Index an array using specific indices

I have a vector

x = [0,1,1,0,1,...,N] % x is vector containing either 1s or 0s of size N = 107199.

then I have another vector s, which contains index locations in x.

s= [23,100,500, 9888,....N] % s is of size 1x518

so I want to filter s to contain only indices where the values in x are 1s.

I tried something like this,

  idx = x(s)~=1;
  new_s = s(idx);

but it gives me this error Index exceeds the number of array elements (107520).. any idea what im doing wrongly?

Answer 1

[Daniel Neubauer]

x=[1 0 1 1 0 0 0 0 1 0];
s=[9 4 6];
snew=s(x(s)==1)

snew = 1×2

9 4

[KALYAN ACHARJYA]

Yes, I noticed the array length later.

Answer 2

[DGM]

Try

% some test vectors
x = randi([0 1],1,20)

x = 1×20

0 0 0 1 1 0 1 0 1 0 1 1 1 0 1 0 1 1 1 0

s = randi([1 numel(x)],1,10)

s = 1×10

9 6 20 9 12 9 17 11 7 3

% this is the content selected by s
x(s)

ans = 1×10

1 0 0 1 1 1 1 1 1 0

% and then this would be cases where s selects 1s within x
new_s = s(x(s)==1)

new_s = 1×7

9 9 12 9 17 11 7

Answer 3

[dpb]

Well, if size N = 107199 while s is of size 1x518, then 107199 > 518 so any location in x past numel(s) is going to be an out of bounds reference.

It's not clear in isolation what relationship there is (if any) between x and s so why one would expect those two to be commensurate is puzzling.

Perhaps you're looking for locations that are in s that were found in some other fashion and wanting to know if there's a corresponding matching "1" value in x at the specific location? That's about the only way I can think of at the moment to relate the two -- if that were the case then try

new_s=s(ismember(s,find(~x)));

Answer 4

[fadams18]

Ah i got it. In my original code I was doing something to s before the indexing, which was chanching its values. My code also works.

Thanks for your insights and time guys, @KALYAN ACHARJYA @dpb @Daniel Neubauer @DGM.