Generate an array with specific duplicate elements in MATLAB

Question

I have one array, for example B = [2,5,7], and also have a number C = 10, where C is always larger than or equal to the largest number in B. and I want to generate an array A according to B and C. In this specific example, I have

A = [1, 2, 2, 2, 3, 4, 5, 5, 5, 6, 7, 7, 7, 8, 9, 10]

that is, I generate an array [1:C], but with each element in B are duplicated 3 times. Is there any good way that does not use for loop to generate array A?

Thank you!

The largest number of A is fixed. For example, 10. – ezheng Oct 21 '15 at 05:19 — ezheng, Oct 21 '15 at 05:19

score 3 · Answer 1 · answered Oct 21 '15 at 10:12

3

You can use repelem (introduced in Matlab R2015a):

B = [2 5 7]
C = 10;
n = 3;

r = ones(1,C);
r(B) = n;
A = repelem(1:C, r)

answered Oct 21 '15 at 10:12

Luis Mendo

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score 2 · Accepted Answer · answered Oct 21 '15 at 06:14

2

How about...

B = [2,5,7];
C = 10;
A = sort([1:C,B,B])

answered Oct 21 '15 at 06:14

RPM

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Overall, I like it. The main drawback is calling a sort function, which may be time-consuming. I wait if there are better answers. Thanks. – ezheng Oct 21 '15 at 06:16
Luis Mendo's answer is also good, but I do not have Matlab 2015. I accept this answer because it is easy and not error-prone. – ezheng Oct 25 '15 at 22:40

Dennis Klopfer · Answer 3 · 2015-10-21T08:01:57.980

0

I think the answer of @RPM could be faster. But because you specifically asked for a solution without sort:

B = [2,5,7];
C = 10;
D = setdiff(1:C,B)-1;

A = reshape(repmat(1:C,3,1),1,3*C);
A([3*D+1,3*D+2]) = [];

which will also return the correct result. I'm not to sure about the order setdiff() has. It might be worse than sort() in all cases. Especially with A = sort([1:C,B,B]) as the input is already almost in order.

edited Oct 21 '15 at 08:01

answered Oct 21 '15 at 07:55

Dennis Klopfer

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@RPM's solution is 2 orders of magnitude faster than this. – IKavanagh Oct 21 '15 at 08:11
Good to know, thanks. Thats also why I upvoted his answer. Though, if the task is reversed and the numbers not in ``B`` should be trippled, this should be a reasonable solution avoiding ``sort()`` as one could get rid of ``setdiff()`` as well. – Dennis Klopfer Oct 21 '15 at 08:21

score 0 · Answer 4 · edited May 23 '17 at 11:44

Following the same philosophy as this solution to Repeat copies of array elements: Run-length decoding in MATLAB, you can do something similar here, like this -

%// Get increment array (increments used after each index being repeated in B) 
inc = zeros(1,C);
inc(B+1) = N-1

%// Calculate ID array (places in output array where shifts occur)
id = zeros(1,C+(N-1)*numel(B));
id(cumsum(inc) + (1:C)) = 1

%// Finally get cumulative summation for final output 
A = cumsum(id)

Sample run -

B =
     2     5     7
C =
    10
N =
     3
inc =
     0     0     2     0     0     2     0     2     0     0
id =
     1     1     0     0     1     1     1     0     0     1     1     0     0     1     1     1
A =
     1     2     2     2     3     4     5     5     5     6     7     7     7     8     9    10

Generate an array with specific duplicate elements in MATLAB

4 Answers4