Why won't cells populate via if-else statements?

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Kyle Lyman 2021-5-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/835758-why-won-t-cells-populate-via-if-else-statements

评论： J. Alex Lee 2021-5-22

I am taking the transpose of the last three rows of matrix "con". What I want to do is search each newly transposed column for specific numbers using find() (i.e. 1,2,3,4,5,6,7,8,9). I want each of these specific numbers to correspond to a 2x1 matrix, each of which is to be inserted into a cell structure. Then I want to use cell2array to form a matrix. For example [1;2;3;4;5;6] maps to [1;2;3;4;5;6;7;8;9;10;11;12]. My issue is that when I run the code T is displaying as an empty matrix (T=[ ]). I have attached the code down below. Thank you in advance!

clc
clear all
close all
con=[1 2 3 4 5 6;                                                           %Connectivity matrix
    1 2 3 4 5 6;
    3 5 4 8 7 9;
    4 5 6 2 5 7];
Con_Transpose=con(2:end,:).'                                                %Transpose of last three rows of connectivity matrix
width=1:1:size(Con_Transpose,2);
for i=1:length(width)
    
    a={};
    if Con_Transpose(:,width(i))==1;
     
        a{find(Con_Transpose==1,1),1}=[1;2]
        
    end
    
    if Con_Transpose(:,width(i))==2;
        a{find(Con_Transpose==2,1),1}=[3;4]
        
    end
        
    if Con_Transpose(:,width(i))==3;
        
        a{find(Con_Transpose==3,1),1}=[5;6]
        
    end
    
    if Con_Transpose(:,width(i))==4;
  
        a{find(Con_Transpose==4,1),1}=[7;8]
        
    end
    
    if Con_Transpose(:,width(i))==5;
        
        a{find(Con_Transpose==5,1),1}=[9;10]
        
    end
    if Con_Transpose(:,width(i))==6;
        
        a{find(Con_Transpose==6,1),1}=[11;12]
        
    end
    
    if Con_Transpose(:,width(i))==7;
        
        a{find(Con_Transpose==7,1),1}=[13;14]
        
    end
    
    if Con_Transpose(:,width(i))==8;
        
        a{find(Con_Transpose==8,1),1}=[15;16]
        
    end
    
    if Con_Transpose(:,width(i))==9;
      
        a{find(Con_Transpose==9,1),1}=[17;18]
        
    end
    
    T=cell2mat(a)
    
end

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Answer 1

J. Alex Lee 2021-5-21

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https://ww2.mathworks.cn/matlabcentral/answers/835758-why-won-t-cells-populate-via-if-else-statements#answer_705138

编辑：J. Alex Lee 2021-5-21

在 MATLAB Online 中打开

The if statements are probably your problem...you are comparing a column vector with scalars, which off the top of my head I don't know the behavior of.

I won't attempt to fix your code, but consider this strategy that's based on indexing, doesn't need to loop, and doesn't touch cell arrays...does it do what you want:

% generate your "map" from 1->[1;2], 2->[3;4], 4->[7;8], etc.
rmap = reshape(1:18,2,[])
rmap = 2×9
     1     3     5     7     9    11    13    15    17
     2     4     6     8    10    12    14    16    18
% test the mechanism
x = [1 4 6]
x = 1×3
     1     4     6
a = reshape(rmap(:,x),[],1)
a = 6×1
     1
     2
     7
     8
    11
    12
% now solve your problem
con=[1 2 3 4 5 6;                                                           %Connectivity matrix
    1 2 3 4 5 6;
    3 5 4 8 7 9;
    4 5 6 2 5 7];
Con_Transpose=con(2:end,:).'
Con_Transpose = 6×3
     1     3     4
     2     5     5
     3     4     6
     4     8     2
     5     7     5
     6     9     7
sz = [2,1].*size(Con_Transpose)
sz = 1×2
    12     3
T = reshape(rmap(:,Con_Transpose),sz)
T = 12×3
     1     5     7
     2     6     8
     3     9     9
     4    10    10
     5     7    11
     6     8    12
     7    15     3
     8    16     4
     9    13     9
    10    14    10

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Kyle Lyman 2021-5-22

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Hey J. Alex Lee! Thank you for the prompt response. The method that you presented certainly works for matrices that have an even number of elements (i.e. size(con,2)*(size(con,1)-1)=18 in the example you replied to). However, when I change the matrix "con" to have an odd number of elements, I receive the error:

"Error using reshape

Product of known dimensions, 2, not divisible into total number of elements, 9.

Error in DegreeOfFreedom (line 8)

rmap = reshape(1:range,2,[])"

The code that reproduces this error is listed below:

con=[1 2 3;                         %The first line is local degrees of freedom, thus we focus on the second row through the end row, which sums to 9 total elements.
    6 7 9;
    4 1 6;
    9 7 2]
Con_Transpose=con(2:end,:).'
range=numel(con)-size(con,2);
% generate your "map" from 1->[1;2], 2->[3;4], 4->[7;8], etc.
rmap = reshape(1:range,2,[])
x=[con(2:end,:)];
a = reshape(rmap(:,x),[],1);
sz = [2,1].*size(Con_Transpose);
T = reshape(rmap(:,Con_Transpose),sz)

J. Alex Lee 2021-5-22

So what does your conversion map look like...

1->1,2

2->3,4

3->5,6

4->7,8

5->9

6->?

7->?

8->?

9->?

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Why won't cells populate via if-else statements?

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