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How do I generate a given Matrix in one command?

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marciuc
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

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Jan
Jan on 23 Feb 2011
For clarification: Does "I give you a beer" belong to the list of helpful commands or is this a different topic?
Paulo Silva
Paulo Silva on 24 Feb 2011
+1 vote to marciuc just because of the beer :)
Jan
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.

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Answers (16)

Jan
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.

Sean de Wolski
Sean de Wolski on 23 Feb 2011
Probably the most compact:
A = toeplitz([1 1 0 0 0])

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Jan
Jan on 23 Feb 2011
This is the most compact command, except for the ambitious RAND apporach. I vote it.

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Jan
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.

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Jan
Jan on 23 Feb 2011
You need the Communications Toolbox for RANDINT.
jiro
jiro on 23 Feb 2011
Or if you have MATLAB R2008b or newer,
A = randi([0 1], 5)

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Jan
Jan on 23 Feb 2011
A = dec2bin([24 28 14 7 3]) - '0'

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Matt Tearle
Matt Tearle on 24 Feb 2011
That is sick, wrong, disturbing, and I love it
Jan
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.

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Matt Tearle
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)

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Sean de Wolski
Sean de Wolski on 23 Feb 2011
Nice job with your current favorite!

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Kenneth Eaton
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)));

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Jan
Jan on 28 Feb 2011
*You* have suggested it now. So I can't believe it now, too. +1

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Jan
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.

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Matt Tearle
Matt Tearle on 23 Feb 2011
You can have all the beer that marciuc's teacher sends me. How's that for a deal?

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

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

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Paulo Silva
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')
answer=input('Press y and Enter if your dare to try','s')
if (strcmp(answer,'y'))
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

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Sean de Wolski
Sean de Wolski on 23 Feb 2011
Yay! The infinite monkey makes its second appearance in a week.

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Jan
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.

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Matt Fig
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

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Matt Fig
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.

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Walter Roberson
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)))

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Jan
Jan on 25 Feb 2011
Thanks for line breaking. This solution will drive marciuc's teacher crazy. Such ugly! +1

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Paulo Silva
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

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marciuc
marciuc on 24 Feb 2011
Thank you guys...regards from Romania

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Jan
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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Paulo Silva
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;

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