# Select in DataFrame

## Select Columns

#### Select a Column

There are two possible syntaxes for selecting all values from a column:

1. `table_name['column_name']`&#x20;
2. `table_name.column_name`, if the name of a column follows all of the rules for a variable name (doesn’t start with a number, doesn’t contain spaces or special characters, etc.).

When we select a single column, the result is called a _Series_.

#### Selecting Multiple Columns

To select two or more columns from a DataFrame, we use a list of the column names.&#x20;

```python
new_df = orders[['last_name', 'email']]
```

When we select multiple columns, the result is a new DataFrame.

## Select Rows

#### Select Rows by Index

We can select a row by its index.

```python
# select a row
orders.iloc[2]

# select multiple rows
orders.iloc[3:7]
orders.iloc[:4]
orders.iloc[-3:]
```

When we select a single row, the result is a _Series._ When we select multiple rows, the result is a new DataFram&#x65;_._

#### Select Rows with Logic

You can select a subset of a DataFrame by using logical statements:

```python
df[df.MyColumnName == desired_column_value]
```

Logical statements include:

* Equal to, `==`&#x20;
* Greater Than, `>`&#x20;
* Less Than, `<`&#x20;
* Not Equal, `!=`&#x20;

We can also combine multiple logical statements, as long as each statement is in parentheses.

```python
df[(df.age < 30) |
   (df.name == 'Martha Jones')]
```

We could use the `isin` command to check that `df.name` is one of a list of values:

```python
df[df.name.isin(['Martha Jones',
     'Rose Tyler',
     'Amy Pond'])]
```

## Setting indices

When we select a subset of a DataFrame using logic, we end up with non-consecutive indices. This is inelegant and makes it hard to use `.iloc()`.

We can fix this using the method `.reset_index()`. For example, here is a DataFrame called `df` with non-consecutive indices:

|   | First Name | Last Name |
| - | ---------- | --------- |
| 0 | John       | Smith     |
| 4 | Jane       | Doe       |
| 7 | Joe        | Schmo     |

If we use the command `df.reset_index()`, we get a new DataFrame with a new set of indices:

|   | index | First Name | Last Name |
| - | ----- | ---------- | --------- |
| 0 | 0     | John       | Smith     |
| 1 | 4     | Jane       | Doe       |
| 2 | 7     | Joe        | Schmo     |

Note that the old indices have been moved into a new column called `'index'`. Unless you need those values for something special, it’s probably better to use the keyword `drop=True` so that you don’t end up with that extra column. If we run the command `df.reset_index(drop=True)`, we get a new DataFrame that looks like this:

|   | First Name | Last Name |
| - | ---------- | --------- |
| 0 | John       | Smith     |
| 1 | Jane       | Doe       |
| 2 | Joe        | Schmo     |

Using `.reset_index()` will return a new DataFrame, but we usually just want to modify our existing DataFrame. If we use the keyword `inplace=True` we can just modify our existing DataFrame.
