清空筛选条件
清空筛选条件

Classifying things into kinds

2 次查看（过去 30 天）
John Pryce
John Pryce 2020-12-5
评论： John Pryce 2020-12-5
Increasingly often I want to do the following, so I want a simple vectorised way to do it - no "for" loops.
I have n things. Each thing is of one of m types. Coding the things as 1:n and the types as 1:m I can describe this data by an n-vector x where each x(j) is an integer in 1:m saying what type thing j has.
E.g. let n=7, m=5, then x = [2 3 1 3 5 1 2] says thing 1 has type 2, thing 2 has type 3 etc.
I want to convert between this form and the (obviously equivalent) form of a *partition* of 1:n into m subsets where the i'th subset lists all the things that have type i. This shall be a cell vector c of length m where c{i} is the set {j such that x(j)=i}, written as an ordinary vector in ascending order.
For the example, c is {[3], [1 7], [2 4], [] ,[5]}. Note c{4} is the empty vector because type 4 wasn't used.
Obviously, one can convert x to c and c to x by simple "for" loops but I want to avoid it.
I can do "x to c" by
c = arrayfun(@(i)find(x==i),1:m,'UniformOutput',false);
but (1) it looks clumsy, (2) implicitly it applies "find" to x a total of m times whereas a single pass through x should do.
Imagine n=10,000 and m=10, say. I welcome a better solution.
  2 个评论
John Pryce
John Pryce 2020-12-5
Sorry I should have written c is {[3 6], [1 7], [2 4], [] ,[5]}.
John Pryce
John Pryce 2020-12-5
Thanks! This is just what I was looking for.

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Ameer Hamza
Ameer Hamza 2020-12-5
编辑：Ameer Hamza 2020-12-5
You can use accumarray()
x = [2 3 1 3 5 1 2];
c = accumarray(x(:), (1:numel(x)), [], @(x) {x})
Result
>> c{:}
ans =
3
6
ans =
1
7
ans =
2
4
ans =
[]
ans =
5

更多回答（0 个）

类别

在 Help Center 和 File Exchange 中查找有关 Entering Commands 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by