loop over a matrix by taking the previous index

hi,

i want to loop over a matrix and add the element to a row by taking the index of the previous element.

e.g.

first_element = find(gnbh(:,1) == 0 ) %now i have the first element i need
i = 1:length(first_element)
j = first_element(i)
next_element = gnbh(j,2)
next_element1 = gnbh(next_element,2)
next_element2 = gnbh(next_element1,2)
and so on ... until my next_element is 0

in the end i want to add to every element j the element next_element,next_element1,next_element2 ...

i hope somebody can help me

[Guillaume]

I don't really understand what you're trying to do. Your pseudocode doesn't make much sense. Note that:

i = 1:length(first_element)
j = first_element(i)

is the same as

j = first_element;

Could you give an example of an input matrix and the desired result?

[bbah]

i got a matrix which is 19313x2 called gnbh

from the first column i get my starting element. and the second column shows me the neighbour of my first element in x direction.

e.g. my starting element is 1 and the neighbour is 10. Now i want to loop over the matrix to get the neighbour of 10 and so on. in the end i want to have a row containing the neighbours [1,10 ...]. I want to loop until my next element is 0 and stop

i can get my first element and the neighbour (second element) like this but how do i loop to get the neighbour of my second and so on ?

first_element = find(gnbh(:,1) == 0 )
for i = 1:length(first_element)
j = first_element(i)
next_element = gnbh(j,2);
row = [j,next_element]
end

thank you

[Stephen23]

@bbah: it would help a lot if you could also supply the requested data:

"Could you give an example of an input matrix and the desired result?"

[bbah]

my input would be e.g.

Matrix (0 10; 0 5; 2 6; ... the 10th row would be ;1 5)

like i said: my second column shows me the neighbour of my element (here for the 1st element it is the 10th element ) in +x direction

my first column is the neighbour in -x direction ( here for the 10th element it is my 1st element)

this would be my input matrix

my output would be a matrix with my first column as my first element and second column my 2nd element

output = (1 10 next neighbour next neighbour; ... )

i hope u can understand what im trying to do

[Guillaume]

You make it extremely hard to help you. Why can't you use valid matlab syntax to give us a complete example of output and inputs?

demoinput = [0 10; 0 5; 2 6; ? ?; ? ?; ? ?; ? ?; ? ?; ? ?; 1 5; ? ?];  %fill in the ? 
desiredoutput = [1 10 ? ?];  %fill in the ?

Why is the first value of the desired output 1? Particularly as 0 is present in at least 2 rows.

[bbah]

sorry

demoinput = [0 2;1 3; 2 4:3 0]

u see the 1st elements neighbour is the 10th element in +x direction. the -x neighbour of the 4th element is the 6th element and so on]

0 would be there is no neighbour

desiredoutput = [1 2 3 4 0; 2 3 4 0; 3 4 ? ?]

the problem here is i do not know how to put the neighbour of the next neighbour in a row and how to deal when no neighbour is there.

[Guillaume]

bbah's comment originally posted as an answer moved here:

this would be my input for the first 34 rows

-x    +x
0	10
0	11
0	12
0	13
0	14
0	15
0	16
0	17
0	0
1	18
2	19
3	20
4	21
5	22
6	0
7	23
8	24
10	25
11	26
12	27
13	28
14	29
16	30
17	31
18	32
19	33
20	34
21	35
22	36
23	38
24	39
25    41
26    42
27    44

0 = no neighbour in the -x or +x direction

the result i want is:

matrix =

Output = [1 10 18 25 41;2 11 26 42;3 12 27 44 ... ]

if there is no neighbour just add 0

[Guillaume]

What is the significance of the number in the first column? It doesn't appear to have any meaning? other than maybe you want to start at every 0. The non-zero values of the first column don't appear to be used anywhere.

[bbah]

good question. the 0 in both columns actually stays for a different type of elements. that means for every 0 the neighbour element in +x or -x direction will be a completely different type of element which im not interested in. absolutely no neighbour would be -1.

what i am trying to do is to collect the elements by starting at -x = 0 ( which means my next element in -x direction is not my type of element) and add every element until i reach my last element ( which means my next element in x direction would be again a different element and it has the value +x=0)

with the loop i did in the beginning i can get the 1st elements that start at -x=0 and my next elements at +x but i dont know how to get from my next element the next element in +x direction until i reach +x=0.

i hope u can understand me