MapReduce-MPI WWW Site - MapReduce-MPI Documentation

### KeyValue add() method

    void KeyValue::add(char *key, int keybytes, char *value, int valuebytes)
    void KeyValue::add(int n, char *keys, int keybytes, char *values, int valuebytes)
    void KeyValue::add(int n, char *keys, int *keybytes, char *values, int *valuebytes)

The methods are called by the mymap(), mycompress(), and myreduce() functions in your program to register key/value pairs with the KeyValue object stored by the MapReduce object whose map(), compress(), or reduce() method was invoked. The first version registers a single key/value pair. The second version registers N key/value pairs, where the keys are all the same length and the values are all the same length. The third version registers a set of N key/value pairs where the length of each key and of each value is specified.

As explained here, from the perspective of the MR-MPI library, keys and values are variable-length byte strings. To register such strings, you must specify their length in bytes. This is done via the keybytes and valuebytes arguments, either as a single length or as a vectors of lengths. Note that if your key or value is a text string, it should typically include a trailing "0" to terminate the string.

See the Settings and Technical Details sections for details on the byte-alignment of keys and values you register with these add methods.