Finding all indices by ismember

Question

This is what is described in one of the examples for `ismember`: Define two vectors with values in common. `A = [5 3 4 2]; B = [2 4 4 4 6 8];` Determine which elements of `A` are also in `B` as well as their corresponding locations in `B`. `[Lia,Locb] = ismember(A,B)` The result is: ``` Lia = 0 0 1 1 Locb = 0 0 2 1 ``` The element in `B` with the lowest index that matches `A(3)` is `B(2)`. `A(4)` equals `B(1)`. Is there a way by which we could find all the indices of the elemets of `B` matching the same element in `A`?

Accepted Answer

You can swap the input arguments to `ismember`:

```
[tf, ia] = ismember(B, A)
```

For your example, you should get:

```
tf =
     1     1     1     1     0     0

ia = 
     4     3     3     3     0     0
```

This allows you to find, say, the indices of all the elements of `B` that equal `A(3)` simply by doing:

```
find(ia == 3)
```

Here's a nifty solution for the general case:

```
[tf, ia] = ismember(B, A);
idx = 1:numel(B);
ib = accumarray(nonzeros(ia), idx(tf), [], @(x){x});
```

Note that the output is a cell array. For your example, you should get:

```
ib = 
    []
    []
    [2     3     4]
    [      1]
```

which means that there are no elements in `B` matching `A(1)` and `A(2)`, `A(3)` matches elements `B(2)`, `B(3)` and `B(4)`, and `A(4)` equals `B(1)`.

Finding all indices by ismember

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