如何拆分包含字符串的数据框列

Posted 2023-04-13

【中文标题】如何拆分包含字符串的数据框列

【英文标题】：How do I split a dataframe column that contains strings

【发布时间】：2018-09-22 16:36:03

【问题描述】：

我正在使用 spark 2.2 和 我正在尝试从 pyspark 中的 tsv 文件中读取数据集，如下所示：

    student_id subjects result
    "1001" "[physics, chemistry]" "pass"
    "1001" "[biology, math]" "fail"
    "1002" "[economics]" "pass"
    "1002" "[physics, chemistry]" "fail"

我想要以下结果：

    student_id subject result
    "1001" "physics" "pass"
    "1001" "chemistry" "pass"
    "1001" "biology" "fail"
    "1001" "math" "fail"
    "1002" "economics" "pass"
    "1002" "physics" "fail"
    "1002" "chemistry" "fail"

我做了以下，但它似乎不起作用

    df = spark.read.format("csv").option("header", "true").option("mode", "FAILFAST") \
    .option("inferSchema", "true").option("sep", ' ').load("ds3.tsv")
    df.printSchema()

我在执行“printSchema”时看到以下结果

   root
    |-- student_id: integer (nullable = true)
    |-- subjects: string (nullable = true)
    |-- result: string (nullable = true)

当我执行以下操作时，即使用爆炸功能：

    df.withColumn("subject", explode(col("subjects"))).select("student_id", "subject", "result").show(2)

我得到以下异常：

    AnalysisException: "cannot resolve 'explode(`subjects`)' due to data type mismatch: input to function explode should be array or map type, not string;;\n'Project [student_id#10, subjects#11, results#12, explode(subjects#11) AS subject#30]\n+- AnalysisBarrier\n      +- Relation[student_id#10,subjects#11,result#12] csv\n"

我在某处读到 pyspark 不支持字符串的 ArrayType。

编写一个从“subjects”列值的两端修剪“[]”字符，然后使用“split”函数并使用“explode”的 UDF 是否是个好主意？

【问题讨论】：

很明显，您只有一个主题字符串。

【参考方案1】：

第二列是String，可以拆分，然后“explode”使用：

val df = List(
  ("1001", "[physics, chemistry]", "pass"),
  ("1001", "[biology, math]", "fail"),
  ("1002", "[economics]", "pass"),
  ("1002", "[physics, chemistry]", "fail")
).toDF("student_id", "subjects", "result")

df
  .withColumn("clearedFromLeftBracket", expr("substring(subjects,2,length(subjects))"))
  .withColumn("clearedFromBrackets", expr("substring(clearedFromLeftBracket,1,length(clearedFromLeftBracket)-1)"))
  .withColumn("splitted", split($"clearedFromBrackets", ", "))
  .withColumn("subjectResult", explode($"splitted"))
  .drop("clearedFromLeftBracket", "clearedFromBrackets", "splitted","subjects")

输出：

+----------+------+-------------+
|student_id|result|subjectResult|
+----------+------+-------------+
|1001      |pass  |physics      |
|1001      |pass  |chemistry    |
|1001      |fail  |biology      |
|1001      |fail  |math         |
|1002      |pass  |economics    |
|1002      |fail  |physics      |
|1002      |fail  |chemistry    |
+----------+------+-------------+

【讨论】：

对于外行来说，.show 表示不返回 DF

这取决于口味，已删除

非常感谢@pasha701，感谢您的帮助！它工作得很好......:)