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### MapReduce sort_values() method

    int MapReduce::sort_values(int (*mycompare)(char *, int, char *, int))

This calls the sort_values() method of a MapReduce object, which sorts a KeyValue object by its values to produce a new KeyValue object. The mycompare() function you provide compares pairs of values for the sort, since the MapReduce object does not know how to interpret the content of your values. The method returns the total number of key/value pairs in the new KeyValue object which will be the same as in the original.

This method is used to sort key/value pairs by value before a KeyValue object is transformed into a KeyMultiValue object, e.g. via the clone(), collapse(), or convert() methods. Note that these operations preserve the order of pairs in the KeyValue object when creating a KeyMultiValue object, which can then be passed to your application for output, e.g. via the reduce() method. Note however, that sort_values() does NOT sort values across all processors but only sorts the values on each processor within the KeyValue object. Thus if you gather() or aggregate() after performing a sort_values(), the sorted order will be lost, since those methods move key/value pairs to new processors.

In this example the user function is called mycompare() and it must have the following interface

    int mycompare(char *value1, int len1, char *value2, int len2)

Value1 and value2 are pointers to the byte strings for 2 values, each of length len1 and len2. Your function should compare them and return a -1, 0, or 1 if value1 is less than, equal to, or greater than value2, respectively.

This method is an on-processor operation, requiring no communication. When run in parallel, each processor operates only on the key/value pairs it stores.

Related methods: sort_keys(), sort_multivalues()