Using of symprod fuction

Question

HSUAN 2023-7-30

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2002442-using-of-symprod-fuction

评论： HSUAN 2023-8-1

采纳的回答： Torsten

在 MATLAB Online 中打开

I want to calculate the following formula

So I wrote out the following code.(r=7)

syms Nb Nv K n Tb_1 Hb t Sb F Sv Hv D P Cj  r q i z j
Nv(i)=[0 5 1 17 4 8 9];
Nb(i)=[1 ;1 ;1 ;1 ;1 ;1 ;1];
Sb(i)=[25 ;28 ;32 ;36 ;35 ;38 ;0];
for i=1:6
 z(i)=symprod((Nv(j))*(Nb(i)*Sb(i)),j,2,(i+1));
disp(z(i))
end
Error using *
Dimensions do not match.

It display error using of * .

I tried modifying sb(i)=[25 28 32 36 35 38 0]; but still got the same result.

So I guess maybe it's not possible to multiply three matrices using.

syms Nb Nv K n Tb_1 Hb t Sb F Sv Hv D P Cj  r q i z j
Nv(i)=[0 5 1 17 4 8 9];
Nb(i)=[1 ;1 ;1 ;1 ;1 ;1 ;1];
Sb(i)=[25 ;28 ;32 ;36 ;35 ;38 ;0];
for i=1:6
 z(i)=symprod((Nv(j))*(Nb(i)),j,2,(i+1));
disp(z(i))
end

I try to multiply two matrices using.

The result is

44

1936

85184

3748096

164916224

7256313856

While it works, the result doesn't seem to match the formula I wrote above.

Please help me fix the code so that I can calculate the correct result, thank you for taking the time to answer.

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Dyuman Joshi 2023-7-30

编辑：Dyuman Joshi 2023-7-30

在 MATLAB Online 中打开

Why are you using symprod for that? A simple for loop will be good enough. You might even be able to vectorize it.

Also, this is just bad

Nv(i)=[0 5 1 17 4 8 9];

i is by default assigned as the imaginary constant in MATLAB, see below

i
ans = 0.0000 + 1.0000i

However, since you have defined i as a symbolic variable, the above line of code works. If you had not done that, and simply use that line of code, you would have gotten error.

If you want to index a vector or an array, you will have to define the indices before using them.

If you want to define an array simply assign the array to a variable name -

Nv = [0 5 1 17 4 8 9]
Nv = 1×7
     0     5     1    17     4     8     9

Walter Roberson 2023-7-30

在 MATLAB Online 中打开

It is not possible to use any of the following as an array index:

symbolic variables
symbolic expressions
symfun
symmatrix

So in the context of

symprod((Nv(j))*(Nb(i)),j,2,(i+1))

where j is a symbolic variable (otherwise you could not symprod() over it) then Nv and Nb could not be arrays and would have to be symfun instead. For example,

Nv(i) = piecewise(i == 1, 0, i == 2, 5, i == 3, 1, i == 4, 17, i == 5, 4, i == 6, 8, i == 7, 9);

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Answer 1

Torsten 2023-7-30

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2002442-using-of-symprod-fuction#answer_1280817

编辑：Torsten 2023-7-30

在 MATLAB Online 中打开

Nv=[0 5 1 17 4 8 9];
Nb=[1 1 1 1 1 1 1];
Sb=[25 28 32 36 35 38 0];
n = numel(Nv);
% Easy to follow
Z = 0;
for i = 1:n-1
   p = 1;
   for j = 2:i+1
     p = p*Nv(j)*Nb(i)*Sb(i);
   end
   Z = Z + p;
end
Z
Z = 7.3851e+13
% Harder to follow
Z = sum((Nb(1:n-1).*Sb(1:n-1)).^(1:n-1).*cumprod(Nv(2:n)))
Z = 7.3851e+13