Repeating elements (vector)

Hi everyone,

Suppose I have matrix A

A = [1; 2; 3; 4]

And I want to have matrix B

B = [1; 1; 2; 1; 2; 3; 1; 2; 3; 4]

I don't want to use loop to do so because I have lots of observations. So how can I do this efficiently without using loop? Thank you very much!

Jeff

[Stephen23]

@Tsz Heung: what is the rule to generate B from A ?

[Tsz Heung]

What do you mean? My question is how can I generate B from A.

[Adam]

I assume the rule is you want just the 1st row, then the first 2 rows, then the first 3 rows, etc, all collapsed into a single vector.

[Jan]

@Tsz Heung: There is an infinite number of methods to create B based on A from your example. To create a program, we have to know the definition of the procedure. One example is not enough to define this unequivocally.

@Adam: You mean:

• one time the first row
• one time the first 2 rows
• two times the first 3 rows
• one time the last row

???

[Adam]

No, I meant

• one time the first row,
• one time the first 2 rows,
• one time the first 3 rows,
• one time the first 4 rows,...

Answer 1

[Stephen23]

There are most likely much more efficient algorithms than this:

>> N = 4;
>> X = (1:N)'*ones(1,N);
>> X(triu(true(N)))
ans =
   1
   1
   2
   1
   2
   3
   1
   2
   3
   4

You can use X as indices into your vector to select the values that you want.

[Jan]

+1: An excellent guess.

[Tsz Heung]

Really Appreciated! Thank you Stephen!

Answer 2

[Walter Roberson]

nonzeros(triu(repmat(A,1,length(A))))

Note: this assumes that none of the entries are 0.

[Stephen23]

+1 very nice. It would be interesting to know if repmat or multiply is faster:

 nonzeros(triu((1:N)'*ones(1,N)))

[Tsz Heung]

Really Appreciated! Thank you Walter!