Cannot pre-allocate an empty matrix?

4 次查看（过去 30 天）
Tuvshee
Tuvshee 2018-5-14
编辑： Stephen23 2018-5-14
Hello, I'd like to create a matrix that increases with every loop by concatenation the matrix to itself, starting from an empty matrix as shown below
Q1s = [];
alphas = [];
for y = ymin:0.5:ymax
x = (xmin:2.5e-13:xmax)';
Q1s = vertcat(Q1s,x);
alphas = vertcat(alphas,ones(size(x))*y);
end
Now I'm getting the famous "consider pre-allocating for speed" message. But I need Q1 and alpha to be empty matrices to begin with because I'm using vertcat. Is there a way to go about this?

请先登录，再回答此问题。

采纳的回答

Stephen23
Stephen23 2018-5-14
编辑：Stephen23 2018-5-14
There are many ways:
  1. tell the editor to ignore the warning: right click, "suppress ... on this line"
  2. use a preallocated cell array, concatenate after the loop.
  3. figure out the final size (the total number of elements you require is just numel(x)*numel(y)).
  4. use simpler, more efficient code:
[Q1s,alphas] = ndgrid(xmin:2.5e-13:xmax,ymin:0.5:ymax);
Q1s = Q1s(:);
alphas = alphas(:);
Those three lines give the exactly same output as your code, yet avoids all the bother of loops and expanding arrays and all that jazz.

更多回答（0 个）

类别

在 Help Center 和 File Exchange 中查找有关 Loops and Conditional Statements 的更多信息

标签

产品


版本

R2017b

Community Treasure Hunt

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

Start Hunting!

Translated by