1. Midterm Review Programming in Fortran
Yi Lin
Oct 12, 2006
11/24/2018
Comp208 Computers in Engineering

2. Comp208 Computers in Engineering
What we have learned
Units of Fortran programs
Variable
Expression
Statement
Control statements (IF-ELSE)
IF-THEN-ELSE-ENDIF
IF-THEN-ELSEIF-THEN-ELSE-ENDIF
Repetition statements (DO LOOP)
Count DO LOOP
INFINITE DO LOOP
Function and Subroutine
11/24/2018
Comp208 Computers in Engineering

3. Units of Fortran programs
Smallest: Variables and constants
Constants: the values are the same
E.g., “hello”, 34,
Variables: is a unique name which a FORTRAN program applies to a word of memory and uses to refer to it. Its values can be reassigned.
E.g. a, b,
Variable types
INTEGER
REAL
LOGICAL TRUE. .FALSE.
CHARACTER
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Comp208 Computers in Engineering

4. Comp208 Computers in Engineering
Expression
Composed of variables, constants and operators. For example:
3 + ½
3 > 2 .AND. 4 >=3
“Prof. “ // ”Friedman”
An expression has a value which is of a specific type (e.g., INTEGER, REAL, LOGICAL, CHARACTER)
3+1/2 has a value of 3.5
3 > 2 .AND. 4 >=3 has a value of .TRUE.
“Prof. “ // “Friedman” has a value of “Prof. Friedman”
11/24/2018
Comp208 Computers in Engineering

5. Arithmetic Expressions
An arithmetic expression is formed using the operations:
+ (addition), - (subtraction)
* (multiplication), / (division)
** (exponentiation)
If the operands are integers, the result will be an integer value
If the operands are real, the result will be a real value
If the operands are of different types, the expression is called mixed mode.
11/24/2018
Comp208 Computers in Engineering

6. Evaluate Simple Expressions
> 4
> 0.78
6.5/ > 5.2
8.4/4.2 --> 2.0 rather than 2, since the result must be of REAL type.
-5**2 --> -25
12/4 --> 3
13/4 --> 3 rather than Since the operands are of INTEGER type, so is the result. The computer will truncate the mathematical result to make it an integer.
3/5 --> 0 rather than 0.6.
11/24/2018
Comp208 Computers in Engineering

7. Evaluate Complex Expression Arithmetic operators: precedence
2+3*4 = ?
= 5 * 4 = 20 Or
= 2+12 = 14
(2+3)*4 = 20
operators
Precedence () 1 ** 2
*, / 3
+, - 4
11/24/2018
Comp208 Computers in Engineering

8. Evaluation Complex Expression Arithmetic operators: associativity()
Left to right **
Right to left
*, /
+, -
associativity resolves the order of operations when two operators of the same precedence compete for three operands:
2**3**4 = 2**(3**4) = 2**81,
72/12/ 3 = (72/12)/3 = 6/3 = 2
30/5*3 = (30/5)*3 = 18
if *,/ associativity is from right to left
30/5*3 = 30/(5*3) = 2
11/24/2018
Comp208 Computers in Engineering

9. Mixed Mode Expressions
If one operand of an arithmetic operator is INTEGER and the other is REAL
the INTEGER value is converted to REAL
the operation is performed
the result is REAL 
1/2.0 
2.0/8 
-3**2.0 
1 + 5/2 
4.0**(1/2) 
3.5
0.5
0.25
-9.0
3 (since 5/22)
1.0 (since ½  0)
11/24/2018
Comp208 Computers in Engineering

10. Logical Expressions (.TRUE. .FALSE.)
Relational operators
lower precedence than arithmetic operator
No associativity (illegal: 4 > 3 >2)
<, <=, >, >=, ==, /=
Logical operators
lower precedence than Relational operators
From left to right except .NOT.
.NOT.
.AND.
.OR.
.EQV., .NEQV.
High
Low
11/24/2018
Comp208 Computers in Engineering

11. Comp208 Computers in Engineering
Examples
Suppose we have the declaration:
INTEGER :: age=34, old=92, young=16
What is the value of the following expressions?
age /= old
age >= young
age==56 .and. old/=92
age==56 .or. old/=92
age==56 .or. old/=92 .and. young==16
.not. age==56 .or. old/=92
11/24/2018
Comp208 Computers in Engineering

12. Comp208 Computers in Engineering
Control statements
IF-THEN-ELSE-END IF
Syntax:
IF (logical-exp) THEN
first statement block,
ELSE
second statement block,
END IF
.TRUE.
.FALSE.
Log Exp
1st Block
2nd block
Stmt following END IF
11/24/2018
Comp208 Computers in Engineering

