Problem 42405. Divisible by 3

Created bygoc3

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Pursuant to the <a href = "//www.tianjin-qmedu.com/matlabcentral/cody/problems/42404-divisible-by-2">first problem</a> in this series, this one involves checking for divisibility by 3.Write a function to determine if a number is divisible by 3. This can be checked by at least two different methods:<ol><li>Sum the digits in the number. If the resulting sum is divisible by 3, then so is the original number. This routine can be applied recursively until the resulting sum is a single digit.</li><li>From the <a href = "https://en.wikipedia.org/wiki/Divisibility_rule">wikipedia reference page</a>: "Subtract the quantity of the digits 2, 5, and 8 in the number from the quantity of the digits 1, 4, and 7 in the number." If the resulting sum (absolute value) is a multiple of 3, then so is the original number.</li></ol>Previous problem: <a href = "//www.tianjin-qmedu.com/matlabcentral/cody/problems/42404-divisible-by-2">divisible by 2</a>. Next problem: <a href = "//www.tianjin-qmedu.com/matlabcentral/cody/problems/42406-divisible-by-4">divisible by 4</a>.

Solve

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1141解万博尤文图斯决方案
347求解器

Last解决方案submitted on May 25, 2022

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7条评论

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节目 4 older comments

让·玛丽·塞恩蒂尔（Jean-Marie Sainthillier）on 25 Jun 2015

I don't understand your restrictions. OK for Java functions, but here you need mod/rem no ?

goc3on 25 Jun 2015

No, mod() or rem() isn't necessary because you can recursively apply the method, down to a single-digit number. The small number of single-digit numbers (i.e., 10, assuming absolute value) then makes checking against known divisible numbers trivial.

Alfonso Nieto-Castanonon 25 Jun 2015

I understand the java restriction, but the rest seem a bit heavy handed. Also the large number of similar problems might encourage solvers to build a general "divisible by n" routine instead

goc3on 26 Jun 2015

I'm not trying to be authoritarian; just providing awareness to interesting mathematical trivia. There are some numbers, such as 7 and 11, for which a range of methods are possible. Without blocking a lot of those basic functions, very easy workarounds can be used, defeating the purpose of trying to optimize your score (akin to regexp, eval, etc. being banned due to defeating the scoring system).

goc3on 26 Jun 2015

Also, building a general "divisible by n" routine is a good idea and I had thought about posting that at the end of the group of problems. I just think that it's too big for a standalone problem. There are many problems on Cody that are extremely simple and require no thought (like the recent string of beginner problems). There are also quite a few that require significant time, thought, and/or programming to solve, but they are all worth the same number of points. I prefer to break more difficult problems into simpler modules (just like is done in programming) to provide an encouraging approach, especially to those not already expert in Matlab. There are similarities between many of the problems in this set, but some numbers require quite different methods, such as 7 and 11.

Jan Orwat2015年7月2日

Solved problems before reading comments and must agree with Alfonso. "divisible by N" approach worked nicely through all of them.

goc3on 6 Jul 2015

Thanks for the comment. I'll be adding a few more problems to this set in the near future to round out and finish up the "divisible by N" approach.

解决方案Comments

解决方案1890020

1 Comment

1 Comment

Nikolaos Nikolaouon 2 Aug 2019

为什么不可见测试套件？是我（Firefox显示问题）还是应该这样？

解决方案1705227

2 Comments

2 Comments

克里斯·布雷西克（Chris Breshike）on 9 Jan 2019

我唯一的失败吗test 12?
I get tf=0 when i run this in Matlab...

goc3on 10 Jan 2019

That test case has the longest string input, by far. Your solution is coded to only sum digits twice. After those two steps, the resulting sum in this specific case is still greater than 9, and, therefore, fails your test of equal to a single-digit multiple of three (3, 6, or 9).

解决方案695335