Using map reduce to count word frequency.

https://hadoop.apache.org/docs/r1.2.1/mapred\_tutorial.html\#Example%3A+WordCount+v1.0

Have you met this question in a real interview?

Yes

Example

chunk1: "Google Bye GoodBye Hadoop code"
chunk2: "lintcode code Bye"


Get MapReduce result:
    Bye: 2
    GoodBye: 1
    Google: 1
    Hadoop: 1
    code: 2
    lintcode: 1

/**
 * Definition of Input:
 * template<class T>
 * class Input {
 * public:
 *     bool done(); 
 *         // Returns true if the iteration has elements or false.
 *     void next();
 *         // Move to the next element in the iteration
 *         // Runtime error if the iteration has no more elements
 *     T value();
 *        // Get the current element, Runtime error if
 *        // the iteration has no more elements
 * }
 */
class WordCountMapper: public Mapper {
public:
    void Map(Input<string>* input) {
        // Write your code here
        // Please directly use func 'output' to 
        // output the results into output buffer.
        // void output(string &key, int value);
        while (!input->done()) {
            vector<string> words = split(input->value(), " ");
            for (string& word : words) {
                output(word, 1);
            }
            input->next();
        }
    }

private:
    vector<string> split(const string& value, string delim) {
        vector<string> words;
        int last = 0, index;
        while ((index = value.find(delim, last)) != string::npos) {
            words.push_back(value.substr(last, index - last));
            last = index + delim.length();
        }
        if (last < value.length()) {
            words.push_back(value.substr(last, value.length() - last));
        }
        return words;
    }
};


class WordCountReducer: public Reducer {
public:
    void Reduce(string &key, Input<int>* input) {
        // Write your code here
        // Please directly use func 'output' to 
        // output the results into output buffer.
        // void output(string &key, int value);

        int sum = 0;
        while (!input->done()) {
            sum += input->value();
            input->next();
        }

        output(key, sum);
    }
};