changing a value in an array with matrix indices

hello all, I want to make such that

if true
i_i=zeros(48,1);
a_a=[3 9 11;25 2 4;5 7 1];
i_i(a_a(2,1) a_a(2,2) a_a(2,3))=1
end

it gives error of Unexpected MATLAB expression. if anyone knows please help thanks

[dpb]

It's a problem w/ the interface but please edit your question to remove the 'if true' and 'end' wrapper on your code. It shows up when one uses the CODE button w/ a blank area. It's simpler to just type two blanks in front of the first line of code instead...

Answer 1

[Roger Stafford]

You need to replace

 i_i(a_a(2,1) a_a(2,2) a_a(2,3))=1

with

 i_i([a_a(2,1),a_a(2,2),a_a(2,3)]) = 1;

(I assume you want to set the 25th, the 2nd, and the 4th elements of i_i to 1.)

[Metin]

thanks:)

[dpb]

But note in your case since it's the 2nd row, can use the colon operator and write just

 i_i(a_a(2,:))=1;

instead.

Answer 2

[dpb]

Other than you preallocated a 1D (column) array and then wrote access to it w/ three subscripts, the expression

i_i(a_a(2,1) a_a(2,2) a_a(2,3))=1;

would expand to be

i_i(25 2 4)=1;

That clearly isn't proper array addressing syntax as it's missing the commas between the three locations.

Now it comes to me that perhaps you intended to address the three elements in the vector instead of a 3D expression is more likely -- in that case what you need is to turn the row of a_a into a vector itself.

Explicitly it could be written as

 i_i([a_a(2,1) a_a(2,2) a_a(2,3)])=1;

but since it's the 2nd row, use the colon operator and write

 i_i(a_a(2,:))=1;

instead.