## Double Dabble

### July 31, 2018

Over at ComputerPhile, Professor Brailsford gives a beautiful explanation of the double-dabble algorithm for converting a number from binary to binary-coded decimal. If you don’t want to watch the video — you should — there is also an explanation at Wikipedia, or you can read the original description by C. B. Falconer.

Your task is to implement the double-dabble algorithm as shown by Professor Brailsford. 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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### July 27, 2018

By starting from the number 1 and repeatedly either adding 5 or multiplying by 3, an infinite set of numbers can be generated. For instance, starting from 1, you can triple to get 3, then add five to get 8, and finally triple to get 24, so 24 is reachable. On the other hand, 15 is not reachable, as a little thought will show.

Your task is to write a program that determines if a number n is reachable by tripling or adding five, and if the number is reachable to give the sequence of operations by which it can be reached. 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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## Fenwick Trees

### July 20, 2018

There are some algorithms in coding theory where cumulative frequency tables must be maintained. This requires two tasks to be performed on an array of integers:

1) Calculate the cumulative sum of the first n elements of the array.

2) Modify the value of an element of the array.

One approach is to maintain an array of the elements, so that an element can be modified in O(1) time but calculating the cumulative sum requires O(n)time. Another approach is maintain an array of the cumulative sumes, so that a cumulative sum can be calculated in O(1) time but modifying a single element of the array takes O(n) time.

A Fenwick tree performs both operations in O(log n) time. The tree is normally embedded in a 1-based array, with the children of node i at nodes 2i and 2i + 1. Each element with index i a power of 2 contains the sum of the first i elements. Each element with index i a sum of two powers of 2 contains the sum of the elements since the preceding power of 2. In general, each element contains the sum of the values since its parent in the tree, found by clearing the least-significant bit in the index. Wikipedia has a good description of Fenwick trees.

For example, to find the sum of the first 1110 = 10112 items in the array, note that 11 has three bits set, so add elements 10002, 10102, and 10112. To modify the 11th element, modify 10112, 11002, 100002, and all higher powers of 2 up to the size of the array.

Your task is to implement Fenwick trees. 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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## Top Heavy Perfect Powers

### July 17, 2018

Over at his blog, Ken is investigating top-heavy perfect powers to investigate increasing the breadth of the Cunningham project. He makes a list of 2413 perfect powers bn with bn in increasing order, up to a googol, omitting bases that are themselves perfect powers (for instance, 4n, 8n or 9n).

Your task is to make a list of the 2413 top-heavy perfect powers less than a googol. 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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## Data Laundry, Again

### July 13, 2018

Data laundry is the act of cleaning data, as when it arrives in one format and must be translated to another, or when external data must be checked for validity. We looked at data laundry in a previous exercise. We return to it today because I have been doing data laundry all week, handling data from a new vendor. Today’s task is similar to one I have been doing this week; convert the input to the output shown below, changing all appearances of the string ABCDE to an incrementally-numbered string with a prefix:

```ABCDE This is some text.
This is more text. ABCDE, ABCDE.
ABCDE And this is [ABCDE] still more text.```
```X1 This is some text.
This is more text. X2, X3.
X4 And this is [X5] still more text.```

Your task is to write a program to perform the data laundry shown 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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## 8/10 Palindromes

### July 10, 2018

Sometimes I enjoy browsing oeis.org (I might be a little bit weird). While browsing the other day, I found A029804, the sequence of numbers that are palindromes in both decimal and octal.

Your task is to write a program that writes the sequence of numbers that are palindromes in both decimal and octal. 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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## Top Five Test Scores

### July 3, 2018

We frequently base exercises on student assignments. Today we have something for the teachers:

A student’s final store is the average of the five best test scores the student achieved during the school term. Given a list of student name/score pairs, determine the final score for each student. You may assume each student has at least five scores.

Your task is to write a program to determine each student’s final score, 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.

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