基于向量的每个元素的向量创建-Matlab [英] Vector creation based on each element of a vector - Matlab

问题描述

我想对向量的每个元素应用一个行距函数，并最终创建一个整个向量，有什么方便的方法吗?

I want to apply a linespace function for each element of a vector, and create a whole vector in the end, is there some way convenient to do it?

具体来说，我想使用循环在矩阵 A 没有的情况下通过矩阵 A 创建矩阵 B A = [1; 2; 3; 4] B = [1; 1; 2; 1; 2; 3; 1; 2; 3; 4]

Specifically,I want to create matrix B by matrix A without using a loop, where A = [1; 2; 3; 4] B = [1; 1; 2; 1; 2; 3; 1; 2; 3; 4]

如您所见，如果向量中的第三个元素为3，则输出向量 B 中的第三向量"将为[1:1:3].我想要的步骤是固定的，对于任何元素都将是精确的1.

As you can see, if the third element in the vector is 3, then the "third vector" in the output vector B will be [1:1:3]. The step I desired is fixed and will be exact 1 for any element.

请问有人可以帮我吗?

推荐答案

这里有两种避免循环的方法.

Here are two ways that avoid loops.

• 使用 bsxfun :这种方法是记忆(或时间)效率低下:

• Using bsxfun: this approach is memory- (and perhaps time-) inefficient:

t = 1:max(A);
B = nonzeros(bsxfun(@times, t.', bsxfun(@le, t.', t)));

使用 cumsum :这是内存效率，但更复杂一些:

Using cumsum: this is memory-efficient, but a little more convoluted:

A = nonzeros(A); % remove zeros, if any
B = ones(1, sum(A)); % initiallize
B(cumsum(A(1:end-1))+1) = 1-A(1:end-1); % write appropriate values...
B = cumsum(B).'; % ... so that the cumulative sum gives the desired output

循环可能是最好的方法:最简单，最清晰.

A loop is probably the best way: simplest and clearest.

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