I am not certain what you intend by ‘numbers’, so guessing:
D = load('TargetSet.mat');
TargetSet = D.TargetSet;
ActivityCode = table(findgroups(TargetSet.Activity), 'VariableNames',{'ActivityCode'});
TargetSet = [TargetSet, ActivityCode];
First15Rows = TargetSet(1:15,:)
producing:
First15Rows =
Activity ActivityCode
___________ ____________
{'LAYING' } 1
{'LAYING' } 1
{'LAYING' } 1
{'LAYING' } 1
{'LAYING' } 1
{'LAYING' } 1
{'LAYING' } 1
{'LAYING' } 1
{'LAYING' } 1
{'LAYING' } 1
{'SITTING'} 2
{'LAYING' } 1
{'LAYING' } 1
{'LAYING' } 1
{'LAYING' } 1
If you want a different result, please describe it in some detail.