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# Time for action – manipulating arrays

1. To delete the second column in `A`, we use:
```octave:14> A(:,2) = []

A =
1   3
1   3
```
2. We can extend an existing array, for example:
```octave:15 > b = [b 4 5]

b =
1 2 3 4 5
```
3. Finally, try the following commands:
```octave:16> d = [2 4 6 8 10 12 14 16 18 20]

d =
2    4    6    8    10    12    14    16    18    20

octave:17> d(1:2:9)

ans =
2    6    10   14    18

octave:18>  d(3:3:12) = -1

d =
2    4    -1    8    10    -1    14    16    -1    20    0    -1
```

## What just happened?

In Command 14, Octave interprets `[]` as an empty column vector and column 2 in A is then deleted in the command. Instead of deleting a column, we could have deleted a row, for example as an empty column vector and column 2 in A is then deleted in the command.

```octave:14> A(2,:)=[]
```

On the right-hand side of the equal ...

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