Can I find set of values in larger set in one step?

Hi,

I am working on a code that takes a very long time to implement, so I try to optimize it

If I look for a set of values (x) in a larger set (z), can I do that in a way faster than using for and find?

x=[1 2 3];

z=[10 2 1 5 7 .....];

thanks

[Walter Roberson]

ismember() with two outputs

[the cyclist]

ismember is almost certainly the way to go, as @Walter Roberson says. A more specific answer could be given, if you tell us more specifically what output you want from the inputs x and z.

[Walter Roberson]

For example are there multiple copies of the values, and is what you are looking for as output a cell array, one entry for each item in x, giving the indices in z ?

[huda nawaf]

Thanks,

In current problem there are no copies of the values,and the output shoud be in this example 2 and 3, in other word their positions.

But if there are copies of the values, are there function to do that?

[huda nawaf]

thanks @Walter Roberson, but there is a problem when removing their positions

x=[1 2 3];

z=[10 2 1 5 7];

[r r1] =ismember(x,z); the output is :

r = 1 1 0

r1 = 3 2 0

r1=r1(r1~=0);

z(z==z(r1 ))=[ ]

why this error:

Arrays have incompatible sizes for this operation.

the output should

z=[10 5 7]

[Bruno Luong]

x=[1 2 3]; 
z=[10 2 1  5 7];

z(ismember(z,x)) = []

z = 1×3

10 5 7

[huda nawaf]

many thanks @Bruno Luong

[Jan]

If Bruno's method solves the problem, this works also:

x = [1 2 3]; 
z = [10 2 1  5 7];
z = setdiff(z, x, 'stable')

z = 1×3

10 5 7

[huda nawaf]

what if I have c ,when I find 1 and 2 in z , I want to remove it from c

x=[1 2 3];

>> z=[10 2 1 5 7];

>> c=[9 4 2 1];

I need the output is

c=[9 4]

[Bruno Luong]

x=[1 2 3];
z=[10 2 1  5 7];
c=[9 4 2 1];

c(ismember(c,intersect(z,x)))=[]

c = 1×2

9 4

Answer 1

[Ishan Ghosekar]

As per my understanding, you want to look for the set of values in x in another larger set, z in an optimized way.

The following functions can help you achieve this:

• The intersect function returns common values between x and z in sorted order:

>> x=[1 2 3];
>> z=[10 2 1 5 7 4 6 9];
>> intersect(x,z)
ans =
  1	2

For more information regarding the intersect function, refer to the following documentation link:

https://www.mathworks.com/help/matlab/ref/double.intersect.html

• The ismember function returns an array containing logical 1 (true) where the data in A is found in B.

>> ismember(x,z)
ans =
  1×3 logical array
  1   1   0

For more information regarding the ismember function, refer to the following documentation link:

https://www.mathworks.com/help/matlab/ref/double.ismember.html