datatable.first()

datatable.

first

(

cols

)

sourcetests

Return the first row for each column from cols.

Parameters

cols

Expr

Input columns.

return

Expr

f-expression having one row, and the same names, stypes and number of columns as in cols.

Examples

first() returns the first column in a frame:

from datatable import dt, f, by, sort, first df = dt.Frame({"A": [1, 1, 2, 1, 2], "B": [None, 2, 3, 4, 5]}) df

	A	B
0	1	NA
1	1	2
2	2	3
3	1	4
4	2	5

5 rows × 2 columns

dt.first(df)

	A
0	1
1	1
2	2
3	1
4	2

5 rows × 1 column

Within a frame, it returns the first row:

df[:, first(f[:])]

	A	B
0	1	NA

1 row × 2 columns

Of course, you can replicate this by passing 0 to the i section instead:

df[0, :]

	A	B
0	1	NA

1 row × 2 columns

first() comes in handy if you wish to get the first non null value in a column:

df[f.B != None, first(f.B)]

	B
0	2

1 row × 1 column

first() returns the first row per group in a by() operation:

df[:, first(f[:]), by("A")]

	A	B
0	1	NA
1	2	3

2 rows × 2 columns

To get the first non-null value per row in a by() operation, you can use the sort() function, and set the na_position argument as last:

df[:, first(f[:]), by("A"), sort("B", na_position="last")]

	A	B
0	1	2
1	2	3

2 rows × 2 columns

See Also

• last() – function that returns the last row.