# NumPy selecting specific column index per row by using a list of indexes

I'm struggling to select the specific columns per row of a `NumPy`

matrix.

Suppose I have the following matrix which I would call `X`

:

```
[1, 2, 3]
[4, 5, 6]
[7, 8, 9]
```

I also have a `list`

of column indexes per every row which I would call `Y`

:

```
[1, 0, 2]
```

I need to get the values:

```
[2]
[4]
[9]
```

Instead of a `list`

with indexes `Y`

, I can also produce a matrix with the same shape as `X`

where every column is a `bool`

/ `int`

in the range 0-1 value, indicating whether this is the required column.

```
[0, 1, 0]
[1, 0, 0]
[0, 0, 1]
```

I know this can be done with iterating over the array and selecting the column values I need. However, this will be executed frequently on big arrays of data and that's why it has to run as fast as it can.

**I was thus wondering if there is a better solution?**

Thank you.

If you've got a boolean array you can do direct selection based on that like so:

```
>>> a = np.array([True, True, True, False, False])
>>> b = np.array([1,2,3,4,5])
>>> b[a]
array([1, 2, 3])
```

To go along with your initial example you could do the following:

```
>>> a = np.array([[1,2,3], [4,5,6], [7,8,9]])
>>> b = np.array([[False,True,False],[True,False,False],[False,False,True]])
>>> a[b]
array([2, 4, 9])
```

You can also add in an `arange`

and do direct selection on that, though depending on how you're generating your boolean array and what your code looks like YMMV.

```
>>> a = np.array([[1,2,3], [4,5,6], [7,8,9]])
>>> a[np.arange(len(a)), [1,0,2]]
array([2, 4, 9])
```

Hope that helps, let me know if you've got any more q.

From: stackoverflow.com/q/23435782

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