how to read the data from vector

Question

Chaoyang Jiang 2018-3-27

评论： Chaoyang Jiang 2018-3-28

a=randi(10000,[10000,1]);
b=randi(10000,[10000,1]);
c=randi(100,[10000,10000]);

My question is how to find the row index of c where the c equals 5 (a is the x coordinate and b is the y coordinate).

I can do it using loop:

index=zeros(length(a),1);
for i=1:length(a)
index(i)=(c(a(i),b(i))==5);
end
index1=find(index);

May I know how to do it more efficiently whithout the use of for loop?

Answer 1

Walter Roberson 2018-3-27

You cannot. Your a values are up to 10000 but your c array is only 1000 on a side.

If you were to fix the ranges of a and b then

d = c(sub2ind(size(c), a, b));

Walter Roberson 2018-3-28

You changed your question.

index1 = find( c(sub2ind(size(c), a, b)) == 5);

Chaoyang Jiang 2018-3-28

Thank you very much for your answer!!