Introductory Fortran Programming Part 3 -...

dynamic arrays datatypes lists

Introductory Fortran Programming Part 3

Gunnar Wollan1

Dept. of Geosciences, University of Oslo1

January 2007

Wollan Introductory Fortran Programming Part 3

Outline

1 Dynamic arrays

2 Derived datatypes

3 Linked lists

List of Topics

1 Dynamic arrays

2 Derived datatypes

3 Linked lists

Dynamic arrays

What is a dynamic array?

A dynamic array is an array where we can alocate the space atrun-time

We do that by declaring the array as ALLOCATABLE

An allocatable array of a real type can be declared like this:

The code is like this:

REAL, ALLOCATABLE, DIMENSION(:,:) :: arrayINTEGER :: rstat....ALLOCATE(array(100,100),STATUS=rstat))

Dynamic arrays

The ALLOCATE function reserves space in memory for ourarray

It is important to test if the allocation was successful

We do that by using the STATUS= which returns an integersignaling the result of the operation

If a zero was returned the operation was successful, but anyother value signals an error

If the allocation was ok the dynamic array can be used justlike a static array

Dynamic arrays

Another way of using dynamic allocation of memory is to usea POINTER variable

In Fortran 95 we can declare a variable as a pointer. thepointer can be of any variable type

For those who has programmed in other languages likeC/C++, a pointer in Fortran 95 is not the same as a pointerin C/C++

In Fortran 95 a pointer is an alias pointing to a target of thesame datatype and shape as the pointer

A poiter can also be allocated to be used as an ordinaryvariable it beeing a single variable or an array ov one or moredimensions

Dynamic arrays

So how can we use both dynamic arrays and pointers

Let us make a small example to illustrate this

We have an allocatable array like in the first code example

In addition we declare a pointer variable of the same datatypeand shape as the array

The code can be like this

REAL,ALLOCATABLE,TARGET,DIMENSION(:,:) :: arrayREAL,POINTER :: ptr(:,:)INTEGER :: rstat....ALLOCATE(array(100,100),STATUS=rstat))....ptr(:,:) => array(10:20,10:20)

Dynamic arrays

So what have we done here?

We have declared the array as both ALLOCATABLE and aTARGET

This means that we can hava a pointer pointing to the wholearray or only to a part of the array

In this example the pointer ptr is pointing to the 10x10elements of the 100x100 elements of the array variablestarting at position 10,10

So what have we gained here??

We can now index the ptr variable from 1 to 10 in eachdirection instead of remembering that we have to startindexing from ten 10 to 20This is also a convenient way of passing parts of an array to asubroutine or a function without passing the index values asarguments

Dynamic arrays

Let us make a small program based on the array declarationsusing this technique to passparts of an arry to a subroutine

We will read the data from a file where the first two numbersare the size of the array

The code extension can look like this

PROGRAM array_testCHARACTER(LEN=80() :: filenameREAL,ALLOCATABLE,TARGET,DIMENSION(:,:) :: arrayREAL,POINTER :: ptr(:,:)INTEGER :: rstatINTEGER :: i, j, m, nINTEGER,PARAMETER :: lun = 10

Note the use of a static variable lun. It will be used for filereference

Dynamic arrays

Let us open the file and read in the dimensions of the arrayand allocate the array

....OPEN(UNIT=lun,FILE=filename,FORM=’UNFORMATTED’,&

IOSTAT=rstat)READ(UNIT=lun,IOSTAT=rstat) m, nALLOCATE(array(m,n),STAT=rstat)....

Here we use the two integer numbers containing thedimensions from the binary file to allocate the array

Even if it is not in this code we should always test the returnstatus from all file operations to check if it was successful

Dynamic arrays

Then let us read the data from the file into the array, closethe file

and sett the pointer to point to a part of the array (weassume the array size is 100x100)

....DO j = 1, n

DO i = 1, mREAD(UNIT=lun,IOSTAT=rstat) array(i,j)

END DOEND DOCLOSE(lun)ptr => array(1:m-20,1:n-20)....

Now the pointer is associated with the array(1:80,1:80)

Dynamic arrays

....DO j = 1, n

Dynamic arrays

....DO j = 1, n

Dynamic arrays

Now we will pass the pointer to the subroutine and makesome calculations on the partial array

....CALL calculate(ptr,m-20,n-20)....END PROGRAM array_test

SUBROUTINE calculate(A,m,n)REAL, POINTER :: A(:,:)INTEGER :: m, nINTEGER :: i, jm = SIZE(A,1)n = SIZE(A,2)DO j = 1, nDO i = 1, m

A(i,j) = A(i,j) * SQRT(3.1415)END DO

END DO....

Dynamic arrays

So what have we really done in this program?

