# Using multiple functions on a variable in a table

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Stef1998 on 26 Nov 2021
Commented: Dave B on 27 Nov 2021
I have a table (T) of data 365x3. With column variables A,B,C lets say. I am trying to create a new column (D) where 2 functions would be necessary for the new variable. I run into an error that the size of the left and right do not match. I have tried 3 different approaches with no success, shown below. I am not too sure how to fix this?
%Method 1
T.D = 1000 * (2834.1 - 0.29 * T.B - 0.004 * T.B.^(2.0));
idx = T.B >= 0;
T.D(idx) = 1000 * (2501 - 2.36 * T.B);
%Method 2
idx = T.B < 0;
T.D(idx) = 1000 * (2834.1 - 0.29 * T.B - 0.004 * T.B.^(2.0));
idx2 = T.B >= 0;
T.D(idx2) = 1000 * (2501 - 2.36 * T.B);
%Method 3
idx = T{:,'B'} < 0;
T{idx,'D'} = 1000 * (2834.1 - 0.29 * T{:,'B'} - 0.004 * T{:,'B'}.^(2.0));
idx2 = T{:,'B'} >= 0;
T{idx2,'D'} = 1000 * (2501 - 2.36 * T{:,'B'});

Stef1998 on 27 Nov 2021
A bit tedious and Im sure there might be a faster method out there but this solves the problem
T.D1 = 1000 * (2834.1 - 0.29 * T.B - 0.004 * T.B.^(2.0));
T.D1(T.B>=0) = 0;
T.D2 = 1000 * (2501 - 2.36 * T.B);
T.D2(T.B<0) = 0;
T.D = T.D1 + T.D2;

Dave B on 26 Nov 2021
Edited: Dave B on 26 Nov 2021
How about just initializing the new table variable first (e.g. with NaN values) and then populating it using your indices?
T = table(rand(100,1));
T.Var2 = nan(height(T), 1);
T.Var2(T.Var1<.5) = -1;
T.Var2(T.Var1>=.5) = 1
T = 100×2 table
Var1 Var2 ________ ____ 0.59525 1 0.89591 1 0.3016 -1 0.94513 1 0.38489 -1 0.3707 -1 0.20688 -1 0.066268 -1 0.07012 -1 0.97702 1 0.23667 -1 0.97863 1 0.56713 1 0.9572 1 0.93443 1 0.5219 1
##### 2 CommentsShowHide 1 older comment
Dave B on 27 Nov 2021
Your snippet was just missing the index in the right hand side
T = table(rand(100,1));
T.Var2 = nan(height(T), 1);
ind = T.Var1<.5;
T.Var2(ind) = 3.2*T.Var1(ind);
T.Var2(~ind) = 4.2*T.Var1(~ind)
T = 100×2 table
Var1 Var2 ________ ________ 0.61477 2.582 0.022891 0.073253 0.51972 2.1828 0.40734 1.3035 0.10431 0.3338 0.50238 2.11 0.69675 2.9263 0.82957 3.4842 0.41558 1.3298 0.12382 0.39623 0.50392 2.1164 0.9992 4.1967 0.61337 2.5762 0.69751 2.9295 0.96018 4.0328 0.82302 3.4567

R2021b

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