13. Comp208 Computers in Engineering
Control statements
IF-THEN-END IF
Syntax:
IF (logical-exp) THEN
first statement block,
END IF
.TRUE.
.FALSE.
Log Exp
1st Block
Stmt following END IF
11/24/2018
Comp208 Computers in Engineering

14. Comp208 Computers in Engineering
Control statements
Logical IF
Syntax:
IF (logical-exp)
one statement
.TRUE.
.FALSE.
Log Exp
One statement
Stmt following END IF
11/24/2018
Comp208 Computers in Engineering

15. Comp208 Computers in Engineering
Control statements
.FALSE.
IF-THEN-ELSEIF-THEN-ELSE-END IF
Syntax:
IF (log-exp1) THEN
first statement block,
ELSEIF (log-exp2)THEN
second statement block, ……
ELSE
else block
END IF
Log Exp1
.FALSE.
.TRUE.
Log Exp2
.TRUE.
1st Block
2nd block ……
Stmt following END IF
11/24/2018
Comp208 Computers in Engineering

16. Comp208 Computers in Engineering
IF statement example
Prints a season associated with a given month in terms of value. If the value of the integer month is not in the range 1-12, print “not a month!”
1, 2, 3: Winter
4, 5: Spring
6, 7, 8: Summer
9, 10: Fall
11, 12: Winter
11/24/2018
Comp208 Computers in Engineering

17. IF statement example (cont.)
INTEGER::month
READ(*,*) month !input an integer from keyboard
IF (month ==4 .OR. month==5) THEN
WRITE(*,*) “Spring”
ELSEIF (month >=6 .AND. month <=8) THEN
WRITE(*,*) “Summer”
ELSEIF (month==9 .OR. Month==10) THEN
WRITE(*,*) “FALL”
ELSEIF (month==11 .OR. month==12 .OR. (month>=1 .AND. month<=3)) THEN
WRITE(*,*) “WINTER”
ELSE
WRITE(*,*) “Not a month!”
ENDIF
11/24/2018
Comp208 Computers in Engineering

18. Control statement: SELECT CASE
The SELECT CASE construct provides an alternative to a series of repeated IF ... THEN ... ELSE IF statements.
Syntax:
SELECT CASE( expression )
CASE( value 1)
block 1
...
CASE (value i)
block I …
[CASE DEFAULT
block default]
END SELECT
11/24/2018
Comp208 Computers in Engineering

19. SELECT CASE statement example
INTEGER::month
READ(*,*) month !input an integer from keyboard
SELECT CASE(month)
CASE (1)
WRITE(*,*) “WINTER”
CASE (2)
CASE (3)
CASE (4)
WRITE(*,*) “Spring”
CASE (5)
CASE (6)
WRITE(*,*) “Summer”
CASE (7)
CASE (8)
CASE (9)
WRITE(*,*) “FALL”
CASE (10)
CASE (11)
CASE (12)
CASE DEFAULT
WRITE(*,*) “Not a month!”
END SELECT
11/24/2018
Comp208 Computers in Engineering

20. SELECT CASE statement example
Can be INTEGER, CHARACTER, LOGICAL
No REAL
INTEGER::month
READ(*,*) month !input an integer from keyboard
SELECT CASE(month)
CASE (4,5)
WRITE(*,*) “Spring”
CASE (6:8)
WRITE(*,*) “Summer”
CASE (9,10)
WRITE(*,*) “FALL”
CASE (11, 12, 1:3) THEN
WRITE(*,*) “WINTER”
CASE DEFALUT
WRITE(*,*) “Not a month!”
END SELECT
(value1, value2)
(min:max) i.e., 6, 7, 8
(value1, value2, min:max)
11/24/2018
Comp208 Computers in Engineering

21. Repetition, DO statement
Count loop
uses a control clause to repeat a block of statements a predefined number of times. Note that count variable should not be modified within loop body.
Syntax:
DO count = start, stop [,step]
block of statements
END DO
Infinite loop
Use EXIT to get out. DO
11/24/2018
Comp208 Computers in Engineering

22. Comp208 Computers in Engineering
Count DO Loop examples
Example 1:
DO i=1, 10, 1
WRITE(*,*) i !write numbers 1, 2, …, 10
END DO
Example 2:
DO i=1, 10 ! Default step = 1
Example 3:
DO i=1, 10, 2 ! i increased by 2 for each step
WRITE(*,*) i !write numbers 1,3,5,7,9
11/24/2018
Comp208 Computers in Engineering

