Merge cell array of maps by key in Matlab

Question

Given a cell array of `containers.Map` objects containing counts (values) for given events (keys). I would like to get a single map containing the sum of the counts (as values) for each key (or something allowing me to calculate the counts). For example: ``` maps = { containers.Map({'a','b'},{1,2}), containers.Map({'b','c'},{4,1}) }; mergeMaps(maps) ans = Map(a -> 1, b -> 6, c -> 1) ``` The only thing I could come up with so far is: ``` maps = { ... }; res = containers.Map(); for cMapC = maps cMap = cMapC{1}; for cKeyC = keys(cMap{1}) cKey = cKeyC{1}; if isKey(res, cKey) res(cKey) = res(cKey) + cMap(cKey); else res(cKey) = cMap(cKey); end end end ``` Which should work (untested), but whose "efficiency" and readability is questionable to say the least. What is a better solution? Please note that currently there is no index attributed to events, so using sparse matrices instead of maps is cumbersome.

Accepted Answer

Not sure about readability, but it is probably more efficient:

```
allKeys0   = cellfun(@keys, maps, 'UniformOutput', false);
[allKeys, ~, m] = unique([allKeys0{:}]);
allValues0 = cellfun(@values, maps, 'UniformOutput', false);
allValues = cell2mat([allValues0{:}]);
sumValues = arrayfun(@(x) sum(allValues(m==x)), 1:numel(allKeys));

mergedMap = containers.Map(allKeys, sumValues);
```

Merge cell array of maps by key in Matlab

Solution

Problem