How to use loop to fill in specific numbers?

Question

Daniel Holden 2024-3-25

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https://ww2.mathworks.cn/matlabcentral/answers/2098501-how-to-use-loop-to-fill-in-specific-numbers

编辑： John D'Errico 2024-3-25

I want to make a matrix that maps out all positive integer based fractions up to 4/8 as shown below.

I have done this in a laboriously manual way as can be seen below. I am quite certain this is not the best way nor the most efficient as I intend to make this table up with larger numbers. I have a vague idea of putting a loop in a loop, but this also seems convoluted. So any ideas or suggestions to make this as efficent as possible would be appreciated!

for i = 1:8
        n_d(i,1) = 1;
        n_d(i,2) = i;
end
for j = 9:16
        n_d(j,1) = 2;
        n_d(j,2) = j-8;
end
for k = 17:24
        n_d(k,1) = 3;
        n_d(k,2) = k-16;
end
for l = 25:32
        n_d(l,1) = 4;
        n_d(l,2) = l-24;
end

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VBBV 2024-3-25

在 MATLAB Online 中打开

You can use if-else statements to make the code simpler with only one loop

for i = 1:32
        if i>=1 & i<=8
            n_d(i,1) = 1;
            n_d(i,2) = i;
        elseif i > 8 & i <= 16
            n_d(i,1) = 2;
            n_d(i,2) = i-8;
        elseif i > 16 & i <= 24
            n_d(i,1) = 3;
            n_d(i,2) = i-16;
        else 
            n_d(i,1) = 4;
            n_d(i,2) = i-24;
        end
end
disp(n_d)
     1     1
     1     2
     1     3
     1     4
     1     5
     1     6
     1     7
     1     8
     2     1
     2     2
     2     3
     2     4
     2     5
     2     6
     2     7
     2     8
     3     1
     3     2
     3     3
     3     4
     3     5
     3     6
     3     7
     3     8
     4     1
     4     2
     4     3
     4     4
     4     5
     4     6
     4     7
     4     8

John D'Errico 2024-3-25

编辑：John D'Errico 2024-3-25

If these are indeed meant to represent "fractions", then do you want to have both the pairs {1,4} and {2,8} in there as separate items in the list? Of course, there are other examples too of fractions in your list that are not reduced.

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Answer 1

Bruno Luong 2024-3-25

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https://ww2.mathworks.cn/matlabcentral/answers/2098501-how-to-use-loop-to-fill-in-specific-numbers#answer_1430476

编辑：Bruno Luong 2024-3-25

在 MATLAB Online 中打开

T = combinations(1:4,1:8)
T = 32x2 table
    Var1    Var2
    ____    ____

     1  
     2  
     3  
     4  
     5  
     6  
     7  
     8  
     1  
     2  
     3  
     4  
     5  
     6  
     7  
     8  
% for alder release that does not support combinations
[v1 v2] = meshgrid(1:4,1:8);
v1 = v1(:);
v2 = v2(:);
T = table(v1,v2)
T = 32x2 table
    v1    v2
    __    __

   1 
   2 
   3 
   4 
   5 
   6 
   7 
   8 
   1 
   2 
   3 
   4 
   5 
   6 
   7 
   8 

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