I fixed it as follows:

from pyspark.sql.functions import udf

from pyspark.sql.types import StringType

def string_to_float(x):

  return float(x)

udfstring_to_float = udf(string_to_float, StringType())

rawdata.withColumn("name",udfstring_to_float("numberfloat") )

Out[8]: DataFrame[name: string, number_int: int, numberfloat: double]