Inserting 0s in rows of an array

Question

Alberto Acri 2023-7-18

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https://ww2.mathworks.cn/matlabcentral/answers/1997493-inserting-0s-in-rows-of-an-array

评论： Dyuman Joshi 2023-7-18

采纳的回答： Dyuman Joshi

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Hi. I have a matrix A1 and a vector B1.

If I have a 0 in any row of vector B1, I need to insert 0s in that same row of matrix A (see picture).

With this code the 0s are applied only to the first row of A. How come?

A1 = importdata("A1.mat");
B1 = importdata("B1.mat");
rows_B1 = height(B1);
for C = 1:height(rows_B1)
    
    if B1(C) == 0
        t1 = 0;
        t2 = 0;
        A1(C,1) = t1;
        A1(C,2) = t2;
    end
end

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Answer 1

Dyuman Joshi 2023-7-18

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https://ww2.mathworks.cn/matlabcentral/answers/1997493-inserting-0s-in-rows-of-an-array#answer_1275048

编辑：Dyuman Joshi 2023-7-18

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"With this code the 0s are applied only to the first row of A. How come?"

In your code, you have already defined rows_B1 to be the height of B1. That will return a scalar value.

rows_B1 = height(B1);

When you use height on rows_B1, it will just return 1 (because it is a scalar), and your loop will go from 1 to 1 i.e. only the 1st row.

for C = 1:height(rows_B1)

Correct your code as follows -

%Corrected
for C = 1:rows_B1

A better approach would be indexing -

%Sample data
A1 = [309 126; 310 126; 289 309; 209 309;291 309];
B1 = uint8([0 0 68 70 72])';
%Comparing values of B1 to 0
idx = B1==0
idx = 5×1 logical array
   1
   1
   0
   0
   0
%Assigning all columns of corresponding rows to be 0
A1(idx,:) = 0
A1 = 5×2
     0     0
     0     0
   289   309
   209   309
   291   309

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Alberto Acri 2023-7-18

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Thanks! Could I also ask you how to modify the created A1 matrix so that the rows containing 0s are removed? For example, using: A1_n = A1(A1~=0) and the result is:

A1 = [289 309; 209 309;291 309];

Dyuman Joshi 2023-7-18

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%Sample data
A1 = [309 126; 310 126; 289 309; 209 309;291 309];
B1 = uint8([0 0 68 70 72])';
%Comparing values of B1 to 0
idx = B1==0
idx = 5×1 logical array
   1
   1
   0
   0
   0
%Assigning all columns of corresponding rows to be 0
A1(idx,:) = 0
A1 = 5×2
     0     0
     0     0
   289   309
   209   309
   291   309
%Removing the rows where rows in A are zeros is same as 
%getting the rows where B1 is not equal to zero
A1_n = A1(~idx,:)
A1_n = 3×2
   289   309
   209   309
   291   309