如何计算nx3向量中每100个点的标准差？

Question

假设我有一个包含 n 行的矩阵，每行包含三个坐标（x、y 和 z）。我想计算 MATLAB 中每 100 个点的标准差。例如，对于前 100 个 x 坐标，我应用 std，然后对 y 和 z 坐标应用相同，依此类推……最终，我每 100 个点都有一组 x、y 和 z 值。我该怎么做？

Answer 1

我会这样做：

M = randn(120,3); % substitute this for the actual data; 3 columns
N = 100; % number of elements in each set for which std is computed

cols = size(A,1);
for n = 1:ceil(cols/N)
  row_ini = (n-1)*N+1;
  row_fin = min(n*N, cols); % the "min" is in case cols is not a multiple of N
  std(A(row_ini:row_fin,:))
end

如果速度是一个问题，“for”循环可能会被矢量化。

编辑：如果要将所有结果存储在三列矩阵中，只需修改“std”行并添加一些初始化，如下所示：

M = randn(120,3); % substitute this for the actual data; 3 columns
N = 100; % number of elements in each set for which std is computed

cols = size(A,1);
n_max = ceil(cols/N);
result = repmat(NaN,ceil(cols/N),3); % initialize
for n = 1:n_max
  row_ini = (n-1)*N+1;
  row_fin = min(n*N, cols); % the "min" is in case cols is not a multiple of N
  result(n,:) = std(A(row_ini:row_fin,:));
end