Can anyone tell what is this line doing

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Kamil Kacer
Kamil Kacer 2020-11-23
评论： Kamil Kacer 2020-11-23
It seems these two first lines two the exact same thing can anyone help
I cannot figure out what they are doing
a = d{1}(f(1,2));
a = f(1,2);
...
f = [1 2 4];
d = {[4 2 1] [4 1 2] [4 3 5]};
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Kamil Kacer
Kamil Kacer 2020-11-23
> d{1}(f(1,3))
Index exceeds the number of array elements (3).

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John D'Errico
John D'Errico 2020-11-23
编辑：John D'Errico 2020-11-23
Ran in:
You asked again, why you cannot use f(1,3), instead of f(1,2).
LOOK CAREFULLY AT WHAT YOU HAVE. TAKE IT ONE STEP AT A TIME.
f = [1 2 4];
d = {[4 2 1] [4 1 2] [4 3 5]};
Now, what is f(1,3)? TRY IT!
f(1,3)
ans = 4
It is the number 4. How are you using it? As an index into a cell of the cell array d. So what is d{1}?
d{1}
ans = 1×3
4 2 1
●
Does d{1} have a 4th element? NO.
Why was this not a problem with f(1,2)? Because f(1,2) is 2.
d{1}(f(1,2))
ans = 2
d{1}(f(1,3))
Index exceeds the number of array elements (3).
The latter case fails because you cannot access the 4th element of a vector that has only 3 elements.
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John D'Errico
John D'Errico 2020-11-23
Excellent.
For most problems in MATLAB where you need to understand a complicated line of code, you take apart the line from the inside and work out. Figure out what the inside piece is doing. Then look backwards and outwards. In this case, it is perhaps not fully obvious, that (f(1,2)) was an index into what came before.

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KSSV
KSSV 2020-11-23
f = [1 2 4]; % this is an array can be access by f (i) where i = 1,2,3
d = {[4 2 1] [4 1 2] [4 3 5]}; % this is a cell array, can be access by d {i} where i = 1,2,3

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