Find index of multiple entry in array

General usage
1

Dear,

I need some hep finding the index of multiple entries within an array of integers.

e.g:


A = [1 2 3 4 4 5 6 7 8 8 9]
// expected index
indx = [4 5 9 10]

// The entries of A are to be expected to be consecutive...
// e.g something like 
// A = [1 2 3 4 4 5 6 7 8 8 9 1 1]
// is not expected

// my current approach:

[nrOfEntries dummy] = size(Data);

uniqueEntries = unique( Data);
[nrOfUniqueEntries dummy]= size(uniqueEntries);

I guess I could now iterate through every unique entry by using something like:

for i = 1: nrOfUniqueEntries 
       pos = find (Data = uniqueEntry(i) )
end

but I expect this to be quite slow.

Thank you,
Philipp

2

Hello, I would propose the following two liner:

A = [1 2 3 4 4 5 6 7 8 8 9];
b = A(2:$)==A(1:$-1);
indx = find([%f, b] | [b, %f])

   4.   5.   9.   10.
3

Hello,

Thank you indeed.

If you have some extra time:
Can you explain the magic behind the 3rd line?

4

You select a given index k if A(k)==A(k+1) or A(k-1)==A(k), for relevant values of k, hence if A is of length n you can write this for k=2:n-1 only. For the border cases you have only A(1)==A(2) (first index) and A(n)==A(n-1) (last index). If you want to compute the vector by a single vector operation you can use vector b and its shifted version but in that case you have to add a neutral %f second operand for border cases.

5

What about?

union(find(diff(A)==0),find(diff(A)==0)+1)

1 Like
6

Yes, but I think there is more work (in terms of complexity) when computing the union.

S.

7

Hi Scilab Team,

i looked a little bit around and find, that the find() command does not search for %F, could this be a problem depending on a matrix with multiple inputs? And also i found nothing really visually handsome, on how Scilab make indexes at default. from left to right or top to down and the left. would be cool if in the help, there were some visual, how Scilab indexes

M.f.G. Pascal

8

Hi Scilab team i also noticed that for legend and legend, defining the position is handled differently. Having there also a common handle would be nice. :slight_smile:

M.f.g. Pascal

9

Please start a new topic.

S.