## Pythagorean Quadruple

### February 25, 2020

A Pythagorean quadruple consists of four positive integers *a*, *b*, *c* and *d* such that *a* ≤ *b* ≤ *c* ≤ *d* and *a*² + *b*² + *c*² = *d*². For instance, (2 3 6 7) is a Pythagorean quadruple because 2² + 3² + 6² = 4 + 9 + 36 = 49 = 7².

Your task is to write a program that counts the Pythagorian quadruples with *a*, *b*, *c* less than or equal to some given *N*, and compute the number of Pythagorian quadruples with *N* = 1000. 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.

## Formatted Dates

### February 21, 2020

Regular readers of this blog know that I work with a team of programmers, sysadmins and database administrators to maintain a large legacy database application, running on Oracle and HP-UX, and Scheme is nowhere in sight. Lately I have been “stealth programming” by writing awk programs in shell wrappers, because shell programming is a normal part of our environment. One thing I have been doing is formatting reports with awk. That frequently requires a formatted date string, either for today or some other day; gawk provides the `strftime`

function to format dates, but Posix awk, which is what HP-UX provides, doesn’t. So I wrote my own.

Your task is to write a function that formats dates; use any convention you like to determine how the date is formatted. 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.

## Square Triple

### February 14, 2020

We have a simple homework problem today:

Given a list of distinct integers, find all triples (

xyz) wherex,yandzare in the list andx*y=z².

Your task is to find the list of triples. 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.

## Removing Spaces

### February 11, 2020

We have a simple task today, with dozens of potential solutions:

Write a program to remove all spaces from a string.

Your task is to write a program to remove all spaces from a 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.

## My Mailbag

### February 7, 2020

I had two interesting emails from readers this week. I’ll discuss both of them, but suppress the names of the writers; they can identify themselves in the comments below if they wish.

One writer saw Johnny Ball’s video about Russian Multiplication on Numberphile and suggested it would make a good exercise. Indeed it would; in fact, we have already done it twice ([1] [2]).

Another writer suggested that the `compose`

function in the Standard Prelude should return the identity function if called with no arguments, rather than reporting an error. That is, of course, correct; my apologies for the bug.

Your task is to write programs to perform russian multiplication and compose functions, as suggested 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.

## Password Generator

### February 4, 2020

Many web sites require passwords that have particular combinations of various types of characters; for instance, a web site might require a password that has at least ten characters, including at least one each from the sets of lower-case letters, upper-case letters, digits, and special characters.

Your task is to write a program that generates passwords according to specifications that you define. 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.

## Anna’s Homework

### January 31, 2020

Let’s help Anna with her homework:

Given an array of

nelements, find an elementxthat appears in the array at leastn/3 times, or indicate that no such element exists. Your program may take no more than O(n) time and O(n) space.

Your task is to write a program to solve Anna’s homework 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.

## Non-Abundant Sums

### January 28, 2020

It’s been a long time since we did an exercise from Project Euler; here is number 23:

A perfect number is a number for which the sum of its proper divisors is exactly equal to the number. For example, the sum of the proper divisors of 28 would be 1 + 2 + 4 + 7 + 14 = 28, which means that 28 is a perfect number.

A number

nis called deficient if the sum of its proper divisors is less thannand it is called abundant if this sum exceedsn.As 12 is the smallest abundant number, 1 + 2 + 3 + 4 + 6 = 16, the smallest number that can be written as the sum of two abundant numbers is 24. By mathematical analysis, it can be shown that all integers greater than 28123 can be written as the sum of two abundant numbers. However, this upper limit cannot be reduced any further by analysis even though it is known that the greatest number that cannot be expressed as the sum of two abundant numbers is less than this limit.

Find the sum of all the positive integers which cannot be written as the sum of two abundant numbers.

Your task is to solve Project Euler 23; in the spirit of Project Euler, please do not display 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.

## Mertens’ Conjecture

### January 24, 2020

Dr Holly Krieger discussed Mertens’ Conjecture on Numberphile yesterday:

Mertens’ Conjecture: The absolute value of the Mertens function M(

n), computed as the sum forkfrom 1 tonof the Moebius function μ(k), is less than the square root ofn. The Moebius function μ(n) is (-1)^k, wherekis the number of prime factors ofn, but 0 ifnhas any repeated prime factors.

The conjecture has been proved false, though no counter-examples are known. You can read more about Mertens’ Conjecture at MathWorld or Wikipedia.

Your task is to write a program to compute Mertens’ function M(*n*) and use it to explore some of the sequences at OEIS. 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.

## Mini Markdown

### January 21, 2020

Today’s exercise is an interview question. The situation is an interviewee at home, sharing his screen with an interviewer at his office. The interviewee has one hour to write the following program while the interviewer watches:

Write a program that takes a text file as input and writes an html file as output. The input consists of one or more paragraphs (maximal lines of text separated by blank lines). The following elements of John Gruber’s Markdown language are supported:

– ###### up to six levels of headings

– unordered lists introduce by a dash (like this paragraph)

– plain text paragraphs

Your task is to write a mini-Markdown processor; you have one hour to complete it. 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.