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How do I add a value to a field in each element of a struct array?

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Brandon Kuczenski
Brandon Kuczenski on 29 Apr 2014
Commented: Image Analyst on 29 Apr 2014
Say I have a struct array with a numeric field:
>> a=struct('bar',{47 52})
I want to increment that field across each element of the array, something like the following:
>> [a.bar] = [a.bar] + 1; % this fails
The nearest I can figure out is the following, which is cumbersome:
>> inc = num2cell([a.bar]+1);
>> [a.bar] = inc{:};
Is there a way to do this without creating an intermediate variable? Thanks in advance.

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Answers (1)

Image Analyst
Image Analyst on 29 Apr 2014
Well why are you messing around with cell arrays? Why make it way more complicated than it needs to be??? I don't see any reason for a cell array. I think you need to read the FAQ: http://matlab.wikia.com/wiki/FAQ#What_is_a_cell_array.3F
Try using just regular numerical arrays:
% Create structure.
a=struct('bar',[47 52])
a.bar % Report to command window.
% Add 1
a.bar = a.bar+1
a.bar % Report to command window.
In the command window:
a =
bar: [47 52]
ans =
47 52
a =
bar: [48 53]
ans =
48 53

  2 Comments

Brandon Kuczenski
Brandon Kuczenski on 29 Apr 2014
Your 'a' is not a struct array- it is a struct with one field whose contents are an array. In my version, a(1).bar = 47, a(2).bar=52, and a.bar(2) gives an error.
Image Analyst
Image Analyst on 29 Apr 2014
Sorry, I misunderstood. A fast, straightforward and intuitive solution is to simply use a for loop:
for k = 1:length(a)
a(k).bar = a(k).bar+1;
end
Don't believe the hype about a for loop being slow. It's not. Assuming you have less than several million structures in the array, it should be very fast.

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