How do I merge two arrays in MATLAB?

There are two ways of doing this:
A) Using brackets operator
B) Using concatenating functions

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                      A.  Concatenation Of Arrays

                                             (Using brackets operator)

The brackets operator ( [] ) , that is used for creating an array in MATLAB , also serves the purpose of concatenating arrays .

Consider :
A =                                                             B = 
       1            2                                                       5               6
       3            4                                                       7               8

CASE I : Horizontal Concatenation 

C = [A  B]
C  =
          1            2            5          6
          3            4            7          8

CASE II : Vertical Concatenation

C = [A ; B ]
C =
         1              2
         3             4
         5             6
         7             8 

Note : 
 1. If you are concatenating arrays horizontally then each component arrays must have same number of rows .
2. If you are concatenating arrays vertically then each component arrays must have same number of columns. 

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                     B. Concatenation Of Arrays 

                                         (Using concatenating functions)

There are mainly three functions used for concatenation namely cat , horzcat , vertcat .

Again,  consider 
  A =                                                             B = 
       1            2                                                       5               6
       3            4                                                       7               8

A .  cat

This function concatenate arrays along specific dimension .

Syntax : 

C = cat (dim , A1 , A2 , A3 .... An)

If  dim = 1 then vertical concatenation.
     dim = 2 then horizontal concatenation.

Example :

B.  horzcat
This function concatenates arrays horizontally .

Syntax :

C = horzcat( A1 , A2 , A3 ,..... An)
where ,
           A1 , A2 , A3 ......An must have same number of rows .

C. vertcat
This function concatenates arrays vertically .

C = vertcat(A1 , A2 , A3 ,...... An )
where ,
          A1 , A2 , A3 ,.......An must have same number of columns .