23. Comp208 Computers in Engineering
Count DO loop examples
Example 4:
DO j=10,2,-2 ! j decreased by 2
WRITE(*,*) j !write even numbers 10,8,6,4,2
END DO
Example 5:
i=1
DO WHILE(i<10)
write(*,*) I
i=i+1
11/24/2018
Comp208 Computers in Engineering

24. Infinite DO loop example
INTEGER::I=0 DO
IF(I>10) EXIT ! Loop terminated at I==11
WRITE(*,*) I ! WRITE number 1 to 10
I=I ! I increased by 1 at each step
END DO
Without IF(i>10) EXIT, the program will not be able to stop.
11/24/2018
Comp208 Computers in Engineering

25. Comp208 Computers in Engineering
Array
An array is a collection of individual data elements, all of the same type. E.g.,
The subscript (or index) of an array element is the position of that element within the array, for example:
the first element is 51 and has a subscript 1,
the second element is 6 and has a subscript 2.
array
Index: 1 2 3 4 5 6 7 8 51 34 61 75 53
element
11/24/2018
Comp208 Computers in Engineering

26. Comp208 Computers in Engineering
Declare an array
Syntax
type, DIMENSION(bound ) :: name ! Fortran 90 only
type :: name(bound)
Where, bound = [lower:]upper
lower: smallest index of the elements, by default=1
upper: largest index of the elements
E.g., to declare the previous array example:
INTEGER, DIMENSION(8)::a
INTEGER, DIMENSION(1:8)::a
INTEGER::a(8)
INTEGER::a(0:7) ! Then 51’s index=0, 6’s index=1, 5’s index=7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 8 51 34 61 75 53
11/24/2018
Comp208 Computers in Engineering

27. Multi-dimensional array
Consider the following array
This is one-dimensional array so it can only represent a vector. However, some data are more than one dimensional, e.g., matrix
Syntax:
TYPE, DIMENSION([1lb:][1ub], [2lb:][2ub])::name
TYPE::name([1lb:][1ub], [2lb:][2ub]) 51 6 34 61 75 4 53 5 51 61 53 6 75 5 34 4 10
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Comp208 Computers in Engineering

28. Comp208 Computers in Engineering
Two dimensional array
To declare an integer matrix with 3 rows and 4 columns
They are equivalent
INTEGER::a(1:3, 1:4)
INTEGER::a(3,4)
INTEGER, DIMENSION(1:3, 1:4)::a
INTEGER, DIMENSION(3,4)::a
a(i, j): to refer to an element at row i and column j, e.g., a(2, 3)
j=3
i=2
a(2,3)
11/24/2018
Comp208 Computers in Engineering

29. Two dimensional array, example
! To set a matrix with 3 rows and 4 columns to zero
PROGRAM test
IMPLICIT NONE
INTEGER::a(3,4), i, j
DO i=1,3
DO j=1,4
a(i, j)=0
END DO
END PROGRAM
DO j=1,4
a(1, j)=0
END DO
a(2, j)=0
a(3, j)=0
11/24/2018
Comp208 Computers in Engineering

30. Function and Subroutine
type FUNCTION function-name (arg1, arg2, ..., argn)
IMPLICIT NONE
[declarations]
[statements]
[other subprograms]
END FUNCTION function-name
SUBROUTINE subroutine-name (arg1, arg2, ..., argn)
END SUBROUTINE subroutine-name
11/24/2018
Comp208 Computers in Engineering

31. Where Do Function/Subroutine Definitions Go?
External
PROGRAM program-name
IMPLICIT NONE
INTERFACE
[declaration of function/subroutine]
END INTERFACE
[declarations]
[statements]
END PROGRAM program-name
[Function/subroutine definitions]
INTERNAL
PROGRAM program-name
IMPLICIT NONE
[declarations]
[statements]
CONTAINS
[function/Subroutine definitions]
END PROGRAM program-name
11/24/2018
Comp208 Computers in Engineering

32. Rules for Argument Association
Rule 1: If an actual argument is an expression or a constant, it is evaluated and the result is saved into a temporary location. Then, the value in this temporary location is passed.
INTEGER :: a = 10, b = 3, c = 37
WRITE(*,*) Minimum(18,c-a,a+b)
When the function is invoked, new temporary variables we can call x, y and z are created. The value of x is initialized to 18, y to 27 and z to 13.
The function returns 13.
11/24/2018
Comp208 Computers in Engineering

