flatten matlab table by key - matlab

Question

I have a large table whose entries are
KEY_A,KEY_B,VAL
where KEY_A and KEY_B are finite sets of keys. For arguments sake, we'll have 4 different KEY_B values and 4 different KEY_A values. And example table:
KEY_A KEY_B KEY_C
_____ _____ _________
1 1 0.45054
1 2 0.083821
1 3 0.22898
1 4 0.91334
2 1 0.15238
2 2 0.82582
2 3 0.53834
2 4 0.99613
3 1 0.078176
3 2 0.44268
3 3 0.10665
3 4 0.9619
4 1 0.0046342
4 2 0.77491
4 3 0.8173
4 4 0.86869
4 5 1
I want to elegantly flatten the table into
KEY_A KEY_B_1 KEY_B_2 KEY_B_3 KEY_B_4 KEY_B_5
_____ _________ ________ _______ _______ _______
1 0.45054 0.083821 0.22898 0.91334 -1
2 0.15238 0.82582 0.53834 0.99613 -1
3 0.078176 0.44268 0.10665 0.9619 -1
4 0.0046342 0.77491 0.8173 0.86869 1
I'd like to be able to handle missing B values (set them to a default like -1), but I think if I get an elegant way to do this to start then such things will fall into place.
The actual table has millions of records, so I do want to use a vectorized call.
The line I've got (which doesn't handle int invalid 5) is:
cell2mat(arrayfun(#(x)[x,testtable{testtable.KEY_A==x,3}'],unique(testtable{:,1}),'UniformOutput',false))
But
it doesn't output a different table
If there are missing keys in the table, it doesn't handle that
I would think that this isn't that uncommon of an activity...has anyone done something like this before?

Answer 1

If the input table is T, then you could try this for the given case -
KEY_B_ =-1.*ones(max(T.KEY_A),max(T.KEY_B))
KEY_B_(sub2ind(size(KEY_B_),T.KEY_A,T.KEY_B)) = T.KEY_C
T1 = array2table(KEY_B_)
Output for the edited input -
T1 =
KEY_B_1 KEY_B_2 KEY_B_3 KEY_B_4 KEY_B_5
_________ ________ _______ _______ _______
0.45054 0.083821 0.22898 0.91334 -1
0.15238 0.82582 0.53834 0.99613 -1
0.078176 0.44268 0.10665 0.9619 -1
0.0046342 0.77491 0.8173 0.86869 1
Edit by MadScienceDreams: This answer lead me to write the following function, which will smash together pretty much any table based on the input keys. Enjoy!
function [ OT ] = flatten_table( T,primary_keys,secondary_keys,value_key,default_value )
%UNTITLED Summary of this function goes here
% Detailed explanation goes here
if nargin < 5
default_value = {NaN};
end
if ~iscell(default_value)
default_value={default_value};
end
if ~iscell(primary_keys)
primary_keys={primary_keys};
end
if ~iscell(secondary_keys)
secondary_keys={secondary_keys};
end
if ~iscell(value_key)
value_key={value_key};
end
primary_key_values = unique(T(:,primary_keys));
num_primary = size(primary_key_values,1);
[primary_key_map,primary_key_map] = ismember(T(:,primary_keys),primary_key_values);
secondary_key_values = unique(T(:,secondary_keys));
num_secondary = size(secondary_key_values,1);
[secondary_key_map,secondary_key_map] = ismember(T(:,secondary_keys),secondary_key_values);
%out =-1.*ones(max(T.KEY_A),max(T.KEY_B))
try
values = num2cell(T{:,value_key},2);
catch
values = num2cell(table2cell(T(:,value_key)),2);
end
if (~iscell(values))
values=num2cell(values);
end
OT=repmat(default_value,num_primary,num_secondary);
OT(sub2ind(size(OT),primary_key_map,secondary_key_map)) = values;
label_array = num2cell(cellfun(#(x,y)[x '_' mat2str(y)],...
repmat (secondary_keys,size(secondary_key_values,1),1),...
table2cell(secondary_key_values),'UniformOutput',false),1);
label_array = strcat(label_array{:});
OT = [primary_key_values,cell2table(OT,'VariableNames',label_array)];
end