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for loop to subtract every element from the first element for a given trial

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Mirthand
Mirthand on 29 Apr 2021
Commented: Mirthand on 30 Apr 2021
I have two columns with TrialNumber and Minutes in the table tt (please see attached).
I'm trying to get the Minutes to start from 0 for each of the trials. To do this, I am trying to subtract every element from the first element in a trial and that will restart time for each trial and have each trial start from 0.
I've attempted this with a for loop but it doesn't seem to be working:
allTrials = unique(tt.TrialNumber);
for ii = 1:length(allTrials)
bb = find(tt.TrialNumber == allTrials(ii));
for jj = 2:length(bb)
nn(jj) = tt.Minutes(bb(jj)) - tt.Minutes(bb(1));
end
end

Accepted Answer

dpb
dpb on 30 Apr 2021
Edited: dpb on 30 Apr 2021
Another job for varfun and grouping variables--
tt.TrialTime=cell2mat(rowfun(@(m)m-m(1),tt,'GroupingVariables','TrialNumber', ...
'InputVariables','Minutes','OutputFormat','cell'));
results in
>> head(tt)
ans =
8×3 table
TrialNumber Minutes TrialTime
___________ _______ _________
15.00 7.79 0.00
15.00 7.84 0.05
15.00 7.86 0.07
15.00 8.17 0.38
15.00 8.17 0.38
15.00 8.17 0.38
15.00 8.17 0.38
15.00 8.17 0.38
>>
To illustrate it "did the right thing" overall,
>> i30=find(tt.TrialNumber==30);
>> tt(i30:i30+7,:)
ans =
8×3 table
TrialNumber Minutes TrialTime
___________ _______ _________
30.00 16.15 0.00
30.00 16.16 0.01
30.00 16.36 0.21
30.00 16.66 0.51
30.00 16.66 0.51
30.00 16.66 0.51
30.00 16.66 0.51
30.00 16.66 0.51
>>
The dataset appears to be short of precision in the measured time -- many times are not distinguishable.

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