First of all we have declared a dynamic array which c an bepointed to by a pointer variable

In addition to other variable declarations we openend a binaryfile, read in two integers containing the array size

Then we allocated the array, read in the data from the file andset the pointe variable to point to a part of the array

The last part was that we passed the pointer to the subroutineas an argument

In the subroutine we used the intrinsic function SIZE() to getthe array size of the argument

finally we performed a calculation on the contents of theargument which was of course the pointer from the callingprogram

Dynamic arrays

The use of dynamic arrays are a valuable tool since we do nothave to know the array size in advance

With the introduction of the pointer we can simplify the codeby not having to pass array sizes, where to start an index etc.as arguments to a function or subroutine, but just pass thearray and let all indexes start at 1

Click here to see the source code for this example program

Dynamic arrays

What have we learned in this lesson?

We have learned about dynamic arrays, target arrays andpointers

In addition we have learned that a pointer can be set up topoint to a part of the target array an dnot the whole array

Also with the use of a pointer in this way we can simplify theuse of indexing in subroutines and functions

To test what we have learned we can again answer a multiplechoice quiz

This requires that you have enabled JavaScript in your browser

Click here for the quiz

The answers without taking the quiz is here

Dynamic arrays

List of Topics

1 Dynamic arrays

2 Derived datatypes

3 Linked lists

Derived datatypes

What is a derived datatype?

It is a compund datatype made up from the basic data typesor other derived datatypes

To declare a derived datatype we use the TYPE keyword

An example of such a derived datatype is as follows

TYPE (mytype)CHARACTER(LEN=20) :: nameDOUBLE PRECISION :: measure_valueINTEGER :: year, month, day

END TYPE mytypeTYPE(mytype) :: station

This derived data type consists of a character string to hold aname, a double precision number to hold a mesured value andthree integers to hold the date of the measurement

We use the TYPE(mytype) to declare a variable of the deriveddata type

Derived datatypes

The example on the previous slide contained three integers tohold the year, month and day

It would of course be nice to have a single variable called datetho hold these values

An implementation could look like this

TYPE dateINTEGER :: year, month, day

END TYPE dateTYPE mytype

CHARACTER(LEN=20) :: nameDOUBLE PRECISION :: measure_valueTYPE(date) :: today

Derived datatypes

First we defined another variable type called date whichcontains three integer variables

Then we defined our variable mytype which in addition to theoriginal variables contains a variable of the new date typeinstead of the three integers in the first definition of mytype

This is all very fine, but how do we get access to the internalvariables in our new types?

Let us take another code example

TYPE(mytype) :: stationstation%name = "Site One"station%today%year = 2007

Derived datatypes

As we can see we access the internal variables by using thevariable name of the new variable type with the percent-signand then the name of the internal variable

If we have nested types (as we have here) we just add anotherpercent-sign and then the name if the nested variable likewhen we set the value of the year

Derived datatypes

As we can see we access the internal variables by using thevariable name of the new variable type with the percent-signand then the name of the internal variable

If we have nested types (as we have here) we just add anotherpercent-sign and then the name if the nested variable likewhen we set the value of the year

List of Topics

1 Dynamic arrays

2 Derived datatypes

3 Linked lists

Linked lists

What is a linked list?

In this context we use the term linked list to consist of a listof objects linked together by pointers pointing to the next andprevious element in the list

We use a derived datatype to hold the data and pointers ineach list element

An example of such a list element can be like this

TYPE list_elementTYPE(list_element), POINTER :: prevTYPE(list_element), POINTER :: nextCHARACTER(LEN=80) :: nameDOUBLE PRECISION :: measured_valueTYPE(date) :: today

END TYPE list_element

Linked lists

Next we defina a new variable to contain the linked list

We call this new variable just list

TYPE linked_listTYPE(list_element), POINTER :: headTYPE(list_element), POINTER :: tailTYPE(list_element), POINTER :: this

END TYPE linked_list....TYPE(linked_list) :: listTYPE(list_element), POINTER :: element

The list type contains three pointer variables which will be setto point to the first element of the list the head, the lastelement of the list tail and the current element of the list this

Linked lists

Then we will see how we can put things together

ALLOCATE(element)element%name = "nr1"element%measured_value = 3.14list%head => elementlist%tail => elementlist%this => element

Here we have created a new element and added it to our list

Since the element is a pointer we have to allocate space for it

If we do not allocate space the element will have no space forthe variables

Linked lists

Performing the same procedure we can fill our list with variouselements

The printout in the test program with two elements in the listis this

PRINT *, TRIM(list%head%name)PRINT *, list%head%measured_valuePRINT *, TRIM(list%this%name)PRINT *, list%this%measured_valuePRINT *, TRIM(list%tail%name)PRINT *, list%tail%measured_value

Note the use of the TRIM function whch will suppress theprinting of trailing spaces int the character string

Linked lists

The ouput from the test program was this

Nr13.14000010490417

Nr23.14149999618530

Linked lists

The ouput from the test program was this

Nr13.14000010490417

Nr23.14149999618530

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