MATLAB: interpolate vector

Question

How can I interpolate a vector in MATLAB?

For example, I have the following matrix:

M=    
 1 10  
 2 20  
 3 30  
 4 40  

The first column of M denotes the independent parameter of x coordinate while the second column of M denotes the output or y coordinate.

I also have the following input vector:

a =
 2.3  
 2.1  
 3.5  

For each value of a, I wish to determine what the output interpolated result would be. In this case, given a, I wish to return

23   
21   
35

Answer 1

Here's the answer to the question after the edit, i.e. "how to interpolate"

You want to use interp1

M = [1 10;2 20;3 30;4 40];
a = [2.3;2.1;3.5;1.2];

interpolatedVector = interp1(M(:,1),M(:,2),a)
interpolatedVector =
    23
    21
    35
    12

---

Here's the answer to the question "find the two closest entries in a vector", i.e. the original question before the edit.

x=[1,2,3,4,5]'; %'#
a =3.3;

%# sort the absolute difference
[~,idx] = sort(abs(x-a));

%# find the two closest entries
twoClosestIdx = idx(1:2);

%# turn it into a logical array
%#   if linear indices aren't good enough
twoClosestIdxLogical = false(size(x));
twoClosestIdxLogical(twoClosestIdx) = true;
twoClosestIdxLogical =
     0
     0
     1
     1
     0