arrays - 矢量化- Matlab

Question

给定一个向量

X = [1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3]

我想生成一个这样的向量

Y = [1 2 3 4 5 1 2 3 4 5 6 1 2 3 4 5]

到目前为止我得到的是

idx = find(diff(X))
Y   = [1:idx(1) 1:idx(2)-idx(1) 1:length(X)-idx(2)]

但我想知道是否有更优雅(稳健)的解决方案？

Answer 1

针对一般情况使用 diff、find 和 cumsum 的一种方法 -

%// Initialize array of 1s with the same size as input array and an 
%// intention of using cumsum on it after placing "appropriate" values
%// at "strategic" places for getting the final output.
out = ones(size(X))

%// Find starting indices of each "group", except the first group, and
%// by group here we mean run of identical numbers.
idx = find(diff(X))+1

%// Place differentiated and subtracted values of indices at starting locations
out(idx) = 1-diff([1 idx])

%// Perform cumulative summation for the final output
Y = cumsum(out)

sample 运行-

X =
     1     1     1     1     2     2     3     3     3     3     3     4     4     5
Y =
     1     2     3     4     1     2     1     2     3     4     5     1     2     1

---

只是为了好玩，但是习惯bsxfun的替代解决方案-

%// Logical mask with each column of ones for presence of each group elements
mask = bsxfun(@eq,X(:),unique(X(:).'))  %//'

%// Cumulative summation along columns and use masked values for final output
vals = cumsum(mask,1)
Y = vals(mask)