# ismember

ismember is used to constrain an sdpvar object to be selected from a set of constants, or to be inside MPT polytopes.

### Syntax

```
F = ismember(x,Y)
```

### Examples

To constrain a scalar sdpvar variable to take a value from a finite set, ismember can be used.

```
sdpvar x
F = ismember(x,[1 2 3 4]);
```

Of course, this can also be obtained with a standard integer variable

```
intvar x
F = [1 <= x <=4];
```

or an integrality constraint.

```
sdpvar x
F = [integer(x), 1 <= x <= 4];
```

The functionality is more useful when the set is more complicated.

```
sdpvar x
F = ismember(x,[1 pi 12 -8]);
```

The function can also be used together with the polytope object in MPT to constrain a variable to be inside at least one of several polytopes.

```
x = sdpvar(3,1);
P1 = polytope(randn(10,3),rand(10,1));
P2 = polytope(randn(10,3),rand(10,1));
F = ismember(x,[P1 P2]);
```

### Comments

Note that ismember will introduce binary variables if the cardinality of the set **Y** is larger than 1

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