Husk is a dialect of Scheme written in Haskell that adheres to the R5RS standard. Advanced R5RS features are provided including continuations, hygienic macros, and a full numeric tower.
For the complete post, please visit http://jeapostrophe.github.com/2013-05-20-forth-post.html.
It is a classic problem from computer science to determine the number of ways in which coins can be combined to form a particular target; as an example, there are 31 ways to form 40 cents from an unlimited supply of pennies (1 cent), nickels (5 cents), dimes (10 cents) and quarters (25 cents), ranging from the 3-coin set (5 10 25) to the 40-coin set consisting only of pennies.
The solution is usually stated in recursive form: if c is the first coin in the set of coins cs and n is the target, the solution is the number of ways to reach the target after removing c from cs plus the number of ways to reach n − c using all the coins in cs. The algorithm to make a list of the coins, instead of the count, is the same, but keeping track of the list of coins instead of the count.
Your task is to write two functions, one to determine the count and one to determine the list of coins. 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.
The ISLISP programming language is a member of the Lisp family of programming languages. It attempts to bridge the gap between the various incompatible members of the Lisp family of languages (most notably Common Lisp, Eulisp, LeLisp, and Scheme) by focusing on standardizing those areas of widespread agreement. It is a small language, closer to the size of Scheme than to Common Lisp, for example.
The most recent change to the specification occurred in 2007.
The design of ISLISP had these design goals:
- Compatible with existing Lisp dialects where feasible.
- Provide basic functionality.
- Designed with extensibility in mind.
- Gives priority to industrial needs over academic needs.
- Promotes efficient implementations and applications.
John Cowan mentioned it on scheme-reports, so I was curious.
Today’s exercise comes from the book Making the Alphabet Dance: Recreational Wordplay by Ross Eckler, but I found it at http://www.math.cmu.edu/~bkell/alpha-order/:
Assume we have a list of all the words in the English language. Under the normal ordering of the alphabet in English (A, B, C, …, Z), some number of these words have all their letters in alphabetical order (for example, DEEP, FLUX, and CHIMPS). However, if we change the ordering of the alphabet, we will change the number of words having their letters in “alphabetical” order. What arrangement of the letters of the alphabet maximizes this number?
Your task is to write a program to find such an arrangement. 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.
For the complete post, please visit http://jeapostrophe.github.com/2013-05-13-vi-post.html.
According to their masthead, the MindCipher website is “a social repository of the world’s greatest brain teasers, logic puzzles and mental challenges.” Some of the problems lend themselves to brute force computation, others are better solved with pencil-and-paper or solely with mental effort. We look at three MindCipher exercises today.
1: Coin Flip: On Monday, you flip a coin all day. You start flipping it until you see the pattern Head, Tail, Head. You record the number of flips required to reach this pattern, and start flipping again (and counting up from 1 again) until you see that pattern again, you record the second number, and start again. At the end of the day you average all of the numbers you’ve recorded. On Tuesday you do the EXACT same thing except you flip until you see the pattern Head, Tail, Tail.
Will Monday’s number be higher than Tuesday’s, equal to Tuesday’s, or lower than Tuesday’s?
2: 1978: The year 1978 is such that the sum of the first two digits and the latter two digits is equal to the middle two digits, i.e. 19 + 78 = 97. What is the next year (after 1978) for which this is true?
3: Sum of Two Prime Numbers: If p, q > 2 are consecutive in set of primes. Since p,q can only be odd number, (p+q) is an even number.
Can (p+q)/2 be prime?
Your task is to solve the three problems given above; write a computer program to help you if you wish. 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.
Today’s exercise provides three little exercises on linked lists, designed to help beginning programmers learn more about how lists work; there is also a special invitation for more experienced programmers.
1. Write a function that takes an input list and an interval n and returns a new list with all the elements of the original list, in order, except that every nth item has been removed. For instance, given the input list (1 2 3 4 5 6 7 8 9 10) and n = 4, the function should return the list (1 2 3 5 6 7 9 10).
2. Write a function that takes an input list and returns a new list with all the elements of the original list, in order, except that in the case of duplicate elements all of the duplicates except the first has been removed. For instance, all of the following lists should be transformed into the list (1 2 3 4 5): (1 2 3 4 5), (1 1 2 3 4 5), (1 2 1 3 1 4 1 5 1), and (1 2 2 3 3 3 4 4 4 4 5 5 5 5 5).
3. Write a function that takes an input list and splits the list in half; for instance, given the input list (1 2 3 4 5) the two outputs are the lists (1 2) and (3 4 5). If the list has odd length the middle element can be placed in either half, at your option, so the lists (1 2 3) and (4 5) are an alternate acceptable solution for the example problem.
Your task is to write the three indicated functions. If you find that to be too simple, write one or more exercises for your student friends. 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.
For the complete post, please visit http://jeapostrophe.github.com/2013-05-06-texture--post.html.