How can I find the index of a point in an array?

Hi! I have the following point: tmp = [-119.3 1042.5] and I want the index of this point in a grid. I have two separate grids, xdata and ydata and I'm trying to find the indexes, ipix and jpix. When I'm looking for ipix using

ipix = find(xdata == tmp(1,1))

I get the following:

ipix =

   Empty matrix: 0-by-1

The numbers on the xdata and ydata grids are the form -1.193846913204592e+02 and when I write

ipix = find(xdata == -1.193846913204592e+02)

I get

ipix =

         71361

which makes no sense. Can somebody help me with this? What I need is to find the index of the given point so I can find the displacement of the point in different images.

Thank you;

Answer 1

[Star Strider]

This is due to floating-point approximation error. You have to search within a range of tolerances.

Try something like this:

tol = 0.001;
[xrow,xcol] = find((xdata >= tmp(1,1)-tol) & (xdata <= tmp(1,1)+tol)); 
[yrow,ycol] = find((xdata >= tmp(1,2)-tol) & (xdata <= tmp(1,2)+tol));

For a more extended discussion, see Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero? (link)

[Agustin]

Thank you so much! I will try this.

[Star Strider]

My pleasure!

Also, the second line should be:

[yrow,ycol] = find((ydata >= tmp(1,2)-tol) & (ydata <= tmp(1,2)+tol));

My apologies for not that error and specifying ‘ydata’ earlier.

[Agustin]

Thanks!

[Star Strider]

My pleasure!