I can not find an error in the expression

Question

Ilya Kotov 2019-10-24

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编辑： Jim Riggs 2019-10-25

Hi all,

I'm new to matlab, so sorry for the stupid question.

I tried to fix the errors, but could not find another. I give the code of the expression that I want to get in Python and ask to fix this expression for matlab. Also, if not difficult, please share some good materials for learning matlab.

Thanks!

In MatLab with error:

fun = @(r,t,p) r.^2*sin(p).*(exp(i*(r.*sin(t).*cos(p) + r.*sin(t).*sin(p) + r.*cos(t))))*4*pi/(r.^2 - 4)

q = integral3(fun,0,1.99,0,pi,0,pi*2)

Error using * Incorrect dimensions for matrix multiplication. Check that the number of columns in the first matrix matches the number of rows in the second matrix. To

perform elementwise multiplication, use '.*'.

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Answer 1

Katie 2019-10-24

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在 MATLAB Online 中打开

Hi! It looks like you're really close! You're just missing a couple dots in front of some of your multiplication and division operators. Below is the fixed expression. I also added in a couple sets of parentheses.

fun = @(r,t,p) (r.^2).*sin(p).*(exp(i*(r.*sin(t).*cos(p) + r.*sin(t).*sin(p) + r.*cos(t)))).*((4*pi)./(r.^2 - 4))

Summary of changes:

placed the first term r.^2 in parenteses
Added a dot in front of the first sin()
added a dot in front of the last division operator
added parentheses to the last term as below:

((4*pi)./(r.^2 -4))

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Jim Riggs 2019-10-24

编辑：Jim Riggs 2019-10-24

Note that here it is assumed that t, r and p are all vectors of the same length. If not, this also will not work and will generate a similar error message due to incorrect matrix dimentions

Ilya Kotov 2019-10-24

Thanks a lot, it's hard to get used to the strict syntax with tensors when you write python code

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Answer 2

Jim Riggs 2019-10-24

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/487321-i-can-not-find-an-error-in-the-expression#answer_398127

编辑：Jim Riggs 2019-10-25

It's hard to say without seeing how the variables r, t, and p are defined. If r and p are vectors of different length, then the expression:

r.^2*sin(p)

will cause a problem, because it implies a matrix multiplication of a vector of length r and one of length p. (here r.^2 is a vector with the same length as r, and sin(p) is a vector with the same length as p)

In Matlab, if you want to multiply vector r and vector p, they both must be the same length, and one must be a row vector, and the other a column vector. For example, if r and p are the same size but both are row vectors, then r*p will give an error (This works in python, but not in matlab, although in python, this will perform element-wise multiplication). To perform the matrix multiply in matlab you would have to transform one of them:

r*p'

If you want to perform element-wise multiplication, r and p must still be the same length, and you use .*

r.*p

or

r.^2.*sin(p)

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Katie 2019-10-24

在 MATLAB Online 中打开

For this particular case, you don't need to define r, t,and p. They are terms in the expression 'fun' that is being integrated over from 0<r<1.99, 0<t<pi, and 0<p<2pi. When you run the two lines of code you get the following:

fun = @(r,t,p) (r.^2).*sin(p).*(exp(i*(r.*sin(t).*cos(p) + r.*sin(t).*sin(p) + r.*cos(t)))).*((4*pi)./(r.^2 - 4))
q = integral3(fun,0,1.99,0,pi,0,pi*2)
%output:
q =
   1.1211e-07 - 1.2308e+02i