gr2idx is used with the Statistics Toolbox implementation of a categorical variable type. Unfortunately, while it was useful and ahead of the ultimate native categorical data type later introduced, it is now deprecated and use is discouraged.
Use the base categorical data type and findgroups combined with splitapply to process table data by groups:
gL1={'No Damage','1-9%','10-25%','26-50%','21-75%','Destroyed'};
gC1=categorical(gL1,gL1,'ordinal',1); % create ordinal categorical variable with names given
>> [g,id]=findgroups(gC1) % find the groups and the group id associated with...
g =
1 2 3 4 5 6
id =
1×6 categorical array
No Damage 1-9% 10-25% 26-50% 21-75% Destroyed
>>
ADDENDUM
Hmmm...I thought the computational routines ought to have been amended to handle ordinal categorical variables but apparently that was a unwarranted assumption.
To get the numeric values, just use
Table.Damage=double(Table.Damage_rate);
once you've created the categorical variable--make a new variable for it; no sense in destroying the other.
