Dictionary / Map with array as key

Hi there,

I'm looking for a way to initialize a map with an array as Key.

I am thinking of something like

map = containers.Map;
state = {1,2};
map(state) = 0;

However, the last line fails with

Error using containers.Map/subsasgn
Specified key type does not match the type expected for this container.

Is there any way I can initialize a Map with an array (not converted to char) as keys?

Or is there any other appropriate data structure for doing this?

Looking forward to your ideas;)

[Walter Roberson]

The array is to be a single key, right?

[Verena Heußer]

Yes, I want the key to be an array.

Answer 1

[Mehmed Saad]

EDIT: First Answer is replaced

I donot understand why you dont want to go on characters. see the code below

M = containers.Map('KeyType','char','ValueType','double');
%% Single Key
key1 = [1.8 2.0 0.005];
valueSet = 327.2;
%% array key encoder
keySet =  key_enc(key1); 

M = containers.Map(keySet,valueSet);
%% array key decoder
key1_dbl =  key_dec(M.keys);
assert(isequal(key1_dbl,key1));

where key_enc is double array to character converter. It basically converts double precision to hexadecimal (so that we dont lose any data due to rounding off)

Similarly key_dec is character to array double converter in order to retreive the key

For 2D Array

M = containers.Map('KeyType','char','ValueType','double');
%% More than 1
key1 = [1.8 2.0 0.005;4 5 6];
valueSet = [327.2 550];
%% array key encoder
keySet =  key_enc(key1) 

M = containers.Map(keySet,valueSet);
%% array key decoder
key1_dbl =  key_dec(M.keys);
assert(isequal(key1_dbl,key1));

Point to remember

1. a row is a complete key (i.e. a single row vector is considered as 1 key)
2. key must be double precision (or any other 64 bit type)
3. if you provide a 2D matrix, the number of rows represent number of keys
4. key_inc can take both 1D and 2D input, it will return scalar output for 1D row vector. It's output type is cell array of character vectors
5. key_dec take cell array of character vector as input and will return the double key in that format

Functions

Key Encoder

function key1_char =  key_enc(key1)

[m,n] = size(key1);

key1_char=repmat({char(ones(1,16*n))},m,1);
tmp = num2hex(key1);
for ii=1:m  
key1_char{ii,:} = reshape((tmp(ii:m:end,:).'),1,16*n);
end
end

Key Decoder

function key1_dbl =  key_dec(key1_char)

m = numel(key1_char);
n = numel(key1_char{1});
key1_dbl = zeros(m,n/16);
for ii=1:m
key1_dbl(ii,:) = transpose(hex2num(transpose(reshape(key1_char{ii},16,n/16))));
end
end

Edit:

it is not necessary that assert works everytime because map gives sorted key out

For example

rng(2017)
key1 = randn(2,10);
valueSet =[1 2];
%% array key encoder
keySet =  key_enc(key1)

keySet =

2×1 cell array

{'c00151bd6e3dafe9bfcefd29436d4...7b0bfe72b879a67bbee'}

{'3ff0c59e4de6586abff94c4bcb1ceb...ba03fd58526d0d66e84'}

Now see that they are not sorted in ascending order while

M = containers.Map(keySet,valueSet);
M.keys

ans =

1×2 cell array

{'3ff0c59e4de6586abff94c4bcb1ceb433fd72f384820035ebff29c1…'} {'c00151bd6e3dafe9bfcefd29436d4de93fe6733364db1b1fbfe17d1…'}

so if now you check them error will occur (assertion failed)

[Verena Heußer]

Hi:) Thanks for your answer. I guess it doesen't solve my problem, as I'd need arrays to be the keys in the map.

[Mehmed Saad]

Ok

I have another idea.

Suppose that the array key is of size 8 and of type uint8

then

rng(2019)
key1 = randi([0 255],1,8,'like',uint8(1));

key1 =

1×8 uint8 row vector

231 100 159 163 225 76 179 231

Now key is an array but before feeding

just need a typecast

map = containers.Map('KeyType','uint64','ValueType','double')
state = typecast(key1,'uint64');
map(state) = 0;

[Verena Heußer]

Hi:)

Thanks for the answer! This works fine for ints.

But is there also a way to insert a list of doubles as key?

Such as

key1 = [1.8 2.0 0.005]

[Walter Roberson]

No. Keys can be numeric scalars or character vectors or string() scalars, and cannot be cells and cannot be numeric vectors or non-scalar string objects.

[Mehmed Saad]

see the updated answer for double array

Answer 2

[Walter Roberson]

It isn't nice but ...

A container.Map can be a value. So you can create a top level container.Map that has a key that is 1.8, and the associated value is a second container.Map . The second container you would have a key that is 2.0 and associated value is a third container.Map . That third container.Map would have a key 0.005 and value would be whatever you were trying to associate as a value for key1 .

I would not expect this to be efficient. For efficiency, you would probably be better of doing something like an MD5 of key1 and use that as the key. Or see https://www.mathworks.com/matlabcentral/answers/534528-how-to-save-serialize-matlab-variables-to-a-byte-string#answer_439768 with the idea being that you would apply that to key1 and use the result as the key -- the advantage there would be that you would be able to look at the key list and deserialize the key list entries to find out what the resulting numeric keys were.

Answer 3

[Verena Heußer]

Dear all,

thanks for helping me out :)

I just wanted to share the solution I've come up with (don't know if it's the most efficient / elegant, but it totally serves my purposes).

So, I wanted to have arrays (1xn vectors) as keys, while the values are supposed to be real values.

I've decided to use the java.util.hashtable:

map = java.util.Hashtable;
state = [1, 2];
map.put(num2str(state), 0); 

map.get(num2str(state))
>> ans =

     0