将向量附加到空的MATLAB矩阵 [英] Appending a vector to an empty MATLAB matrix

问题描述

我已经获得了将n维点(n> 1)插入矩阵(myPointMatrix)的MATLAB代码，并且正在考虑如何插入第一个点.

I've got MATLAB code inserting n-dimensional points (n >1) into a matrix (myPointMatrix) and am having thoughts about how to insert the first point.

现在，程序将在插入点之前检查myPointMatrix的大小.如果它是1x1，则将myPointMatrix设置为等于当前点.否则，将附加当前点.该if语句仅适用一次，但是每次插入一个点时都会对其进行评估，这种情况非常常见.

Right now the program checks the size of myPointMatrix before inserting a point. If it is 1x1, myPointMatrix is set equal to the current point. Otherwise the current point is appended. This if-statement is only true once, but is evaluated each time I insert a point, which is very very often.

删除if并尝试附加到myPointMatrix会使MATLAB抱怨矩阵尺寸不一致.删除if语句和myPointMatrix = 0的初始化都将导致MATLAB查找未定义的myPointMatrix.也可以理解.

Removing the if and trying to append to myPointMatrix makes MATLAB understandably complain about matrix dimensions not being consistent. Removing both the if-statement and the inialization of myPointMatrix = 0 causes MATLAB to find myPointMatrix undefined. Also understandable.

如何初始化myPointMatrix，以便删除if语句?还是有其他一些智能解决方案?

How do I initialize myPointMatrix so that I can remove the if-statement? Or is there some other smart solution?

myPointMatrix = 0;
for x=0:limit
    for y=0:limit
        for z=0:limit
            tempPoint = [x y z];
            if (length(myPointMatrix) == 1)
                myPointMatrix = tempPoint;
            else
                myPointMatrix = [myPointMatrix; tempPoint];
            end
        end
    end
end

推荐答案

使用myPointMatrix = [];初始化矩阵.

myPointMatrix越大，附加速度就越慢.它变得越来越慢，因为每当您添加一个点时，matlab都会分配一个新大小的新矩阵，并将信息从旧矩阵+新点复制到新矩阵中.

The bigger myPointMatrix is, the slower appending will be. It gets slower and slower, since for each time you append a point matlab allocates a new matrix of the new size and copies the information from your old matrix + your new point into the new matrix.

然后最好用其最终大小初始化MyPointMatrix，然后将点插入矩阵中的给定位置.

It is then better to initialize MyPointMatrix with its final size, and inserting the points into given positions in the matrix.

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