matlab - usre array elements as a index -

Let's assume we have followed the array declaration in the matrix

  a = [1 2 3; 4 5 6; 7 8 9]   
 which normally appears in the form of matab  
 a =  
  1 2 3 4 5 6 7 8 9 I know how the following code works  
 
  [aa (a)]    
 Here the answer is definitely from the book  
  ans = 1 2 3 1 4 7 4 5 6 2 5 8 7 8 9 3 6 9 < / Code>  
 As I first understand the number of a bracket simple display original one, or 1 to 9, the second case is to use array elements in the array, then it means that the exmaple number For this,    1 2 3 4 5 6 7 8 9   
 This is an index for array, first index  a [1] = 1, a {2] = 2, a [3] = 3 .... a [9] = 9  Is this right? But why is it printed in a way to change? Should not it be like the original matrix? Thanks a lot   
 
 
 In this case the rows are broken before the column  one [2] = 4 ,  a [3] = 7  and  a [4] = 2