MapReduce Transpose

In this example I will do a MapReduce Algorithm to pivot a table.

the input of the algorithm is :

| a | b | c |
| d | e | f |

then the output is :

| a | d |
| b | e |
| c | f |

The key is the row index and the value is the values associated with that row.
The mapper goles is to generate a key_value pair.

{ 0 : a} { 1 : b} { 2 : c} { 0 : d} { 1 : e} { 2 : f}

map(row_index, values):
   
      for (column_index = 1 to length values):
        
           k = column_index
           v ={ row_index :values[column_index]}
      return{ k : v }

After we do a shuffle and sort phase we get : 

{0 , [(0 , a) , (0 , d)]}
{1 , [(1 , b) , (1 , d)]}
{2 , [(2 , c) , (2 , f)]}


then the reducer will return keys will be transpose row indexs and the
values will be the  elements in that row which we refer to as transposed Valuess.


(0 , [a, d])
(1 , [b, e])
(2 , [c, f])
reduce(k, v) :
      Pivotvalues = [] 
      for ( a in v[]) :
           Pivotvalues[a.k] = a.value
           Pivot_index = k
      return {Pivot_index ,Pivotvalues }