# Loop index is changed on a for loop

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Andrea Sbaragli on 21 May 2021
Commented: Image Analyst on 21 May 2021
Dear all, I've been trying to debug this issue but I m not able to solve it.
After having calculated the Euclidean distance between points I have to check whether or not it is below a certain threshold. If yes I save some statistics and I go for the next row. As soon as the Euclidean distance is higher than 2 I save in another table (c- index that must be different than i) these statistics per each coloumn in the row c :
1- date time associated to the first distance below to the threshold
2- date time associated to the last distance below to the threshold
3- date time = difference between 1 and 2
4- Mean distance
Of course, the index c increases by one in a different manner than i. I paste below my code.
function [report12,d12] = interactions(T1,T2, j, k, stamp1, stamp2)
T1 = array2table(T1);
T2 = array2table(T2);
%stamp1 = array2table(stamp1)
stamp2 = array2table(stamp2)
report12 =table;
d12 = array2table(zeros(size(stamp1)));
i = 1;
counter = 0;
n = 0;
c=1;
starter= 0;
start_time = NaT;
end_time = NaT;
duration = NaT;
mean_distance = 0;
stop_contact=0;
% reshape array size
T2(1:k-j,:)=[]; %same dim of T1
stamp2(1:k-j,:)=[];
for i= 1:height(T1)
d12{i,1} = (norm(T1{i,3:end} - T2{i,3:end} ));
if d12{i,1} <= 2.0
starter= starter +1;
counter = counter +d12{i,1};
n= n+1;
if starter == 1
start_time= stamp1(i,1)
end
if starter >1
end_time = stamp1(i,1)
duration = end_time - start_time;
mean_distance = counter/n;
end
end
if (d12{i,1} > 2.0) && (starter ~= 1)
report12{c,1} = start_time;
report12{c,2} = end_time;
report12{c,3} = duration;
report12{c,4} = mean_distance;
c=c+1;
counter = 0;
n=0;
starter=0;
end_time=NaT;
start_time=NaT;
%%duration= NaT;
mean_distance = 0;
%stop_contact = 1;
end
i= i+1;
end
end
##### 2 CommentsShowHide 1 older comment
Andrea Sbaragli on 21 May 2021
This is my output table report12: At this point I cannot understand why some row have null values since I fill the table only under this condition, setting starter again equal to 0:
if (d12{i,1} > 2.0) && (starter ~= 1)
report12{c,1} = start_time;
report12{c,2} = end_time;
report12{c,3} = duration;
report12{c,4} = mean_distance;
c=c+1;
counter = 0;
n=0;
starter=0;
end_time=NaT;
start_time=NaT;
%%duration= NaT;
mean_distance = 0;
%stop_contact = 1;
end

Geoff Hayes on 21 May 2021
Andrea - when you populate a row in a table, you reset the start and end times
end_time=NaT;
start_time=NaT;
but what guarantee is there on the next iteration of the loop, that the first condition will be satisfied so that you re-initialize these variables
for i= 1:height(T1)
d12{i,1} = (norm(T1{i,3:end} - T2{i,3:end} ));
if d12{i,1} <= 2.0
If the distance is greater than 2 then you will skip this if statement and then go immediately to the other and populate the row with these NaT values. Try confirming this behaviour with the MATAB Debugger.
Image Analyst on 21 May 2021
Andrea, you keep forgetting to attach your data. Make it easy for people to help you, not hard.
Here are the guidelines.
save('answers.mat', 'T1','T2', 'j', 'k', 'stamp1', 'stamp2');
Then attach answers.mat with the paperclip icon.