Updating one field in every element of a Matlab struct array

Question

Suppose I have a struct array arr, where each element has a bunch of fields, including one called val. I'd like to increment each element's val field by some constant amount, like so:

for i = 1:length(arr)     arr(i).val = arr(i).val + 3; end 

This obviously works, but I feel there should be a way to do this in just one line of code (and no for loop). The best I've come up with is two lines and requires a temp variable:

newVals = num2cell([arr.val] + 3); [arr.val] = deal(newVals{:}); 

Any ideas? Thanks.

Answer 1

Just a note, the deal isn't necessary there:

[arr.val] = newVals{:}; % achieves the same as deal(newVals{:}) 

The only other way I know how to do this (without the foor loop) is using arrayfun to iterate over each struct in the array:

% make a struct array arr = [ struct('val',0,'id',1), struct('val',0,'id',2), struct('val',0,'id',3) ]  % some attempts [arr.val]=arr.val; % fine [arr.val]=arr.val+3; % NOT fine :(  % works ! arr2 = arrayfun(@(s) setfield(s,'val',s.val+3),arr) 

That last command loops over each struct in arr and returns a new one where s.val has been set to s.val=3.

I think this is actually less efficient than your previous two-liner and the for loop though, because it returns a copy of arr as opposed to operating in-place.

(It's a shame Matlab doesn't support layered indexing like [arr.val]=num2cell([arr.val]+3){:}).

Answer 2

I like Carl's and mathematical.coffee's original ideas. I have multiple similar lines to express, so for concision of my mainline code, I went ahead and made the generic subfunction

function varargout = clist(in) varargout = {in{:}}; end 

then I could express each such line in a fairly readable way

[arr.var]  = clist(num2cell([arr.var]+3));   [arr.var2] = clist(num2cell([arr2.var]/5+33));