Using ismember to get the corresponding elements

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Hari krishnan
Hari krishnan on 13 Nov 2018
Commented: Guillaume on 18 Nov 2018
Hi, I am using a function to convert the time to seconds based on specific input. 'time_data_observation_entry,real_time_vec,frame_vec are column vetors and frame_rate_played_video = 8. time_data_observation_entry have two columns. After performing the 'ismember' operation, i am not able to preserve the real_time corresponding to time_data_observation_entry. It looses the structure of the data. Any help to solve this will be appreciated
time_data_observation = load('time_data_observation.mat')
frame_rate_played_video = 8;
real_time_vec = load('real_time_vec');
frame_vec = load('frame_vec');
function [real_time] = time_from_videos_to_second_convertor(time_data_observation_entry,frame_rate_played_video,real_time_vec,frame_vec)
for ii = 1:length(time_data_observation_entry)
if isnan(time_data_observation_entry)
frame_id = floor(time_data_observation_entry*frame_rate_played_video);
real_time = real_time_vec(ismember(frame_vec,frame_id));

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Accepted Answer

Nick on 14 Nov 2018
Edited: Nick on 14 Nov 2018
Like Guillaume already mentioned you did not perform any indexing inside your loop and are just repeating the entire operation every time. What i assumed you wanted to do is:
function [real_time] = time_from_videos_to_second_convertor(time_data_observation_entry,frame_rate_played_video,real_time_vec,frame_vec)
real_time = nan(size(time_data_observation_entry));
for ii = 1:length(time_data_observation_entry)
if any(isnan(time_data_observation_entry)) % or all instead of any if you only wanna skip if both entries are nan
frame_id = floor(time_data_observation_entry(ii,:)*frame_rate_played_video);
real_time(ii,:) = real_time_vec(ismember(frame_vec,frame_id));
This will return a real_time vector, with the same format as time_data_observation but nans where they the time_data_observations are nans, you can filter them out of the result or inside the function using logical indexing.
  1 Comment
Guillaume on 18 Nov 2018
Never use length on a matrix. In this particular, length means size(time_data_observation_entry, 1). Depending on the height of that matrix, it could return size(time_data_observation_entry, 2).
I've not tried to understand the code fully in that answer. As far as I can tell, it's a slower and more complicated way of obtaining the same result as mine. Notice that my answer doesn't have any loop and only one call to ismember.

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More Answers (1)

Guillaume on 14 Nov 2018
I think I understand what you want, if so, you're using ismember the 'wrong way round':
function real_time = time_from_videos_to_second_convertor(time_data_observation_entry, frame_rate_played_video, real_time_vec,frame_vec)
frame_id = floor(time_data_observation_entry * frame_rate_played_video);
[found, where] = ismember(frame_id, frame_vec);
assert(all(found(:)), 'Some frames were not found in frame_vec');
real_time = real_time_vec(where);
Note that I made the above function error if a frame is not found in frame_vec. Otherwise, I'm not sure what you'd want to do. You could set the corresponding entry to NaN or remove the entire row. It's trivial to do either.

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