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

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.

[Jan]

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

Answer 1

[Jan]

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.

[Gennaro Arguzzi]

The error is:

Subscripted assignment dimension mismatch.

Error in Untitled (line 7)
immagine(i)=frame2im(f);

the whole code is:

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

[per isakson]

The error message says () and your code {}. Thus it's not the code you show that throws the error.

"I don't understand how use () and {}" The answer by Andrey Rubshtein at Stackoverflow is good.

[Gennaro Arguzzi]

I'm sorry @perisakson, I correct the code (the error referrint to (), not to {}).

[Jan]

@Gennaro Arguzzi: If immagine has not been created before, Matlab assumes, it is a double arrays. But then it tries to assign the image replied by frame2im to one element. This must fail. With the curly braces, each image is stored as a cell element.

This would work also:

immagine = uint8([]);
t = 0:0.1:10;   % No square brackets
for i=1:5
  plot(t,t.^i)
  f = getframe(gcf);
  immagine(:, :, :, i) = frame2im(f);
end

About the square brackets: https://www.mathworks.com/matlabcentral/answers/35676-why-not-use-square-brackets

[Gennaro Arguzzi]

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]

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

Clear now?

Again: It is a good programming practice to avoid unnecessary square brackets.