How to call the row number of an element?

suppose I find a value after applying some formula and then need to find the row/column in the matrix where the value appears.

How do I do this?

Answer 1

[Veronica Taurino]

%[row,col] = find(__)

https://it.mathworks.com/help/matlab/ref/find.html

For example:

X = [1 0 2; 0 1 1; 0 0 4]

X = 3×3

1 0 2 0 1 1 0 0 4

[row,col] = find(X==4)

row = 3

col = 3

Answer 2

[Matt J]

With find, but be mindful that direct logical indexing is often faster if you are seeking to modify the matrix. Compare:

%Replace all A(i,j)>=50 with 3

A=randi(100,5e3,5e3);

tic;
 I=find(A>=50);
 B0=A;
 B0(I)=3;
toc

Elapsed time is 0.501259 seconds.

tic;
 B=A;
 B(A>=50)=3;
toc

Elapsed time is 0.301809 seconds.

isequal(B0,B)

ans = logical
   1