[英]Why am I getting a vertcat error? (Matlab)
I am trying to plot the first row of my matrix against time t but I cannot figure out why my matrix yields the error: "vertcat: Dimensions of matrices being concatenated are not consistent." 我试图相对于时间t绘制矩阵的第一行,但是我无法弄清楚为什么矩阵会产生错误:“ vertcat:被连接的矩阵的尺寸不一致。”
t = linspace(0,100);
y_mat = (1./t).*([1, t+(1/2)*exp(-3*t)-(1/2)*exp(-t);
(3/2)*(exp(-t)-exp(-3*t)), 1-(3/2)*exp(-3*t)+
(1/2)*exp(-t)] * [(t-4)/3;1]);
plot(t,y_mat(1,:))
You are thinking in the term of symbolic notation but implementing in matrix notation. 您在用符号表示法思考,但是在矩阵表示法中实现。 When you do
t = linspace(0,100);
当你做
t = linspace(0,100);
it creates a 1x100
matrix (array). 它创建一个
1x100
矩阵(数组)。 So when later on it is used in the definition of y_mat
, each expression used in the definition evaluates to 1x100
matrix. 因此,稍后在
y_mat
的定义中使用它时,定义中使用的每个表达式的计算结果为1x100
矩阵。 So your y_mat
definition is tying to do this : [1x100] * [1 1x100 ; 1x100 1x100] * [1x100 ; 1]
因此,您的
y_mat
定义就是这样做的: [1x100] * [1 1x100 ; 1x100 1x100] * [1x100 ; 1]
[1x100] * [1 1x100 ; 1x100 1x100] * [1x100 ; 1]
[1x100] * [1 1x100 ; 1x100 1x100] * [1x100 ; 1]
which obviously fails. [1x100] * [1 1x100 ; 1x100 1x100] * [1x100 ; 1]
显然失败了。
You have two options: Do all computations in the matrix notation by first computing the matrix multiplication separately and restructuring the matrices to represent the actual multiplication (ensure the 1
s are appropriately replicated). 您有两个选择:以矩阵表示法进行所有计算,方法是首先分别计算矩阵乘法,然后重组矩阵以表示实际乘法(确保正确地复制了
1
s)。
OR 要么
use Matlabs's symbolic variables and expressions probably like this : 使用Matlabs的符号变量和表达式可能像这样:
syms t % creating symbolic variable
% creating symbolic expressions
f0 = 1/t
f1 = t+(1/2)*exp(-3*t)-(1/2)*exp(-t);
f2 = (3/2)*(exp(-t)-exp(-3*t));
f3 = 1-(3/2)*exp(-3*t)+(1/2)*exp(-t);
f4 = (t-4)/3;
% defining y_mat
y_mat = f0 * [1 f1; f2 f3] * [f4 ; 1]
% putting value in symbolic variable
t = linspace(eps,100); % eps to avoid division by 0 error
% substitute values and evaluate y_mat
y_mat_vals = eval(subs(y_mat));
This gives y_mat_vals
a 2x100
matrix, as the answer. 这给
y_mat_vals
一个2x100
矩阵,作为回答。
YOu have messed up your code..you need to be careful when typing such functions. 您弄糟了您的代码..键入此类函数时需要小心。 To make it simple, I have used a loop.
为简单起见,我使用了一个循环。
t = linspace(0,100);
nt = length(t) ;
y_mat = zeros(2,nt) ;
for i = 1:nt
y_mat(:,i) = (1/t(i))*([1 t(i)+(1/2)*exp(-3*t(i))-(1/2)*exp(-t(i));
(3/2)*(exp(-t(i))-exp(-3*t(i))) 1-(3/2)*exp(-3*t(i))+(1/2)*exp(-t(i))])*[(t(i)-4)/3;1];
end
plot(t,y_mat)
You can also write it out more explicitly. 您也可以更明确地将其写出。 The equation reads:
等式如下:
[ 1,pt2 ; pt3,pt4 ] * [ pt5 ; 1 ] = [ pt5 + pt2 ; pt3.*pt5 + pt4 ]
Since each of those terms is a scalar, you can compute them for all t
at the same time using element-wise multiplication: 由于这些项都是标量,因此您可以使用逐元素乘法同时计算所有
t
的项:
t = linspace(0,100);
pt2 = t+(1/2)*exp(-3*t)-(1/2)*exp(-t);
pt3 = (3/2)*(exp(-t)-exp(-3*t));
pt4 = 1-(3/2)*exp(-3*t)+(1/2)*exp(-t);
pt5 = (t-4)/3;
y_mat = (1./t) .* [ pt5 + pt2 ; pt3.*pt5 + pt4 ];
plot(t,y_mat)
This might be a bit more verbose, but I don't think it's any less readable than other solutions. 这可能有些冗长,但我认为它的可读性并不比其他解决方案低。 And it is much more efficient: 0.0571 ms, versus 483.3 ms ( syms solution ) and 0.681 ms ( loop solution) , for a
t
with 500 elements. 而且效率更高:对于500个元素的
t
,效率为0.0571毫秒,而483.3毫秒( 符号解 )和0.681毫秒( 循环解) 。
(Note that multiplying by 1./t
uses implicit singleton expansion. This works in MATLAB R2016b and newer. For older versions of MATLAB, use bsxfun
.) (请注意,乘以
1./t
使用隐式单例扩展。这在MATLAB R2016b和更高版本中适用。对于旧版本的MATLAB,请使用bsxfun
。)
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