Indexing error while using symsum inside a function.

Question

Bear Foot 2023-12-5

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2056349-indexing-error-while-using-symsum-inside-a-function

编辑： Walter Roberson 2023-12-5

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

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Chunru 2023-12-5

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

Bear Foot 2023-12-5

@Chunru, I am trying to get a value.

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Answer 1

Walter Roberson 2023-12-5

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2056349-indexing-error-while-using-symsum-inside-a-function#answer_1365744

在 MATLAB Online 中打开

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.

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Bear Foot 2023-12-5

移动：Walter Roberson 2023-12-5

在 MATLAB Online 中打开

@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 2023-12-5

编辑：Walter Roberson 2023-12-5

在 MATLAB Online 中打开

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

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