Asked by Salvatore Mazzarino
on 20 Oct 2012

I would initialize a M x N matrix with the same number. Which could be the best way in terms of speed?

Es.

[2 2;

2 2

2 2]

Answer by Login_Name
on 20 Oct 2012

Another:

% Make a 3-by-8 matrix of 9s:

A(1:3,1:8) = 9

Jan
on 20 Oct 2012

This method can cause problems, if A has been defined before, e.g. by A = rand(9) or A = 'string'.

John BG
on 28 Sep 2016

if it has been defined before ..

if it coincides with the name of a function ..

if you start looking backward there is not way to move forward.

Matt gave the right answer, get on with it, or prove it wrong.

Walter Roberson
on 29 Sep 2016

? You are arguing with a 4 year old posting ?

Jan did give a counter example:

A = rand(9);

A(1:3, 1:8) = 9;

A

A =

9.0000 9.0000 9.0000 9.0000 9.0000 9.0000 9.0000 9.0000 0.9651

9.0000 9.0000 9.0000 9.0000 9.0000 9.0000 9.0000 9.0000 0.6406

9.0000 9.0000 9.0000 9.0000 9.0000 9.0000 9.0000 9.0000 0.7577

0.5009 0.5300 0.3514 0.0230 0.6206 0.5925 0.8718 0.5488 0.7359

0.8410 0.9315 0.2206 0.2301 0.4299 0.1449 0.7987 0.3064 0.6590

0.9057 0.9739 0.3609 0.8522 0.6744 0.5350 0.7201 0.2121 0.9933

0.2481 0.8476 0.1054 0.9497 0.9710 0.2542 0.0973 0.6881 0.8679

0.1017 0.7075 0.1900 0.1831 0.3252 0.8435 0.3257 0.7090 0.4237

0.5273 0.9981 0.1697 0.2163 0.9954 0.9812 0.1355 0.4648 0.6465

Part of the array was set as required but the rest was left alone, which does not meet the specifications.

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Answer by Azzi Abdelmalek
on 20 Oct 2012

Edited by Azzi Abdelmalek
on 20 Oct 2012

A=zeros(M,N)

Azzi Abdelmalek
on 20 Oct 2012

A=ones(M,N)*yournumber

Azzi Abdelmalek
on 20 Oct 2012

or

A=repmat(12,M,N);

Azzi Abdelmalek
on 20 Oct 2012

Matt's init is the fastest

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Answer by James Tursa
on 20 Oct 2012

Edited by James Tursa
on 20 Oct 2012

Another method if matrix A is not already allocated:

A = uninit(M,N);

A(:) = some_number;

UNINIT can be found here:

If the matrix A is pre-existing, then of course skip the allocation step and just fill the values ala the 2nd line above.

SIDE NOTE: On later version of MATLAB it seems the parser is smart enough to recognize the value*ones(m,n) formulation and not actually do the multiply. At least that is my conclusion based on speed tests.

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Answer by Friedrich
on 14 Aug 2018

Edited by Friedrich
on 15 Aug 2018

I know this is old but I could not let it go. I found

A=zeros(M,N)+10;

to be the fastest. At least on my computer. Heres my code for testing and the results in Matlab 2017b

% produces 6.4GB of data

M = 80e6;

N = 10;

clear A

tic;

A=ones(M,N)*10;

disp(['A=ones(M,N)*10; = ' num2str(toc) 's']);

clear A

tic;

A=uninit(M,N);

A(:) = 10;

disp(['A=uninit(M,N); A(:)=10; = ' num2str(toc) 's']);

clear A

tic;

A=repmat(10,[M,N]);

disp(['A=repmat(10,[M,N]); = ' num2str(toc) 's']);

clear A

tic;

A = mxFastZeros(0,M,N)+10;

disp(['A=mxFastZeros(0,M,N)+10; = ' num2str(toc) 's']);

clear A

tic;

A=zeros(M,N)+10;

disp(['A=zeros(M,N)+10; = ' num2str(toc) 's']);

clear A

tic;

a = 12;

A = a(ones(M, N));

disp(['a=10;A=a(ones(M, N)); = ' num2str(toc) 's']);

clear A

Results

A=ones(M,N)*10; = 3.312s

A=uninit(M,N); A(:)=10; = 2.508s

A=repmat(10,[M,N]); = 2.1169s

A=mxFastZeros(0,M,N)+10; = 1.8326s

A=zeros(M,N)+10; = 1.8487s

a=10;A=a(ones(M, N)); = 25.0576s

Edit: Thank you James for the hint on mxFastZeros. I included that in the benchmark.

James Tursa
on 14 Aug 2018

Here's another one you can try using the undocumented API function mxFastZeros:

A = mxFastZeros(0,M,N)+10;

The behavior of this seems highly version dependent. In some versions it seems to run fast at first and then maybe exhaust the supply of pre-0'ed data and the timings jump up significantly. But just guessing here.

The mex code:

/* mxFastZeros.c generates a zero 2D double matrix

Syntax: z = mxFastZeros( ComplexFlag, M, N )

Where:

ComplexFlag = 0 (real) or 1 (complex)

M = row size

N = column size

Programmer: James Tursa

*/

#include "mex.h"

#undef mxFastZeros

#undef mxCreateSharedDataCopy

mxArray *mxFastZeros(mxComplexity ComplexFlag, mwSize m, mwSize n);

mxArray *mxCreateSharedDataCopy(mxArray *mx);

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])

{

mxArray *mx;

mxComplexity ComplexFlag;

mwSize m, n;

if( nrhs != 3 ) {

mexErrMsgTxt("Syntax: mxFastZeros(ComplexFlag,M,N)");

}

if( nlhs > 1 ) {

mexErrMsgTxt("Too many outputs.");

}

ComplexFlag = mxGetScalar(prhs[0]);

m = mxGetScalar(prhs[1]);

n = mxGetScalar(prhs[2]);

mx = mxFastZeros(ComplexFlag,m,n);

plhs[0] = mxCreateSharedDataCopy(mx);

mxDestroyArray(mx);

}

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Answer by MathWorks Support Team
on 9 Nov 2018

In general, the easiest ways to initialize a matrix with the same number are the following, which produce a 3-by-2 matrix whose elements are all 2:

A = 2*ones(3,2)

A = zeros(3,2) + 2

A = repmat(2,3,2)

The speed of these methods relative to each other can depend on your computing environment.

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