Creating a mxn matrix that is all zeros except for the middle row and middle column which are 1s

Hi,

Given an 7x5 matrix, i am to return a mxn matrix M that is all zeros except for the middle row and middle column, which should be all 1s.

I have gotten as far as creating a zeros matrix using the code M=zeros(7,5), but am clueless on how to change the middle row and middle column to 1s.

I should also mention that the code should be a one-liner.

Thank you

Answer 1

[Wan Ji]

M = [0,0,0,1,0,0,0; 0,0,0,1,0,0,0; 1,1,1,1,1,1,1; 0,0,0,1,0,0,0; 0,0,0,1,0,0,0];

Once for all

Or you can do

a = zeros(5,7);
a((1:numel(a))>size(a,1)*(size(a,2)-1)/2&1:numel(a)<=size(a,1)*(size(a,2)+1)/2|mod(1:numel(a),size(a,1))==(size(a,1)+1)/2) = 1

The result is

a =

     0     0     0     1     0     0     0
     0     0     0     1     0     0     0
     1     1     1     1     1     1     1
     0     0     0     1     0     0     0
     0     0     0     1     0     0     0
     

We can extend it to any matrix with both odd columns and rows.

a = zeros(9,11);
a((1:numel(a))>size(a,1)*(size(a,2)-1)/2&1:numel(a)<=size(a,1)*(size(a,2)+1)/2|mod(1:numel(a),size(a,1))==(size(a,1)+1)/2) = 1

Then a becomes

a =

     0     0     0     0     0     1     0     0     0     0     0
     0     0     0     0     0     1     0     0     0     0     0
     0     0     0     0     0     1     0     0     0     0     0
     0     0     0     0     0     1     0     0     0     0     0
     1     1     1     1     1     1     1     1     1     1     1
     0     0     0     0     0     1     0     0     0     0     0
     0     0     0     0     0     1     0     0     0     0     0
     0     0     0     0     0     1     0     0     0     0     0
     0     0     0     0     0     1     0     0     0     0     0

Answer 2

[Awais Saeed]

M = zeros(7,5)
M(:,ceil(size(M,2)/2)) = 1
M(ceil(size(M,1)/2),:) = 1

[Lavorizia Vaughn]

I see that you had to use more than one line, and I feel that it is necessary to have more than one line to correctly write the code, yet the problem specifically requests a one-liner.

[Awais Saeed]

I do not think that it is possible. Doing M(x,y) would select only a single element, not enitre row and column.

[Lavorizia Vaughn]

Isn’t it possible by just using column vectors or row vectors?

[Awais Saeed]

This is the shortest code that I can come up with. Why do you need a one line answer? You are not doing it through loops, this is vectorisation which is far better and faster than loops. It is more readable as well.

Answer 3

[Zoya]

% create a 6x6 matrix of zeros
matrix = zeros(6);

% set the middle two rows and columns to 1's
matrix(3:4, :) = 1;
matrix(:, 3:4) = 1;