# Combine Categorical Arrays

This example shows how to combine categorical arrays.

### Create Categorical Arrays

Create a categorical array that contains the preferred lunchtime beverage of 25 students in classroom `A`.

```rng("default") A = randi(3,[25,1]); A = categorical(A,1:3,["milk" "water" "juice"])```
```A = 25x1 categorical juice juice milk juice water milk milk water juice juice milk juice juice water juice milk water juice juice juice water milk juice juice juice ```

Summarize the categorical array.

`summary(A)`
```A: 25x1 categorical milk 6 water 5 juice 14 <undefined> 0 ```

Create another categorical array that contains the preferences of 28 students in classroom `B`.

```B = randi(3,[28,1]); B = categorical(B,1:3,["milk" "water" "juice"])```
```B = 28x1 categorical juice juice water water milk juice milk milk milk milk juice juice milk juice milk water water juice juice milk water water water juice juice milk juice water ```

Summarize the categorical array.

`summary(B)`
```B: 28x1 categorical milk 9 water 8 juice 11 <undefined> 0 ```

### Concatenate Categorical Arrays

Concatenate the data from classrooms A and B into a single categorical array, `Group1`.

`Group1 = [A;B]`
```Group1 = 53x1 categorical juice juice milk juice water milk milk water juice juice milk juice juice water juice milk water juice juice juice water milk juice juice juice juice juice water water milk ⋮ ```

Summarize the categorical array, `Group1`.

`summary(Group1)`
```Group1: 53x1 categorical milk 15 water 13 juice 25 <undefined> 0 ```

### Create Categorical Array with Different Categories

Create a categorical array, `Group2`, that contains data from 50 students who were given the additional beverage option of soda.

```Group2 = randi(4,[50,1]); Group2 = categorical(Group2,1:4,["juice" "milk" "soda" "water"])```
```Group2 = 50x1 categorical juice juice milk water milk soda juice water milk soda soda water water soda juice juice milk water milk water juice water milk juice milk soda milk milk water soda ⋮ ```

Summarize the categorical array.

`summary(Group2)`
```Group2: 50x1 categorical juice 12 milk 14 soda 10 water 14 <undefined> 0 ```

### Concatenate Arrays with Different Categories

Concatenate the data from `Group1` and `Group2`.

`students = [Group1;Group2]`
```students = 103x1 categorical juice juice milk juice water milk milk water juice juice milk juice juice water juice milk water juice juice juice water milk juice juice juice juice juice water water milk ⋮ ```

Summarize the resulting categorical array. Concatenation appends the categories exclusive to the second input, `soda`, to the end of the list of categories from the first input, `milk`, `water`, `juice`, `soda`.

`summary(students)`
```students: 103x1 categorical milk 29 water 27 juice 37 soda 10 <undefined> 0 ```

To change the order of the categories in the categorical array, use the `reordercats` function.

```students = reordercats(students,["juice" "milk" "water" "soda"]); categories(students)```
```ans = 4x1 cell {'juice'} {'milk' } {'water'} {'soda' } ```

### Union of Categorical Arrays

To find the unique responses from `Group1` and `Group2`, use the `union` function.

`C = union(Group1,Group2)`
```C = 4x1 categorical milk water juice soda ```

`union` returns the combined values from `Group1` and `Group2` with no repetitions. In this case, `C` is equivalent to the categories of the concatenation, `students`.

All of the categorical arrays in this example were nonordinal. To combine ordinal categorical arrays, they must have the same sets of categories including their order.