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

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

Answer 1

[dpb]

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.

[Mirthand]

Thank you!!! This is great!