Vectorising nested for loops

Question

Melanie VT 2023-3-10

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1926085-vectorising-nested-for-loops

移动：Stephen23 2023-3-11

采纳的回答： Voss

data.zip

在 MATLAB Online 中打开

Hi,

I’m trying to vectorise the following code snippet that works using the attached data. Unfortunately, I couldn’t succeed. Any ideas on how to deal with it?

Cheers,

Mel

k=0;
struct_data = struct('idx',[],'count',[],'size',[]);
for i = 1:size(table_data,1)
    for j = 1:list(i) 
        k=k+1;
        struct_data(k,1).idx = k;
        struct_data(k,1).count = table_data.count(i);
        struct_data(k,1).size = table_data.size(i);
    end
end

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Stephen23 2023-3-10

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Are you intentionally storing lots of scalar cell arrays containing character vectors, or is the intent to actually just store the character vectors without the superfluous scalar cell arrays? If you only need the text, replace

table_data.size(i);

with

table_data.size{i};

https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-cell-array.html

Melanie VT 2023-3-10

Thank you for your contribution!

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Answer 1

Voss 2023-3-10

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1926085-vectorising-nested-for-loops#answer_1189775

编辑：Voss 2023-3-10

在 MATLAB Online 中打开

data.zip

% unzip and load data
unzip data.zip
load data.mat
% run the loop method, for later comparison
k=0;
struct_data = struct('idx',[],'count',[],'size',[]);
for i = 1:size(table_data,1)
    for j = 1:list(i) 
        k=k+1;
        struct_data(k,1).idx = k;
        struct_data(k,1).count = table_data.count(i);
        struct_data(k,1).size = table_data.size{i}; % note this change:
                                                    % I made struct_data(k).size a char, instead of 
                                                    % a scalar cell array containing a char
    end
end
struct_data_save = struct_data;
% a "vectorized" method
idx = find(list);
struct_data = table2struct(removevars(table_data(repelem(idx,list(idx)),:),'district'));
C = num2cell(1:numel(struct_data));
[struct_data.idx] = deal(C{:});
% are the results the same?
isequal(struct_data,struct_data_save)
ans = logical
   1

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Voss 2023-3-10

编辑：Voss 2023-3-10

在 MATLAB Online 中打开

data.zip

If you really want struct_data(k).size to be a 1x1 cell array containing a character vector (as in {'venti'}), as opposed to just a character vector (as in 'venti'), you can do it with a couple of additional lines of code:

% unzip and load data
unzip data.zip
load data.mat
% run the loop method, for later comparison
k=0;
struct_data = struct('idx',[],'count',[],'size',[]);
for i = 1:size(table_data,1)
    for j = 1:list(i) 
        k=k+1;
        struct_data(k,1).idx = k;
        struct_data(k,1).count = table_data.count(i);
        struct_data(k,1).size = table_data.size(i); % scalar cell array this time
    end
end
struct_data_save = struct_data;
% a "vectorized" method
idx = find(list);
struct_data = table2struct(removevars(table_data(repelem(idx,list(idx)),:),'district'));
C = num2cell(1:numel(struct_data));
[struct_data.idx] = deal(C{:});
C = num2cell({struct_data.size});
[struct_data.size] = deal(C{:});
% are the results the same?
isequal(struct_data,struct_data_save)
ans = logical
   1

Melanie VT 2023-3-10

That is awesome! Thank you so much for providing solutions for both scenarios🙏

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Answer 2

Stephen23 2023-3-10

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1926085-vectorising-nested-for-loops#answer_1189795

在 MATLAB Online 中打开

data.mat

S = load('data.mat')
S = struct with fields:
          list: [27×1 double]
    table_data: [27×3 table]
list = S.list
list = 27×1
     0
   100
    60
     0
     0
     0
     0
     0
     0
     0
tdat = S.table_data
tdat = 27×3 table
districtcountsize_______________________

     {[ 1]}       91     {'venti' }
     {[ 2]}      116     {'venti' }
     {[ 3]}      112     {'venti' }
     {[ 4]}        8     {'venti' }
     {[ 5]}       23     {0×0 char}
     {[ 6]}       16     {0×0 char}
     {[ 7]}        7     {0×0 char}
     {[ 8]}        1     {0×0 char}
     {[ 9]}       28     {0×0 char}
     {[10]}       58     {'tall'  }
     {[11]}       43     {'venti' }
     {[12]}       13     {'trenta'}
     {[13]}       68     {'tall'  }
     {[14]}      108     {'grande'}
     {[15]}      102     {'venti' }
     {[16]}       76     {'venti' }
k=0;
out0 = struct('idx',[],'count',[],'size',[]);
for i = 1:size(tdat,1)
    for j = 1:list(i) 
        k=k+1;
        out0(k,1).idx = k;
        out0(k,1).count = tdat.count(i);
        out0(k,1).size  = tdat.size{i};
    end %                          ^ ^ fixed indexing
end
out0
out0 = 160×1 struct array with fields:
    idx
    count
    size

Without an explicit loop:

out1 = repelem(table2struct(removevars(tdat,'district')),list);
idx = num2cell(1:numel(out1));
[out1.idx] = idx{:}
out1 = 160×1 struct array with fields:
    count
    size
    idx
isequal(out0,out1)
ans = logical
   1