Indexing error while using symsum inside a function.

Does anyone know why I am recieving the indexing error below?

x = 0:0.01:5

x = 1×501

0 0.0100 0.0200 0.0300 0.0400 0.0500 0.0600 0.0700 0.0800 0.0900 0.1000 0.1100 0.1200 0.1300 0.1400 0.1500 0.1600 0.1700 0.1800 0.1900 0.2000 0.2100 0.2200 0.2300 0.2400 0.2500 0.2600 0.2700 0.2800 0.2900

y = [2 -1 3 -2 4 -3 2 -4 1 -2]

y = 1×10

2 -1 3 -2 4 -3 2 -4 1 -2

Test(x,y)

Error using indexing
Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments must be symbolic variables, and function body must be sym expression.

Error in solution>Test (line 7)
f = symsum(y(k)*sin(2*pi*k*x),k,1,size(y));

function f = Test(x,y)
syms k
f = symsum(y(k)*sin(2*pi*k*x),k,1,size(y));
end

[Chunru]

What is the desired output? A value or a symbolic expression?

[Bear Foot]

@Chunru, I am trying to get a value.

Answer 1

[Walter Roberson]

y = [2 -1 3 -2 4 -3 2 -4 1 -2]

Your y is a numeric vector.

Test(x,y)

you are passing that numeric vector as the second parameter to Test

function f = Test(x,y)

Internally, Test knows that numeric vector under the name y

syms k
f = symsum(y(k)*sin(2*pi*k*x),k,1,size(y));

You try to index that numeric vector with a symbolic variable. In MATLAB, you can never index an array at a symbolic variable.

What you need to do is

function f = Test(x,y)
    f = sum(y.*sin(2*pi*(1:numel(y)).*x));
end

That is, form a definite list of terms and sum() them.

[Bear Foot]

@Walter Roberson, I appreciate the help, are there any code adjustments I can make to fix the arrays have incompatible sizes error?

x = 0:0.01:5

x = 1×501

0 0.0100 0.0200 0.0300 0.0400 0.0500 0.0600 0.0700 0.0800 0.0900 0.1000 0.1100 0.1200 0.1300 0.1400 0.1500 0.1600 0.1700 0.1800 0.1900 0.2000 0.2100 0.2200 0.2300 0.2400 0.2500 0.2600 0.2700 0.2800 0.2900

y = [2 -1 3 -2 4 -3 2 -4 1 -2]

y = 1×10

2 -1 3 -2 4 -3 2 -4 1 -2

Test(x,y)

Arrays have incompatible sizes for this operation.

Error in solution>Test (line 5)
f = sum(y.*sin(2*pi*(1:numel(y)).*x));

function f = Test(x,y)
    f = sum(y.*sin(2*pi*(1:numel(y)).*x));
end

[Walter Roberson]

x = 0:0.01:5

x = 1×501

0 0.0100 0.0200 0.0300 0.0400 0.0500 0.0600 0.0700 0.0800 0.0900 0.1000 0.1100 0.1200 0.1300 0.1400 0.1500 0.1600 0.1700 0.1800 0.1900 0.2000 0.2100 0.2200 0.2300 0.2400 0.2500 0.2600 0.2700 0.2800 0.2900

y = [2 -1 3 -2 4 -3 2 -4 1 -2]

y = 1×10

2 -1 3 -2 4 -3 2 -4 1 -2

z = Test(x,y)

z = 1×501

0 -1.5185 -2.3932 -2.1983 -0.8690 1.2779 3.6519 5.5843 6.5444 6.3013 4.9798 2.9977 0.9134 -0.7586 -1.6974 -1.8442 -1.3737 -0.5903 0.2024 0.8070 1.1756 1.3802 1.5365 1.7183 1.9072 2.0000 1.8690 1.4455 0.7835 0.0671

plot(x, z)

function f = Test(x,y)
    f = sum(y(:).*sin(2*pi*(1:numel(y)).'.*x),1);
end