matlab - 根据另一列的值将矩阵的一列元素分组到元胞数组中

Question

一段时间以来，我一直在努力想出一个聪明的方法来做这件事。给定具有以下结构的矩阵(或单元格):

A = [-1  1 
     -1  2 
      1  3 
      3  5
      2  3 
      2  4 
      2  7  
      4  5 
      5  6 
      6  7 
      7 -2 ]

(请注意，上面的矩阵/单元格在两列中均未排序且包含负数)。

如何根据特定列的唯一值对其进行分组。例如。按第二列分组所需的输出类似于:

B{1} = [-1]
B{2} = [-1]
B{3} = [1,2]
B{4} = [2]
B{5} = [3,4]
B{6} = [5]
B{7} = [2,6]
B{-2} = [7]

提前致谢!

Answer 1

您可以使用 accumarray :

[~,~,subs] = unique(A(:,2));
values  = accumarray(subs,A(:,1),[],@(x) {x});
ofGroup = accumarray(subs,A(:,2),[],@(x) {x(1)});

out = [ofGroup values]

---

out = 

    [-2]    [         7]
    [ 1]    [        -1]
    [ 2]    [        -1]
    [ 3]    [2x1 double]
    [ 4]    [         2]
    [ 5]    [2x1 double]
    [ 6]    [         5]
    [ 7]    [2x1 double]

---

如果您真的坚持建议的订单，您可以执行以下操作，但我认为这不是必需的。

% positives
pos = A( A(:,2) >= 0 , :);

[~,~,subs] = unique(pos(:,2));
posvalues  = accumarray(subs,pos(:,1),[],@(x) {x});
posofGroup = accumarray(subs,pos(:,2),[],@(x) {x(1)});

% negatives
neg = A( A(:,2) < 0 , :);

[~,~,subs] = unique(neg(:,2));
negvalues  = flipud( accumarray(subs,neg(:,1),[],@(x) {x}) );
negofGroup = flipud( accumarray(subs,neg(:,2),[],@(x) {x(1)}) );

out = [posofGroup posvalues; negofGroup negvalues ]

---

out = 

    [ 1]    [        -1]
    [ 2]    [        -1]
    [ 3]    [2x1 double]
    [ 4]    [         2]
    [ 5]    [2x1 double]
    [ 6]    [         5]
    [ 7]    [2x1 double]
    [-2]    [         7]