## Exercise 6

### September 27, 2019

Today’s task is a beginning programmer’s exercise from Bjarne Stroustrup’s book Programming: Principles and Practice Using C++:

Write a program that prompts the user to enter 3 integer values, and then outputs the values in numerical sequence separated by commas. So: if the user enters the values 10 4 6, the output should be 4, 6, 10. If two values are the same, they should just be ordered together. So, the input 4 5 4 should give 4, 4, 5.

Your task is to write the indicated program; you may use C++ or any other language, but your solution should be at the level of a beginning programmer who has not yet learned about arrays or user-defined functions. 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.

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## Eureka

### September 24, 2019

Somebody’s homework today:

A number is a eureka number if the sum of the powers of its digits, with powers increasing, is equal to the number. For instance, 89 is a eureka number because 8**1 + 9**2 = 89, and 1306 is a eureka number because 1**1 + 3**2 + 6**4 = 1306.

Your task is to write a program to identify eureka numbers, and determine how many eureka numbers exist that are less than a million. 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.

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## What Comes Next?

### September 20, 2019

This is a Microsoft interview question:

What is the next number in the sequence: 8, 13, 17, 22, 24, 26, 29, 31, 35, …?

Your task is to solve the puzzle. 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.

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## Getopt

### September 17, 2019

As I have said previously on this blog, I’m not allowed to write Scheme code at work; it would never make it past code review. But I can write shell scripts, and Awk is a natural language to use in shell scripts, so lately I have been writing a lot of Awk programs.

I recently had occasion to write an Awk program that took command-line arguments, so naturally I googled “getopt.awk” and found an implementation in the Gnu Awk manual. But that version of `getopt` was very C-like and not at all Awk-ish, so I decided to write my own. My `getopt` function extracts command-line arguments into an associative array `OPTS` that has the flag as its key and the value associated with the flag as its value; the value is null if the flag has no associated value. The `getopt` function takes two arguments — a string defining the available flags, in the same format as the C `getopt` function, and a string containing a message to be printed on error — and returns the number of arguments found.

Your task is to write a `getopt` function for your favorite langauge; if your language already provides command-line argument handling, write a version for Awk. 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.

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## Alchemical Reduction

### September 10, 2019

Today’s exercise is from the 2018 Advent of Code, Day 5:

You’ve managed to sneak in to the prototype suit manufacturing lab. The Elves are making decent progress, but are still struggling with the suit’s size reduction capabilities.

While the very latest in 1518 alchemical technology might have solved their problem eventually, you can do better. You scan the chemical composition of the suit’s material and discover that it is formed by extremely long polymers (one of which is available as your puzzle input).

The polymer is formed by smaller units which, when triggered, react with each other such that two adjacent units of the same type and opposite polarity are destroyed. Units’ types are represented by letters; units’ polarity is represented by capitalization. For instance, r and R are units with the same type but opposite polarity, whereas r and s are entirely different types and do not react.

For example:

– In aA, a and A react, leaving nothing behind.

– In abBA, bB destroys itself, leaving aA. As above, this then destroys itself, leaving nothing.

– In abAB, no two adjacent units are of the same type, and so nothing happens.

– In aabAAB, even though aa and AA are of the same type, their polarities match, and so nothing happens.

Now, consider a larger example, dabAcCaCBAcCcaDA:

```dabAcCaCBAcCcaDA  The first 'cC' is removed.
dabAaCBAcCcaDA    This creates 'Aa', which is removed.
dabCBAcCcaDA      Either 'cC' or 'Cc' are removed (the result is the same).
dabCBAcaDA        No further actions can be taken.```

After all possible reactions, the resulting polymer contains 10 units.

How many units remain after fully reacting the polymer you scanned?

Your task is to write a program that solves the alchemical reduction. 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.

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## Semi-Primes

### September 6, 2019

We have on several occasions seen functions for generating the primes less than n. Sometimes, what you want instead of primes are semi-primes, numbers which are the product of two primes. For instance, the six semi-primes less than 25 are 2×3=6, 2×5=10, 2×7=14, 3×5=15, 3×7=21 and 2×11=22.

Your task is to write a function that makes a list of the semi-primes less than n; use it to discover the number of semi-primes less than 10,000. 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.

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## Date Conversion

### September 3, 2019

Where I work, the accounting department does a survey of its vendors once a year, or every three years, or something, I’m not sure of the details. All of the vendors send CSV files with multiple rows, from a few rows to several hundred; each row contains 3 dates. Different vendors send dates in different formats: 9/3/19, 9-3-19, 09/03/2019, 3-SEP-19, “September 3, 2019” (quoted to protect the embedded comma), and even 09/03/2019T14:37:26.8493 (because every ten-thousandth of a second matters). That variety of formats caused problems for the accounting department, so they asked for help in reformatting the dates. Here is some sample data:

```John,Smith,9/3/19,9-3-19,09/03/19,100
Sally,Jackson,3-SEP-19,"September 3, 2019",09/03/2019T14:37:26.8493,200```

Your task is to write a program to convert dates from a variety of formats to the standard MM/DD/YYYY format. 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.

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