33. Rules for Argument Association
Rule 2: If an actual argument is a variable, the corresponding formal argument is made to refer to the same memory cell.
INTEGER :: a = 10, b = 3, c = 37
WRITE(*,*) Minimum(a,b,c)
When the function is invoked, there are no new variables created. The parameter x refers to a, y to b and z to c. We say x is an alias for a. There are two names for the same memory cell.
The function returns 3.
11/24/2018
Comp208 Computers in Engineering

34. Argument passing example
REAL::x=1, y=2
WRITE(*,*) "x=", x, “y=", y ! X=1.0 y=2.0
CALL swap(x,y)
1.0 a x
2.0 b y
SUBROUTINE swap( a, b )
REAL, INTENT(INOUT):: a, b
REAL:: temp
temp = a
a = b
b = temp
END SUBROUTINE swap
1.0
temp
2.0 a x
1.0 b y
WRITE(*,*) "x=", x, “y=", y ! x=2.0 y=1.0
11/24/2018
Comp208 Computers in Engineering

35. Example passing array as argument
! Input a list of real number and calculate their sum.
PROGRAM Test
IMPLICIT NONE
INTEGER, PARAMETER :: MAX_SIZE = 1000
INTEGER, DIMENSION(1:MAX_SIZE) :: Data
INTEGER :: ActualSize
INTEGER :: i
READ(*,*) ActualSize
READ(*,*) (Data(i), i=1, ActualSize)
WRITE(*,*) "Sum = ", Sum(Data, ActualSize)
CONTAINS
INTEGER FUNCTION Sum(x, n)
INTEGER, INTENT(IN):: n
INTEGER, DIMENSION(n), INTENT(IN) :: x
INTEGER :: Total
Total = 0.0
DO i = 1, n
Total = Total + x(i)
END DO
Sum = Total
END FUNCTION Sum
END PROGRAM Test
11/24/2018
Comp208 Computers in Engineering

36. Comp208 Computers in Engineering
Implied DO Loops
The implied DO loop can simplify this greatly.
INTEGER :: data(100)
INTEGER :: n, i
READ(*,*) n
READ(*,*) (data(i), i=1, n)
If the value of n is 15, this READ(*,*) statement is equivalent to
READ(*,*) data(1), data(2),. . ., data(15)
What is the difference? The values read can appear on one or more lines since FORTRAN will automatically search for the next input on the current input line or go on to the next line if needed.
11/24/2018
Comp208 Computers in Engineering

37. Comp208 Computers in Engineering
FORMAT statement, F
Example
REAL::x=1.0, y=
write(*, 900) x, y
900 format (F3.1, F9.4)
(F3.1,F9.4):
(F3.1,F10.4): 1.0#
(F3.1,F8.4): 1.0********
*: Width=8 is not wide enough to output y.
4 integer digits + 4 decimal digits + 1 for “.” = 9 digits
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Comp208 Computers in Engineering

38. Comp208 Computers in Engineering
FORMAT statement, I
For integers only the field width is specified, so the syntax is Iw. Similarly, character strings can be specified as Aw but the field width is often dropped.
INTEGER::a=1000
WRITE(*,100) “a=“, a
100 FORMAT(A5,I6)
WRITE(*,200) “a=“,a
FORMAT(A,I4)
WRITE(*,300) “a=“,a
300 FORMAT(A,I3)
###a=##1000 A5 I6
a=1000 A I4
a=*** A I3
11/24/2018
Comp208 Computers in Engineering

39. Comp208 Computers in Engineering
FORMAT statement, READ
Example
INTEGER::a,b
READ(*,100) a,b
FORMAT(2I3) ! eqv. To FORMAT(I3,I3)
Correct inputs for (2I3), e.g.,
“##1##2” a=##1=1, b=##2=2
“1##2##” a=1##=1, b=2##=2
“#1##2#” a=#1#=1, b=#2#=2
11/24/2018
Comp208 Computers in Engineering

40. FILE input/output, Example
! Input 10 integers from keyboard and write them to file “inputData.txt”
PROGRAM fileTest
IMPLICIT NONE
INTEGER::count, a
OPEN(UNIT=10,FILE=“inputData.txt”) ! Open file “inputData.txt”
DO count=1,10
WRITE(*,*) “Input an integer number from keyboard:”
READ(*,*) a
READ(10,100) “a=“, a ! Write to “inputData.txt”
END DO
CLOSE(10); ! Close file “inputData.txt”
FORMAT(A2, I8)
END PROGRAM
a=######51
a=#######6 …
inputData.txt
11/24/2018
Comp208 Computers in Engineering