Creating a vector from nested for loops

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Maaz Madha 2020-3-26

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https://ww2.mathworks.cn/matlabcentral/answers/513176-creating-a-vector-from-nested-for-loops

评论： Cris LaPierre 2020-3-26

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So I basically want to create a column vector from two nested for loops

Initially, i planned my code to be like

for i =1:n
    for j=1:m
        P=(j-1)*n+i;
        b(P,1)=T((i-1)*dx,(j-1)*dy); %%T is an anonymous function T=@(x,y) 20*cos(x)*sin(y);
    end
end

But when I run that I get a singular row vector (rank 1) and I can't use that for later on when I need to divide it by a matrix A.

What am I doing wrong over here?

Many thanks

回答（1 个）

Cris LaPierre 2020-3-26

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Two ideas come to mind.

for i =1:n
    for j=1:m
        b(i,j)=...
    end
end
b=b(:);

Another way might be to add the new result to the end of b

b=[];
for i =1:n
    for j=1:m
        b=[b; T((i-1)*dx,(j-1)*dy)]; 
    end
end

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Cris LaPierre 2020-3-26

Let's walk through the code.

i=1

j=1

store the current value of T in b(1,1)

i=1

j=2

store the current value of T in b(2,1)

...

i=1

j=m

store the current value of T in b(m,1)

i=2

j=1

What row of b should the value of T be stored in? It should be m+1, right?

Plug the current values of i and j into my equation for P.

P=(2-1)*m + 1 which equals m+1

Test this for the first case (i=1,j=1).

P=(1-1)*m + 1 or 1.

The outer loop increments one, then the inner loop increments though all it's loops. You wrote P for the opposite case - the outer loop running through all it's values and then the inner loop increments one.

Cris LaPierre 2020-3-26

As for P, you're doing something somewhere else that is not shown that is causing your error. You can read more about that here, but that's a separate issue that is not related to the question asked in this post.

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