Finding a noninteger value from an array

I have an array which looks like:

 x = linspace(0,1,101);

To find the index of the maximum point from an array, I can write:

 [val,ind] = max(x);

However, I need the index of a particular point, let's say 0.70. How should i write the commands to find this?

My second question is that I have a specific value y = 0.703. I need to do my calculation using the array x, but I have to start from the value in x that is closest to y. Which means I am looking for the index of the point 0.70 in x, but i need to express it in terms of y in my code. How can I do that?

Answer 1

[Matt J]

Both questions have the same answer

 [~,loc]=min(abs(x-y))

Answer 2

[Kye Taylor]

To do (1) and (2) try

y = 0.7; % or y = 0.73;
[~,idx] = min(abs(x-y)); 
% idx contains the index of the value in x closest to y

Answer 3

[Matthew Murphy]

Given the array x, you could also use the find command.

 find(x == .7) --> ans = 71

For the second part, you could try a slightly different way with the same command.

Given some plus/minus window of .01, you could do this:

 find(x < .703 + .01 & x > .703 - .01 ) --> ans = 71 72

Now, there are multiple values, so you could use a while loop with smaller and smaller margins until one value is found (with a catch to widen the margin if no values are found).

[I know this is an old post, but I still wanted to follow up with an approach that worked for me.]

[Matt J]

Using find() without floating point tolerances can be hit or miss, for example,

    >> x=0:0.3:1

    x =

             0    0.3000    0.6000    0.9000

>> find(x==(1-.7))

ans =

     Empty matrix: 1-by-0

That is why I recommended the use of min() for both scenarios raised by the OP,

    >> [~,loc]=min(abs(x-(1-.7)))

    loc =

         2

[Matthew Murphy]

I think my version would work if you had the explicit value (eg .3) used in find.

 x=0:0.3:1
 x =
         0    0.3000    0.6000    0.9000

 find(x == .3)
 ans =
     2

You are definitely right with the tolerance issue, no problem there. In my application, I had points to search for, so I had exact values.