flatten matlab table by key

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

1. it doesn't output a different table
2. 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