extract multiple slices of difference sizes from a vector

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LH 2024-7-10，14:10

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评论： Aquatris 2024-7-10，15:14

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Hi all,

I have a vector A of the shape:

A = [1 4 6 5 4 8 7 8 6 4];

and I want to split this vector into slices. The number of slices is 3 and each slice contains a certain number of elements as follows:

num_groups = 3;
num_points_in_each_group = [2 3 5];

I want the result to be:

. I have tried with the code below, but it's not working:

for i = 1:num_groups
    end_count = num_points_in_each_group(i);
    if i == 1
        start_count = 1;
    else
        start_count = num_points_in_each_group(i-1) + 1;
    end
    A_slice = A(start_count:end_count);
end

Any help would be appreicted.

Thanks

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Aquatris 2024-7-10，15:14

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Here is a simple modification to your code to work as intended to increase your understanding hopefully.

Since you are new, for loops might be tempting since it is easy to understand. However, keep in mind they are not efficient and you should work towards vectorization whereever possible.

A = [1 4 6 5 4 8 7 8 6 4];
num_groups = 3;
num_points_in_each_group = [2 3 5];
for i = 1:num_groups
    % your end count is sum of previous group sizes and current group size
    end_count = sum(num_points_in_each_group(1:i)); 
    
    % your start count is 1+numberOfPointsOfPreviousGroup
    start_count = 1+sum(num_points_in_each_group(1:(i-1)));
   
     % you need to store it as cell since groups can have different number of elements
    A_slice{i} = A(start_count:end_count);
end
A_slice
A_slice = 1x3 cell array
    {[1 4]}    {[6 5 4]}    {[8 7 8 6 4]}

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Answer 1

Stephen23 2024-7-10，14:21

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编辑：Stephen23 2024-7-10，14:21

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The MATLAB approach:

A = [1 4 6 5 4 8 7 8 6 4];
N = [2 3 5];
C = mat2cell(A,1,N);

Checking:

C{:}
ans = 1x2
     1     4
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<mw-icon class=""></mw-icon>
ans = 1x3
     6     5     4
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>
ans = 1x5
     8     7     8     6     4
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>

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Answer 2

LH 2024-7-10，14:25

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I think I have solved it:

A = [1 4 6 5 4 8 7 8 6 4]';
groups = 3;
end_count = cumsum(num(:));
for i = 1:groups    
    if i == 1
        start_count = 1;
    else
        start_count = end_count(i-1) + 1;
    end
    A_slice = A(start_count:end_count(i));
end