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

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

[Salem Almahri]

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

Answer 1

[Walter Roberson]

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.

Answer 2

[Stephan]

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

[Salem Almahri]

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]

You define

syms k

and then use it to index an array V in the next line - this does not work.