Elegant way of creating an array full of one number in MATLAB? - arrays

Question

I know that to get 10 0's, one can do
A = zeros(10, 1);
To get 10 1's, one can do
A = ones(10, 1);
What about any arbitrary number? Say, I want 10 3's. I have come up with a way of doing it.
A = linspace(3, 3, 10);
Is this satisfactory? Is there a more elegant way of doing this?

Answer 1

Some alternatives:
Using repmat:
A = repmat(3, [5 7]); %// 5x7 matrix filled with the value 3
Using indexing:
A(1:m, 1:n) = x;
Following is a timing comparison between all proposed approaches. As you can see, #Dennis' solutions are the fastest (at least on my system).
Functions:
function y = f1(x,m,n) %// Dennis' "ones"
y = x*ones(m,n);
function y = f2(x,m,n) %// Dennis' "zeros"
y = x+zeros(m,n);
function y = f3(x,m,n) %// Luis' "repmat"
y = repmat(x,[m n]);
function y = f4(x,m,n) %// Luis' "dirty"
y(m,n) = 0;
y(:) = x;
function y = f5(x,m,n) %// Luis' "indexing"
y(1:m,1:n) = x;
function y = f6(x,m,n) %// Divakar's "matrix porod"
y = x*ones(m,1)*ones(1,n);
Benchmarking script for square arrays:
x = 3;
sizes = round(logspace(1,3.7,10)); %// max limited by computer memory
for s = 1:numel(sizes)
n = sizes(s);
m = sizes(s);
time_f1(s) = timeit(#() f1(x,m,n));
time_f2(s) = timeit(#() f2(x,m,n));
time_f3(s) = timeit(#() f3(x,m,n));
time_f4(s) = timeit(#() f4(x,m,n));
time_f5(s) = timeit(#() f5(x,m,n));
time_f6(s) = timeit(#() f6(x,m,n));
end
loglog(sizes, time_f1, 'r.-');
hold on
loglog(sizes, time_f2, 'g.:');
loglog(sizes, time_f3, 'b.-');
loglog(sizes, time_f4, 'm.-');
loglog(sizes, time_f5, 'c.:');
loglog(sizes, time_f6, 'k.:');
xlabel('Array size')
ylabel('Time')
legend('ones', 'zeros', 'repmat', 'dirty', 'indexing', 'matrix prod')
For column arrays: just change the following lines:
sizes = round(logspace(1,3.7,10)).^2; %// max limited by computer memory
n = 1;
m = sizes(s);
For row arrays:
sizes = round(logspace(1,3.7,10)).^2; %// max limited by computer memory
n = sizes(s);
m = 1;
Results for a dual-core CPU, 2-GB RAM, Windows Vista, Matlab R2010b:
Square arrays;
Column arrays;
Row arrays.

Answer 2

There are two basic ways to do this:
A = ones(10,1)*3
B = zeros(10,1)+3
The first one is most commonly used in examples, yet if I am not mistaken, the second one performs slightly better. All in all it is just a matter of taste.
Of course, if you have an existing matrix, there is another simple way:
C = zeros(10,1)
C(:) = 3;
And for completeness, the repmat solution as suggested by #Luis is also fine.

Answer 3

As an alternative, a matrix multiplication (which is supposed to be pretty fast on MATLAB) based method could be also be suggested for 2D or multidimensional array assignment work.
Thus, assuming m as rows, n as columns and x as the value to be assigned for all elements, the code would be -
y = x*ones(m,1)*ones(1,n);