{} and () brackets - creating array in for loop

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Gennaro Arguzzi 2017-6-27

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https://ww2.mathworks.cn/matlabcentral/answers/346509-and-brackets-creating-array-in-for-loop

评论： Jan 2017-6-29

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Hi everyone. I read the following answer on stack exchange:

https://stackoverflow.com/questions/9055015/difference-between-accessing-cell-elements-using-and-curly-braces-vs-par

Why this code work:

t=[0:0.01:5];
a=[0:1:5];
n=length(a);
for i=1:n
    plot(t,t.^a(i));
    M(i)=getframe(gcf);
end

while this does not work:

t=[0:0.1:10];
for i=1:5
    plot(t,t.^i)
    f=getframe(gcf);
    immagine(i)=frame2im(f);
end

but immagine{i}=frame2im(f); works?

I don't understand how use () and {}. In M(i) the () brackets work, while in immagine(i) don't work.

Thank you very much for your help.

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Jan 2017-6-27

"Works" and "doesn't work" are lean descriptions. Post the error message instead. Note that it contains important information.

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Answer 1

Jan 2017-6-27

0
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https://ww2.mathworks.cn/matlabcentral/answers/346509-and-brackets-creating-array-in-for-loop#answer_272155

在 MATLAB Online 中打开

It depends on how the variable is defined before the loop:

clear('M');  % Now M is not defined and the first access determines the type
t = 0:0.01:5;
a = 0:1:5;
for i = 1:length(a)
  plot(t,t.^a(i));
  M(i) = getframe(gcf);
end

You could initialize M by this also:

M = {};

Then the assignment M(i) must fail, but M{i} is correct. Cell arrays are indexed using curly braces, all other arrays use round parenthesis.

If immagine(i) fails, it is immagine was a cell array before. Then:

immagine{i} = frame2im(f);

or

immagine(i) = {frame2im(f)};

works, whereby the first is more efficient: The latter creates a scalar cell array and assigns it.

Please read the documentation about cells: doc cell The Getting Started chapters of the documentation are very useful also.

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Gennaro Arguzzi 2017-6-28

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In the case of:

M(i) = getframe(gcf);

why does the code work?

 clear all
close all
t=[0:0.01:5];
a=[0:1:5];
n=length(a);
for i=1:n
    plot(t,t.^a(i));
    M(i)=getframe(gcf);
end

Maybe this behaviour is related to the natur of the output of getframe and frame2im? In the right case, why?

Jan 2017-6-29

在 MATLAB Online 中打开

While

immagine(i)=frame2im(f)

tries to assign an array to a scalar,

M(i)=getframe(gcf)

assigns a scalar struct to a scalar. Try this:

clear('A', 'B', 'C')
A(1) = 15   % Now A is a double
tmp.CData = rand(640, 480, 3);
tmp.Map   = [];
B(1) = tmp;  % Now B is a struct
C(1) = [15, 16]   % Error: cannot assign vector to scalar