I am trying to learn Spark and it has been going well so far, except for problems where I need to use functions like reduceByKey or combineByKey on a pair RDD whose values are lists.
I have been trying to find detailed documentation for these functions, that could explain what the arguments actually are, so that I could solve it myself without going to Stack Overflow, but I just cannot find any good documentation for Spark. I have read chapters 3 and 4 from Learning Spark, but to be honest, the explanations for the most complicated functions are very bad.
The problem I am dealing with right now is the following: I have a pair RDD where the key is a string and the value is a list of two elements which are both integers. Something like this: (country, [hour, count]). For each key, I wish to keep only the value with the highest count, regardless of the hour. As soon as I have the RDD in the format above, I try to find the maximums by calling the following function in Spark:
reduceByKey(lambda x, y: max(x[1], y[1]))
But this throws the following error:
TypeError: 'int' object is not subscriptable
Which does not make any sense to me. I interpreted the arguments x and y as being the values of two keys, e.g. x=[13, 445] and y=[14, 109], but then the error does not make any sense. What am I doing wrong?
Lets say you have [("key", [13,445]), ("key", [14,109]), ("key", [15,309])]
When this is passed to reduceByKey, it will group all the values with same key into one executor i.e. [13,445], [14,109], [15,309] and iterates among the values
In the first iterate x is [13,445] and y is [14,109] and the output is max(x[1], y[1]) i.e. max(445, 109) which is 445
In the second iterate x is 445 i.e. max of previous loop and y is [15,309]
Now when the second element of x is tried to be obtained by x[1] and 445 is just an integer, the error occurs
TypeError: 'int' object is not subscriptable
I hope the meaning of the error is clear. You can find more details in my other answer
The above explanation also explains why the solution proposed by @pault in the comments section works i.e.
reduceByKey(lambda x, y: (x[0], max(x[1], y[1])))
来源：https://stackoverflow.com/questions/49303835/how-to-understand-reducebykey-in-spark