Programming Praxis

[ Merry Christmas to all my readers! And best wishes for you and your families in the coming year! I’ll be taking a few weeks away from the blog to be with family; new exercises will resume the first Tuesday of 2016. In the meantime, enjoy this Christmas card from me to you, with art “The Nativity of Jesus” by Botticelli and text from Luke 2:8-14 (King James Version). ]

And there were in the same country shepherds abiding in the field, keeping watch over their flock by night. And, lo, the angel of the Lord came upon them, and the glory of the Lord shone round about them: and they were sore afraid. And the angel said unto them, “Fear not; for, behold, I bring you tidings of great joy, which shall be to all people. For unto you is born this day in the city of David a Savior, which is Christ the Lord. And this shall be a sign unto you: Ye shall find the babe wrapped in swaddling clothes, lying in a manger.” And suddenly there was with the angel a multitude of the heavenly host praising God, and saying, “Glory to God in the highest, and on earth peace and goodwill towards men.”

Today we have a two-part interview question from Amazon:

First, find the first non-repeated element in an unsorted array. For instance, in the array [3, 5, 3, 2, 1], element 3 is repeated, elements 5, 2, and 1 are non-repeated, and the first of those is 5. Second, find the first element that appears an even number of times in an unsorted array. For instance, in the array [5, 3, 5, 1, 5, 1, 3], elements 1 and 3 appear an even number of times, and the first of those is 3.

Your task is to write programs that find the first non-repeated element and the first even-appearance elements in an unsorted input array. 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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It was a dreary Sunday in St Louis, unseasonably warm, but it rained all day and felt much colder than the actual air temperature, so I sat down to read Donald Knuth’s article, with Svante Janson, Shellsort With Three Increments. After sixteen pages, Janson and Knuth conjecture but do not prove that the gap sequence (h, g, 1) with h ≈ √n and g ≈ √h and h coprime to g leads to time complexity O(n^3/2).

Your task is to implement shellsort, as we did in a previous exercise, and test the Janson/Knuth conjecture. 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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Some numbers can be represented as the sum of a sequence of consecutive squares; for instance, 595 = 6² + 7² + 8² + 9² + 10² + 11² + 12². Other numbers are not so representable.

Your task is to write a program that finds the longest consecutive sequence of squares that sums to a given number, or determines that no such sequence exists. 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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We discussed an algorithm due to Lenore Blum, Manuel Blum and Michael Shub that generates cryptographically-secure random numbers in a previous exercise. A better way to generate these random numbers uses an actual hardware source of entropy, such as thermal noise.

I recently learned that all models of the Raspberry Pi computer include a hardware random number generator on their system-on-a-chip; Stewart Russell gives instructions.

If instead of a Raspberry Pi you have a computer based on the Intel Ivy Bridge family of processors, a hardware random number generator is available as rdrand; Linux exposes that as the /dev/random device.

Your task is to explore the hardware random number generators that are available to you; you might want to write a program to generate keypads for the Diana Cryptosystem. 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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Today’s task is a simple exercise in list handling:

Given a list of integers, find the place where the list can be split in two that minimizes the differences in the sums of the two halves of the list. For instance, given the list [3,7,4,2,1], splitting after the second element of the list gives sums of 10 and 7 for the two halves of the list, for a difference of 3, and no other split gives a lower difference.

Your task is to write a program that splits a list as indicated 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.

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I had too much Thanksgiving; more specifically, the stuffing went bad, and I’ve felt poorly the last few days. In between trips to the bathroom, I picked up my copy of Hofstadter’s GEB and looked again at the FIGURE-FIGURE sequences of Chapter III:

1, 3, 7, 12, 18, 26, 35, 45, 56, 69, …

and

2, 4, 5, 6, 8, 9, 10, 11, 13, 14, …

where the two sequences together contain each of the natural numbers exactly once and the second sequence comprises the differences between the numbers in the first sequence. I love that book; in probably a dozen attempts, I am probably about 5% of the way to the enlightenment it offers.

Your task is to write programs that generate the two sequences. 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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