cudf.DataFrame.apply_rows#

DataFrame.apply_rows(func, incols, outcols, kwargs, pessimistic_nulls=True, cache_key=None)#

Apply a row-wise user defined function.

Parameters:

dfDataFrame: The source dataframe.
funcfunction: The transformation function that will be executed on the CUDA GPU.
incols: list or dict: A list of names of input columns that match the function arguments. Or, a dictionary mapping input column names to their corresponding function arguments such as {‘col1’: ‘arg1’}.
outcols: dict: A dictionary of output column names and their dtype.
kwargs: dict: name-value of extra arguments. These values are passed directly into the function.
pessimistic_nullsbool: Whether or not apply_rows output should be null when any corresponding input is null. If False, all outputs will be non-null, but will be the result of applying func against the underlying column data, which may be garbage.

Examples

The user function should loop over the columns and set the output for each row. Loop execution order is arbitrary, so each iteration of the loop MUST be independent of each other.

When func is invoked, the array args corresponding to the input/output are strided so as to improve GPU parallelism. The loop in the function resembles serial code, but executes concurrently in multiple threads.

>>> import cudf
>>> import numpy as np
>>> df = cudf.DataFrame()
>>> nelem = 3
>>> df['in1'] = np.arange(nelem)
>>> df['in2'] = np.arange(nelem)
>>> df['in3'] = np.arange(nelem)

Define input columns for the kernel

>>> in1 = df['in1']
>>> in2 = df['in2']
>>> in3 = df['in3']
>>> def kernel(in1, in2, in3, out1, out2, kwarg1, kwarg2):
...     for i, (x, y, z) in enumerate(zip(in1, in2, in3)):
...         out1[i] = kwarg2 * x - kwarg1 * y
...         out2[i] = y - kwarg1 * z

Call .apply_rows with the name of the input columns, the name and dtype of the output columns, and, optionally, a dict of extra arguments.

>>> df.apply_rows(kernel,
...               incols=['in1', 'in2', 'in3'],
...               outcols=dict(out1=np.float64, out2=np.float64),
...               kwargs=dict(kwarg1=3, kwarg2=4))
   in1  in2  in3 out1 out2
0    0    0    0  0.0  0.0
1    1    1    1  1.0 -2.0
2    2    2    2  2.0 -4.0