem. When you are

finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

## Summing A String

### June 19, 2020

In a string consisting of digits and other non-digit characters, the digits form an embedded series of positive integers. For instance, the string “123abc45def” contains the embedded integers 123 and 45, which sum to 168.

Your task is to write a program that takes a string and writes the sum of the integers embedded in the string. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

## Counting Fingers

### June 16, 2020

A little girl counts on her fingers in a curious way. She counts 1 on her thumb, 2 on her index finger, 3 on her middle finger, 4 on her ring finger, and 5 on her pinkie finger, then works back, counting 6 on her ring finger, 7 on her middle finger, 8 on her index finger, and 9 on her thumb, when she again turns around and counts 10 on her index finger, 11 on her middle finger, and so on.

Your task is to write a program that determines which finger the little girl will be on when she reaches a thousand. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

## 2Max

### June 5, 2020

Today’s exercise comes from Stack Overflow:

Given an array A consisting of N integers, return the maximum sum of two numbers whose digits add up to an equal sum. If there are not two numbers whose digits have an equal sum, the function should return -1. For example, A = [51, 71, 17, 42] would output 93 because there are two sets of numbers with the same digit-sum, (51, 42) with a digit-sum of 6 and (17, 71) with a digit-sum of 8, and the first pair has the maximum sum of two numbers of 93.

Your task is to write a program to calculated the requested maximum sum. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

## Hidden Squares

### June 2, 2020

We have a simple task today:

A number

nmay have squares hidden among its digits. For instance, in the number 1625649, the consecutive digits 1, 4, 9, 16, 25, 49, 64, 256 and 625 are all squares.

Your task is to write a program that takes a positive integer *n* and finds all of its hidden squares. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

## Decreasing-Increasing Array

### May 29, 2020

Today’s task is somebody’s homework:

Given an array of integers, determine if it is in decreasing-increasing order, with some initial segment of the array in decreasing order and the remainder of the array in increasing order. If the array is in decreasing-increasing order, return the pivot element (the minimum element in the array); otherwise, return an indication the array is not in decreasing-increasing order. Array elements may be duplicated. For example, the array (10 10 10 8 8 6 4 4 3 12 13 22 31 40 59 68) is in decreasing-increasing order, with pivot element 3, but the array (1 2 4 8 12 32 64) is not in decreasing-increasing order.

The student who asked the question suggests a solution using binary search that takes O(log *n*) time, but can’t get it to work.

Your task is to write a program to determine if an array is in decreasing-increasing order as described above. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

## Prime Power Triples

### May 22, 2020

Today’s exercise is Project Euler Problem 87:

The smallest number expressible as the sum of a prime square, prime cube, and prime fourth power is 28. In fact, there are exactly four numbers below fifty that can be expressed in such a way:

28 = 2^{2}+ 2^{3}+ 2^{4}33 = 3^{2}+ 2^{3}+ 2^{4}49 = 5^{2}+ 2^{3}+ 2^{4}47 = 2^{2}+ 3^{3}+ 2^{4}How many numbers below fifty million can be expressed as the sum of a prime square, prime cube, and prime fourth power?

Your task is to solve Project Euler 87; in the spirit of Project Euler, show only your code but not the solution. When you are finished, you are welcome to read

or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

## MinStack

### May 19, 2020

[ My apologies that this exercise is so long delayed. We have been rewriting all of our business practices in the light of the pandemic, and work has been madness. The biggest projects are now complete, and we have retreated to merely super-busy, so maybe I will have time to write some more exercises. I hope everyone is well; I am. ]

Today’s task is to implement a minstack data structure that provides the normal stack operations `push`

, `pop`

and `top`

and also the operation `least`

which returns the smallest item in the stack without altering the stack in any way. All four operations must operate in O(1) time and O(*n*) space, where *n* is the size of the stack.

Your task is to implement a minstack as described above. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

## Nth Item In A Linked List

### April 10, 2020

[ I continue working at home. It’s no fun, and less productive than working in my office, but I’m coping. There remains much emergency work to be done related to moving all of our students online. I hope everyone is doing well during this time. ]

A student recently asked for help on a beginning-programmer forum to write a Scheme program that finds the nth item in a linked list, mimicking the `list-ref`

built-in function. He posted his code, which was awful; I won’t repost it here. Instead of engaging him, I sent a private email suggesting that he consult either his professor or his teaching assistant, as his posted code showed several misconceptions about Scheme. He wrote back, saying he was sure there was only one thing wrong with his code and I could easily point it out. I didn’t respond, as there was far more than one thing wrong with his code.

Your task is to write a program to find the nth item in a linked list; your program must be recursive, as was required by the original assignment. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.