# How do I generate a given Matrix in one command?

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marciuc on 23 Feb 2011
I have to generate a matrix A = [1 1 0 0 0; 1 1 1 0 0; 0 1 1 1 0; 0 0 1 1 1; 0 0 0 1 1]
writing a single command.
Teacher told as some helpful commands would be
EYE(m,n)
ONES(m,n)
ZEROS(m,n)
RAND(m,n)
I give you a beer
Jan on 25 Feb 2011
+1: If you can explain how all solutions given here work, you will pass the complete Matlab course based on just one question. Every single answer is funny, but all answers together are serious.

Jan on 23 Feb 2011
Refering to your former post, which has been deleted now: I know also, who you are: you are marciuc.
At first I suggest this:
A = [1 1 0 0 0;1 1 1 0 0;0 1 1 1 0;0 0 1 1 1;0 0 0 1 1]
This is a single command and it is the most efficient solution: No temporary memory, no overhead for calling commnad, and easy to debug. There is no better solution.
Jos (10584) on 23 Feb 2011
vote +1

Sean de Wolski on 23 Feb 2011
Probably the most compact:
A = toeplitz([1 1 0 0 0])
Jan on 23 Feb 2011
This is the most compact command, except for the ambitious RAND apporach. I vote it.

Jan on 23 Feb 2011
Give the beer to your teacher. He is obviously funny if he suggests RAND - but it really works:
A = round(rand(5))
There is at least a certain chance to get the correct answer.
Jiro Doke on 23 Feb 2011
Or if you have MATLAB R2008b or newer,
A = randi([0 1], 5)

Jan on 23 Feb 2011
A = dec2bin([24 28 14 7 3]) - '0'
Jan on 24 Feb 2011
There is the vuvuzela again. The question is obviously more diabolic than I've expected. Again you see: Subtracting zero can reveal the clandestine information.

Matt Tearle on 23 Feb 2011
A = full(gallery('tridiag',ones(1,4),ones(1,5),ones(1,4)))
But my current favorite:
A = 1-reshape(mod(floor((1:25)/3),2),5,5)
Sean de Wolski on 23 Feb 2011
Nice job with your current favorite!

Kenneth Eaton on 28 Feb 2011
I can't believe no one suggested dilation:
A = imdilate(eye(5),ones(2));
Or convolution:
A = sign(conv2(eye(5),ones(2),'same'));
A = sign(filter2(ones(2),eye(5)));
Jan on 28 Feb 2011
*You* have suggested it now. So I can't believe it now, too. +1

Jan on 23 Feb 2011
A general method to create a diagonal matrix is using DIAG (as the example in "help diag" explains):
A = diag(ones(1, 5)) + diag(ones(1,4), 1) + diag(ones(1,4), -1);
You can discuss, if this is still "a single command".
I do not drink beer. But you can ask your teacher to send me the points gained by solving this homework.
##### 2 CommentsShowHide 1 older comment
Matt Tearle on 23 Feb 2011
You can have all the beer that marciuc's teacher sends me. How's that for a deal?

Paulo Silva on 23 Feb 2011
Here's probably the most awesome way to generate the matrix :D
disp('I dare you to try the Infinite monkey matrix')
disp('Congratulations your are not a coward')
disp('Good luck')
pause(1)
disp('Please wait or press CTRL+C to cancel')
disp('but canceling the operation makes you a coward!!')
a=[1 1 0 0 0; 1 1 1 0 0; 0 1 1 1 0; 0 0 1 1 1; 0 0 0 1 1];
w=0;b=zeros(5,5);
while ~isequal(a,b)
b=randi([0 1],5,5);
w=w+1;
end
disp('Congratulations we found the Infinite monkey matrix for you')
b
disp('after')
w
disp('attempts')
else
disp('You are a coward!!!!')
end
Sean de Wolski on 23 Feb 2011
Yay! The infinite monkey makes its second appearance in a week.

Walter Roberson on 25 Feb 2011
EDIT: line-broken per request.
eval(char(mod(1.0599.^ ...
'i<o<ZC<C<d<d<d_C<C<C<d<d_d<C<C<C<d_d<d<C<C<C_d<d<d<C<Cv', ...
96)))
Jan on 25 Feb 2011
Thanks for line breaking. This solution will drive marciuc's teacher crazy. Such ugly! +1

Jan on 23 Feb 2011
A = abs(bsxfun(@minus, 1:5, transpose(1:5))) < 2

Jan on 23 Feb 2011
A = 1 - (tril(ones(5), -2) + triu(ones(5), 2))

Jan on 24 Feb 2011
I cannot resist to post some variation of the DEC2BIN theme:
A = dec2bin('8<.''#' - 32) - '0'
A = dec2bin('FJ<51' - 46) - '0'
But finally you can even omit the first subtraction, because DEC2BIN operates on CHAR vectors also, but you cannot type the non-printables directly:
q = [100 101 99 50 98 105 110 40 39 24 28 14 7 3 39 41 45 39 48 39];
clipboard('copy', char(q))
==> Ctrl-v in the command window
>> dec2bin('#####')-'0'
Here the '#' are the non-printables with the ASCII codes [24,28,14,7,3]. You can write them even in a M-file.

Paulo Silva on 24 Feb 2011
b=[0 0 1 1 1
0 0 0 1 1
1 0 0 0 1
1 1 0 0 0
1 1 1 0 0]
A=~b;

Matt Fig on 24 Feb 2011
One line, anyway. And since the array is at least dynamically pre-allocated, the code is fast.
for ii = 5:-1:1,for jj = min(ii+1,5):-1:max(ii-1,1),A(ii,jj) = 1;end,end
##### 2 CommentsShowHide 1 older comment
Matt Fig on 25 Feb 2011
LOL Jan. I am surprised that such an answer is faster than both the BSXFUN and EYE + DIAG + DIAG solutions, even for N=1000.

Paulo Silva on 23 Feb 2011
diag(diag(eye(4,4)),1)+diag(diag(eye(4,4)),-1)+eye(5,5)
or
diag(ones(1,4),1)+diag(ones(1,4),-1)+eye(5,5)
It's similar to the Jan solution above

marciuc on 24 Feb 2011
Thank you guys...regards from Romania
Jan on 24 Feb 2011
Please do us the favor and accept one of the answers. Or let your teacher choose one.
Beside the fun, this thread will be really helpful, because it describes the creation of tridiagonal matrices exhaustively.
I really hope you had some fun also.

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