CODEWITHSUNDEEP
matlaboptimization
Turbo
Turbo

Reputation: 75

How do you set given indices to $1$ in MATLAB?

I have following lines of code to create a permutation matrix.

 R=zeros(N^2,N^2);
 R1=randperm(N^2);
 for j=1:N^2
     R(j,R1(j))=1;
 end

Is there a faster way?

Upvotes: 0

Views: 51

Answers (1)

Stewie Griffin
Stewie Griffin

Reputation: 14939

You can use linear indexing like this:

N = 3;
R = zeros(N^2);
R(randperm(N^2) + (0:N^2:(N^4-N^2))) = 1;

This works because the N^2+1 is the first element in the second column, 2*N^2+4 is the fourth element in the third column etc.

Let's define M = N^2 to make the explanation simpler. Now, 0:M:(M^2-M) creates a matrix [0 M 2*M 3*M ... (M-1)*M]. By adding R1 you will get a vector with elements [R1(1), M+R1(2) 2*M+R1(3) ... (M-1)*M+R1(M)]. If you use those numbers as indices, the first number will correspond to row R1(1) in the first column, the second number will correspond to the R1(2) in the second column etc.

Now you can assign the value 1 to the R-matrix in these positions. You might want to have a look at the function ind2sub to understand how this works. ind2sub converts the linear indices into the "normal" [rows cols] indexing style.

pos = randperm(M)+(0:M:(M^2-M))
pos =
     1    15    26    31    38    54    59    70    75
[row, col] = ind2sub(size(R), pos)
row =
     1     6     8     4     2     9     5     7     3
col =
     1     2     3     4     5     6     7     8     9

If you need the R1 variable some other place, you can of course do:

R = zeros(N^2);
R1 = randperm(N^2)
R(R1 + (0:N^2:(N^4-N^2))) = 1;

Upvotes: 1

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