MATLAB: How to determine if the desired value in a vector is the Max or Min of the values around it

max or minvectors

I have the Vector x that I want to find the Max or Min such as:
x = [ 21 19 20 17 16 17 18 16 15 13 15 16]
min(concave up) = [19 16 13]
Max(concave down) = [20 18 ]
This is what I have been trying.
I do not know if their is a fuction already in MatLab, but I would like to do it using a script.
Thank you in advance.
y = [1 2 3 4 5 6 7 8 9 10 11 12];
x  = [21 19 20 17 16 17 18 16 15 13 15 16];
s = 1;
% this to get the Min
for i = 2:size(x,2)-1
    if x(i-1)< x(i) < x(i+1)
           z(S)= x(i);
           S = S + 1;
    end
    
end

Best Answer

Differentiate x and determine which values are falling (negative) or rising (positive). Local minima are where the pattern switches from negative to positive. Local maxima are where the pattern switchs from positive to negative. The first and last samples are not considered minima or maxima.
In the code below, localMinIdx and localMaxIdx are the index values of (x) locating the local mins and max's.
localMins and localMaxs are the local mins and max's. 
x  = [ 21 19 20 17 16 17 18 16 15 13 15 16]; 
localMinIdx = strfind([0,diff(x),1]<0,[1,0]); 
localMins = x(localMinIdx);  % = [19    16    13]
localMaxIdx = strfind([0,diff(x),1]>0,[1,0]); 
localMaxs = x(localMaxIdx);  % = [ 20    18]
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