{"id":4207,"date":"2020-10-20T11:58:14","date_gmt":"2020-10-20T15:58:14","guid":{"rendered":"https:\/\/blogs.mathworks.com\/steve\/?p=4207"},"modified":"2020-10-20T11:58:14","modified_gmt":"2020-10-20T15:58:14","slug":"rediscovering-cody","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/steve\/2020\/10\/20\/rediscovering-cody\/","title":{"rendered":"Rediscovering Cody"},"content":{"rendered":"

Lately, I've been spending more time on MATLAB Central, and I'd like to encourage you to try out some of the resources there, if you haven't already.<\/p>

Have you heard of Cody<\/a>? It is an addictive MATLAB puzzle-solving activity. You can learn a lot about MATLAB and algorithm coding approaches by trying the problems there, and especially by looking at other people's solutions. There's a competitive aspect as well, as your puzzle solutions get scored and ranked against other solutions. (In the Cody world, smaller programs get the better scores.)<\/p>

This morning, for example, this Cody problem attracted my attention:<\/p>

\r\n

\r\n<\/p>

Problem 340. Find the last non-zero in a given dimension<\/a><\/p>

You are given a logical matrix BW<\/tt> of any dimension, and a dimension dim<\/tt> . You need to find the locations of the last non-zero element of BW<\/tt> in that given dimension. If no non-zeros exist at an element location, return a 0 at that location.<\/p>

For example, given:<\/p>

BW = [0 0 0 0; 1 1 1 1; 0 1 1 0]<\/pre>
When dim = 1<\/tt> , you should return ans = [2 3 3 2]<\/tt>, because these are the row numbers of the last non-zeroes in each column.<\/p>
When dim = 2<\/tt> , you should return ans = [0; 4; 3]<\/tt>, because these are the column numbers of the last non-zeroes in each row.<\/p>
Don't forget that the input isn't restricted to 2D matrices.<\/p>
\r\n<\/div>\r\n<\/p>
This problem caught my eye because any time I see BW<\/tt> used as a variable name, especially for an array of 0s and 1s, it says image processing<\/b> to me. I also like thinking about multidimensional array operations in different dimensions.<\/p>
After a little thought, I put together a three-line solution using the functions flip<\/tt>, max<\/tt>, size<\/tt>, and any<\/tt>. (I won't be more specific than that here. Try to solve it yourself!)<\/p>
After I created my solution, I spent a few minutes looking at how other people solved it. I look especially at the solutions that have a smaller code size than mine. I almost always learn something interesting, such a new way to apply familiar MATLAB functions, in this exercise.<\/p>
Solving a Cody problem using the new patterns feature<\/b><\/p>
A few days ago, I was looking at this problem:<\/p>
\r\n
\r\n<\/p>
Problem 31. Remove all the words that end with \"ain\"<\/a><\/p>
Given the string s1<\/tt> , return the string s2<\/tt> with the target characters removed.<\/p>
For example, given<\/p>
s1 = 'the main event'<\/pre>
your code would return<\/p>
s2 = 'the  event'<\/pre>
Note the 2 spaces between \"main\" and \"event\" Only the four letters in the word \"main\" were deleted.<\/p>
\r\n<\/div>\r\n<\/p>
When this problem was originally written, I would have expected to see a lot of solutions using regexp<\/tt> and regular expressions. But the MATLAB R2020b release has the new patterns feature, which aims to make text processing code a easier to write and understand. I wanted to give a try with my Cody solution. (Take a look at Jiro Doke's 15-Oct-2020 post<\/a> about patterns.)<\/p>
Here's what I came up with:<\/p>
p = letterBoundary + asManyOfPattern(lettersPattern) + \"ain\"<\/span> + letterBoundary\r\n<\/pre>
\r\np = \r\n\r\n  pattern\r\n\r\n  Matching:\r\n\r\n    letterBoundary + asManyOfPattern(lettersPattern) + \"ain\" + letterBoundary\r\n\r\n<\/pre>
s = \"The rain in Spain falls mainly on the plain\"<\/span>;\r\nreplace(s,p,\"\"<\/span>)\r\n<\/pre>
\r\nans = \r\n\r\n    \"The  in  falls mainly on the \"\r\n\r\n<\/pre>
Give Cody<\/a> a try -- but not when you have something else due soon! Do you want something image-related to start with? Try Problem 181. Change a specific color in an image.<\/a> Or search for Cody problems that are tagged \"image processing\".<\/a><\/p>
If you come across Cody problem (or solution) that you find especially interesting, let us know here in the comments.<\/p>