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2D Arrays

2 Dimensional arrays are quite similar to 1D arrays. You still must declair the type, the size and the names. The difference is, of course, in the size. 2D arrays have a second part of the DIMENSION statement which gives them a size in the vertical direction. The result looks like...

type, DIMENSION(number_rows, number_columns) :: names

Indexing

Indexing a 2D array is again very similar to 2D indexing. The format for indexing a 2D array is array_name(row_pos, column_pos). Indexes can be used to get the number at a point in the array out as well as to assign values to a position. To reference a series of positions use a colon : between the sets of numbers you are referencing

PROGRAM array_ind
   INTEGER, PARAMETER :: upper=5, lower=1
   INTEGER, DIMENSION(lower:upper,lower:upper) :: A
   INTEGER :: ind
   A(1:5,1)=(/1,2,3,4,5/)
   A(1:5,2)=(/6,7,8,9,10/)
   A(1,3:5)=22
   A(2:5,3:5)=44
   ind= A(1,5)
END PROGRAM array_ind

This would produce an array that looked like

1	6	22	22	22
2	7	44	44	44
3	8	44	44	44
4	9	44	44	44
5	10	44	44	44

and return the value ind=22 by looking in row 1 column 5.

Reading into 2D Arrays/ Implied Do

Reading into a 2D array is very similar to reading into a 1D array. The first way is to of course assign each position individually through the use of the READ. This method can be done several ways using a DO or if you already have the numbers to inputed into each column as vectors already you can index entire columns. The following is a program example of indexing entire columns, followed by an example of using a DO statement.

PROGRAM array_test
   INTEGER :: I
   INTEGER, PARAMETER :: upper=5, lower=1
   INTEGER, DIMENSION(lower:upper) :: A
   INTEGER, DIMENSION(lower:upper, 1:2) :: B
   READ(*,*) (A(I), I=lower,upper)
   B(1:5,1)=A !This is were an index is used to read in an entire column
   B(1:5,2)=(/6,7,8,9,10/)
END PROGRAM array_test

The above example shows using one 1D array to help construct a 2D array. Note: 1D arrays are represented as columns of numbers. It is also important to note how the positions are counted and walked through. In a 3x3 matrix called ’a’ the second position is at the second row first column. Although you can not actually index using one number in a 3x3 matrix, this is important when formating as the WRITE function will walk through in a vertical manner.

PROGRAM array_ex
   INTEGER :: I, J
   INTEGER, PARAMETER :: upper=5, lower=1
   INTEGER, DIMENSION(lower:upper, lower:upper) :: A
   DO I= lower,upper
      DO J= lower,upper
         READ(*,*) A(I,J) !A(row, column)
      END DO
   END DO

Finally the last way of creating arrays this page will discuss is using an Implied DO statement to read in the values of an array

INTEGER :: I,J
INTEGER, PARAMETER :: upper=5, lower=1
INTEGER, DIMENSION(lower:upper, lower:upper) :: A
READ(*,*) ((A(I,J), J=lower,upper), I=lower,upper) 
!Order read into below

The above code reads the data in all in one statement and orders it filling the 1^st row first then going on to the next row and so on.

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