keep element greater than immediate previous element

Hi,

x = [1 2 3 4 3 2 3 4 6 8 5 5 6 8.5 9 11 12 ]; I want to keep one vector whereby the next element is greater than the immediate previous element. keep = [[1 2 3 4 6 8 8.5 9 11 12 ]; to discard if the element is smaller than or equal to the immediate previous element. discard = [3 2 3 4 5 5 6] (the bold elements in x).

I am using loop to do that and I didn't get what I want. could help modify my code or advise any alternative way? thanks in advance!

clc; clear; close all

x = [1 2 3 4 3 2  3 4 6 8  5  5 6 8.5  9  11 12 ];
m = length(x);
k=1; n = 1; i =1; j=2;
while i < m-1  & j < m      
        if x(j) >= x(i)
         keep(n) = x(j);   
          n=n+1; 
          i=i+1;
          j=j+1;
        elseif x(j) < x(i)
            discard(k) = x(j);
            k = k+1;
            i=i;
            j=j+1;
        end
    
end

Answer 1

[Thiago Henrique Gomes Lobato]

This loops does what you want:

x = [1 2 3 4 3 2  3 4 6 8  5  5 6 8.5  9  11 12 ];
m = length(x);
keep = [x(1)];
discard =[];
for idx=2:m
    if x(idx)>keep(end)
        keep = [keep,x(idx)];
    else
        discard = [discard,x(idx)];
    end
    
end
keep
discard
keep =

    1.0000    2.0000    3.0000    4.0000    6.0000    8.0000    8.5000    9.0000   11.0000   12.0000


discard =

     3     2     3     4     5     5     6

Although, depending of your goal, simply sorting the array could solve the problem in a more simplified way:

keep = unique(sort(x))

keep =

    1.0000    2.0000    3.0000    4.0000    5.0000    6.0000    8.0000    8.5000    9.0000   11.0000   12.0000

[HYZ]

Can I ask another question? I am trying the way you suggested. my data are in cell arrays. X is a cell array which has a few vectors. I want to use the loop you suggested for each vector in cell array. I got an error "Operator '>' is not supported for operands of type 'cell'" ... could you suggest why > doesn't work in cell array? Thanks.

for i = 1: length(X)
    keep{i}(1) = [X{i}(1)];
    for idx = 2: length(X{1})
        if X{i}(idx) > keep{i}(end)
          keep{i} = [keep{i},X{i}(idx)];
        end
    end
end

[Thiago Henrique Gomes Lobato]

In mine matlab version it works fine, it can be that older versions have some problems with double indexing in cell array. One thing you could try is to remove the variables from the cell whenever possible until you get no error anymore, ex:

x1 = [1 2 3 4 3 2  3 4 6 8  5  5 6 8.5  9  11 12 ];
x2 = [1 2 3 4 3 2  3 4 6 8  5  5 6 8.5  9  11 12 ];

X = cell(2,1);
X{1} = x1;
X{2} = x2;
keep = cell(2,1);
for i = 1: length(X)
    keep{i}(1) = [X{i}(1)];
    actualX    = X{i};
    for idx = 2:length(actualX)
        actualkeep = keep{i};
        actualkeep = actualkeep(end);
        
        if actualX(idx) > actualkeep
          keep{i} = [keep{i},X{i}(idx)];
        end
    end
end

Although I would remain with the vectorized solution I suggested if all you want is the keep array

Answer 2

[per isakson]

An alternative

%%
x = [1 2 3 4 3 2  3 4 6 8  5   5  6  8.5  9  11 12 ];
%%
dx = 1;
while not( isempty( dx ) )
    dx = find((diff(x)<=0));
    x(dx+1)=[];
end
disp( x )  

output

  Columns 1 through 5
            1            2            3            4            6
  Columns 6 through 10
            8          8.5            9           11           12
>>