How to subtract rows when there is a value repeated more than once ?

I want to fill the interval item (Int_item) variable with the difference between the present trial (trial) and the past trial where the observation (obs) appeared. Leave it with a zero for those observations that are not repeated (appereance_number [ap_n] >1).

a =

270×9 table

         onset         ap_n    obs    response    eval    rtim    trial    class    Int_item
    _______________    ____    ___    ________    ____    ____    _____    _____    ________

    "_10:50:57.315"     1        1       1         1       886       1       0         0    
    "_10:51:21.293"     2        1       0         0         0       7       0         0    
    "_10:53:57.247"     3        1       4         1       825      46       0         0    
    "_10:51:01.313"     1        2       1         1       733       2       0         0    
    "_10:51:29.303"     2        2       4         1      1478       9       0         0    
    "_10:54:09.242"     3        2       4         1       948      49       0         0    
    "_10:51:05.311"     1        3       1         1       929       3       0         0    
    "_10:51:33.301"     2        3       4         1       904      10       0         0    
    "_10:54:25.237"     3        3       4         1      1687      53       0         0    
    "_10:51:09.310"     1        4       1         1       866       4       0         0    
    "_10:51:37.300"     2        4       4         1       758      11       0         0    
    "_10:54:33.234"     3        4       4         1       740      55       0         0    
    "_10:51:13.309"     1        5       1         1      1028       5       0         0    
    "_10:51:45.296"     2        5       4         1      1283      13       0         0    
    "_10:54:45.230"     3        5       4         1       811      58       0         0

It aiming to end up with something similar to this in the Int_item column.

a =

270×9 table

         onset         ap_n    obs    response    eval    rtim    trial    class    Int_item
    _______________    ____    ___    ________    ____    ____    _____    _____    ________

    "_10:50:57.315"     1        1       1         1       886       1       0         0    
    "_10:51:21.293"     2        1       0         0         0       7       0         6    
    "_10:53:57.247"     3        1       4         1       825      46       0         39    
    "_10:51:01.313"     1        2       1         1       733       2       0         0    
    "_10:51:29.303"     2        2       4         1      1478       9       0         7    
    "_10:54:09.242"     3        2       4         1       948      49       0         40    
    "_10:51:05.311"     1        3       1         1       929       3       0         0    
    "_10:51:33.301"     2        3       4         1       904      10       0         7    
    "_10:54:25.237"     3        3       4         1      1687      53       0         43    
    "_10:51:09.310"     1        4       1         1       866       4       0         0    
    "_10:51:37.300"     2        4       4         1       758      11       0         7    
    "_10:54:33.234"     3        4       4         1       740      55       0         44    
    "_10:51:13.309"     1        5       1         1      1028       5       0         0    
    "_10:51:45.296"     2        5       4         1      1283      13       0         8    
    "_10:54:45.230"     3        5       4         1       811      58       0         43

This is the code from what I'm trying:

    a = DATA{1,1}; % CREAT TEMP VAR 
    a.class = zeros(270,1);  %  add a new collumn 
    a.Int_item = zeros(270,1); %  add a new collumn 
    a = sortrows(a,[3 2]); % making it easy to find interval of items between rep
    for i = 2:height(a)
        a.Int_item = a.trial(i)-a.trial(i-1)
    end
  To assign to or create a variable in a table, the number of rows must match the height of the table.

I hope the question is clear.

All best,

Answer 1

[Ankita Bansal]

Hi, there are some errors in for loop. The size of a.Int_item is 270x1 while that of a.trial(i)-a.trial(i-1) is 1x1. Also, you have not inserted the condition of ap_n>1.

for i = 2:height(a) 
       if a.ap_n(i)>1
        a.Int_item(i) = a.trial(i)-a.trial(i-1);
        end
end

[Benjamin Dupre]

Hi Ankita, you are right. revising another answer I came around this expression to avoid that problem.

a_z = a.trial(2:end)-a.trial(1:end-1)