Updating one field in every element of a Matlab struct array

Are all the fields in that struct scalar, or the same size? If so, the idiomatic Matlab way to do this is to rearrange your struct to be a scalar struct with arrays in each of its fields, instead of an array of structs with scalar values in the fields. Then you can do vectorized operations on the fields, like arr.val = arr.val + 3;. See if you can rearrange your data. Doing it this way is much more efficient in both time and memory; that's probably why Matlab doesn't provide convenient syntax for operating over fields of arrays of structs.

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));  

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){:}).