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Error using sym/subsindex () while using the symsum function

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Salem Almahri
Salem Almahri on 24 Nov 2020
Edited: Stephan on 24 Nov 2020
Y = [-12.5j +12.5j 0 0 0 0 0 0;
+12.5j -16.649j +1.297j +1.815j +1.037j 0 0 0;
0 +1.297j 5.899+2.288j -5.899-5.984j 0 +2.399j 0 0;
0 +1.815j -5.899-5.984j 5.899+2.169j 0 0 +2j 0;
0 +1.037j 0 0 -3.537j 0 0 +2.5j;
0 0 +2.399j 0 0 -2.399j 0 0;
0 0 0 +2j 0 0 0.871-1.505j -0.871-0.495j;
0 0 0 0 +2.5j 0 -0.871-0.495j 0.871-2.005j];
V = [0 1+0.5j 1+0.5j 1+0.5j 1+0.5j 1+0.5j 1+0.5j 1+0.5j];
syms k
f = abs(V(4))*abs(V(k))*abs(Y(4,k))*sin(angle(Y(4,k))-angle(V(4)+V(k)));
E = subs (f, k, 0:7);
Q = sum(E);
I got the error in line(13), in the f equation, i dont know what is the reason for that, i tried several things and it still did not work, so if anyone know what is the problem exactly, and could help me, as i need to finish this as soon as possible

  1 Comment

Salem Almahri
Salem Almahri on 24 Nov 2020
Error using sym/subsindex (line 857)
Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments must be symbolic variables,
and function body must be sym expression.
This is the error i got

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Accepted Answer

Walter Roberson
Walter Roberson on 24 Nov 2020
You can never use an unresolved symbolic variable as a subscript.
You must always use definite numeric or logical values for subscripts (though table() extends this, it still does not permit symbolic).
Define your numeric k and then have the expression that uses it, making sure that you have vectorized the expression.

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More Answers (1)

Stephan
Stephan on 24 Nov 2020
Edited: Stephan on 24 Nov 2020
There are two possible ways in Matlab, that allow a syntax like
V(k)
The first is indexing with a variable like in a for loop:
> V = 5:10
V =
5 6 7 8 9 10
>> k = 3
k =
3
>> V(k)
ans =
7
The second one is using symbolic functions:
>> syms k
>> V(k) = 3*k + 1
V(k) =
3*k + 1
>> V(3)
ans =
10
What you do is mixing both possibilities, which is not allowed. So you have to rethink your code to what you want to do

  2 Comments

Salem Almahri
Salem Almahri on 24 Nov 2020
Thank You for the reply, but what i mean by V(k), is the kth number of the V array, or V(4) is the 4th integer in the V array that i defined above, so if i can not do V(k) what should i use, since i want to itterate the value of V from the array and sum them.
Stephan
Stephan on 24 Nov 2020
You define
syms k
and then use it to index an array V in the next line - this does not work.

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