cell array expansion
27 views (last 30 days)
Show older comments
I have just been bitten by some careless coding, but I am surprised that MATLAB lets me do it and that mlint didn't provide a warning. Why does MATLAB let you do this:
x = {1,2};
y = x{:};
what I wanted to do (and I am sure there are a number of other ways of doing it) was
y = [x{:}];
I can see the advantage of cell array expansion for referencing and parameter passing. For example,
z = magic(5);
z(x{:})
xy = {1:10, 0:9};
plot(xy{:});
Is there any reason for
y = x{:};
to be valid. I feel like it should return an error about the RHS returning more arguments than the LHS.
0 Comments
Accepted Answer
Jan
on 21 Jul 2011
This is the standard behaviour of Matlab:
x = {1, 2};
y = x{:}; % y is x{1}
You see the same method for:
a = rand(1, 10);
b = max(a);
Why is this equivalent? Because MAX replies 2 outputs as "x{:}" and if just the 1st is caught, the 2nd is ignored. Therefore these methods are equivalent also:
x = {1, 2};
[y1, y2] = x{:}
a = rand(1, 10);
[b1, b2] = max(a)
So I would not expect an MLint-warning for a standard behaviour.
More Answers (2)
Sean de Wolski
on 21 Jul 2011
One reason: For a function that takes varargin you can feed it all contents of a cell as a separate input.
x = {1,2,3,4}
cat(3,x{:})
Yes, it frustrates me some times too!
3 Comments
Sean de Wolski
on 21 Jul 2011
I disagree; that functionality is quite useful. What if the cell contains different sized elements? Concatenating them as you have done will fail.
Fangjun Jiang
on 21 Jul 2011
Maybe one way to explain it is to treat it as the variable output argument. Like,
MaxValue=max(1:10);
[MaxValue,Index]=max(1:10);
You can do:
x={1,2};
y=x{:}
[a b]=x{:}
0 Comments
See Also
Categories
Find more on String Parsing in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!