Do MATLAB tables remove the need for dictionaries? - matlab

Question

MATLAB tables let you index into any column/field using the row name, e.g., MyTable.FourthColumn('SecondRowName'). Compared to this, dictionaries (containers.Map) seem primitive, e.g., it serves the role of a 1-column table. It also has its own dedicated syntax, which slows down the thinking about how to code.
I'm beginning to think that I can forget the use of dictionaries. Are there typical situations for which that would not be advisable?

Answer 1

TL;DR: No. containers.Map has uses that cannot be replaced with a table. And I would not choose a table for a dictionary.
containers.Map and table have many differences worth noting. They each have their use. A third container we can use to create a dictionary is a struct.
To use a table as a dictionary, you'd define only one column, and specify row names:
T = table(data,'VariableNames',{'value'},'RowNames',names);
Here are some notable differences between these containers when used as a dictionary:
Speed: The struct has the fastest access by far (10x). containers.Map is about twice as fast as a table when used in an equivalent way (i.e. a single-column table with row names).
Keys: A struct is limited to keys that are valid variable names, the other two can use any string as a key. The containers.Map keys can be scalar numbers as well (floating-point or integer).
Data: They all can contain heterogeneous data (each value has a different type), but a table changes how you index if you do this (T.value(name) for homogeneous data, T.value{name} for heterogeneous data).
Syntax: To lookup the key, containers.Map provides the most straight-forward syntax: M(name). A table turned into a dictionary requires the pointless use of the column name: T.value(name). A struct, if the key is given by the contents of a variable, looks a little awkward: S.(name).
Construction: (See the code below.) containers.Map has the most straight-forward method for building a dictionary from given data. The struct is not meant for this purpose, and therefore it gets complicated.
Memory: This is hard to compare, as containers.Map is implemented in Java and therefore whos reports only 8 bytes (i.e. a pointer). A table can be more memory efficient than a struct, if the data is homogeneous (all values have the same type) and scalar, as in this case all values for one column are stored in a single array.
Other differences:
A table obviously can contain multiple columns, and has lots of interesting methods to manipulate data.
A stuct is actually a struct array, and can be indexed as S(i,j).(name). Of course name can be fixed, rather than a variable, leading to S(i,j).name. Of the three, this is the only built-in type, which is the reason it is so much more efficient.
Here is some code that shows the difference between these three containers for constructing a dictionary and looking up a value:
% Create names
names = cell(1,100);
for ii=1:numel(names)
names{ii} = char(randi(+'az',1,20));
end
name = names{1};
% Create data
values = rand(1,numel(names));
% Construct
M = containers.Map(names,values);
T = table(values.','VariableNames',{'value'},'RowNames',names);
S = num2cell(values);
S = [names;S];
S = struct(S{:});
% Lookup
M(name)
T.value(name)
S.(name)
% Timing lookup
timeit(#()M(name))
timeit(#()T.value(name))
timeit(#()S.(name))
Timing results (microseconds):
M: 16.672
T: 23.393
S: 2.609

Answer 2

You can go simpler, you can access structs using string field:
clear
% define
mydata.('vec')=[2 4 1];
mydata.num=12.58;
% get
select1='num';
value1=mydata.(select1); %method 1
select2='vec';
value2=getfield(mydata,select2) %method 2