1. Subroutine
Comp 208
Yi Lin

2. Comp208 Computers in Engineering
SUBROUTINE
SUBROUTINE is a program unit like FUNCTION, except that
It does not have return value, so no TYPE in declaration
To be called:
CALL <name of SUBROUTINE>
Syntax
SUBROUTINE name(arg1, arg2, ..., argn)
IMPLICIT NONE
[declarations]
[statements]
[other subprograms]
END SUBROUTINE name
2018/11/12
Comp208 Computers in Engineering

3. Comp208 Computers in Engineering
SUBROUTINE (cont.)
Since there is no return value for a SUBROUTINE, what is the reason to use a SUBROUTINE?
A subroutine is used to change the value of one or more of its arguments.
2018/11/12
Comp208 Computers in Engineering

4. A Factorial Subroutine
INTEGER FUNCTION Factorial(n)
IMPLICIT NONE
INTEGER :: n, Fact, i
Fact = 1
DO i = 1, n
Fact = Fact * i
END DO
Factorial=Fact
END FUNCTION Factorial
SUBROUTINE Factorial(n, Fact)
IMPLICIT NONE
INTEGER :: n, Fact, i
Fact = 1
DO i = 1, n
Fact = Fact * i
END DO
END SUBROUTINE Factorial ……
READ(*,*) a
CALL Factorial(a, value)
WRITE(*,*) value
CALL
2018/11/12
Comp208 Computers in Engineering

5. Comp208 Computers in Engineering
SUBROUTINE example
PROGRAM testSwap
IMPLICIT none
REAL::x=1, y=2
WRITE(*,*) "x=", x, "y=", y
CALL swap(x,y)
END PROGRAM
SUBROUTINE swap( a, b )
REAL:: a, b, temp
temp = a
a = b
b = temp
END SUBROUTINE swap
2018/11/12
Comp208 Computers in Engineering

6. SUBROUTINE example (cont.)
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:: a, b, 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
2018/11/12
Comp208 Computers in Engineering

7. Function and Subroutine argument passing
Actual argument: the argument(s) in a referencing statement,
e.g., CALL swap(x, y)
x and y are actual arguments
Dummy argument: those in the corresponding function or subroutine definition statement
e.g., SUBROUTINE swap( a, b )
a and b are dummy arguments
2018/11/12
Comp208 Computers in Engineering

8. Function and Subroutine argument passing
If actual arguments are variables, the corresponding dummy arguments refer to the same variables (i.e., using the same memory slot with actual argument)
So if dummy arguments are to be changed in the functions or subroutines, the actual arguments will be changed also.
Programmers must be very careful about argument passing.
2018/11/12
Comp208 Computers in Engineering

9. Comp208 Computers in Engineering
Example
PROGRAM Mid8
IMPLICIT NONE
INTEGER :: Calc, a
a = 10
WRITE(*,*) Calc(a) - Calc(a)
END PROGRAM Mid8
INTEGER FUNCTION Calc(N)
INTEGER :: N, var
var = 0 DO
IF (N<=0) EXIT
var = var + N
N = N - 1
END DO
Calc = var
END FUNCTION Calc
A. 0
B. 10
C. 45
D. 55
E. None of the above
2018/11/12
Comp208 Computers in Engineering

10. Array argument and examples
Yi Lin
Oct 4, 2005

11. Comp208 Computers in Engineering
Array as arguments
Sometimes we need to pass a list of variables with the same type to a function or subroutine. In this case, we can use an array as an argument.
INTEGER::a, b, c, d ……
CALL sub1(a, b, c, d)
Or sometimes we need to process different arrays in the same way by using a function or subroutine. We can use an array as an argument.
INTEGER::array1(10), array2(20)
CALL sub3(array1, array2, …)
INTEGER::array1(4) ……
CALL sub2(array1, …)
2018/11/12
Comp208 Computers in Engineering

12. Comp208 Computers in Engineering
Array as argument
Passing an array as an argument is like passing a variable. The function needs to know the following information about the array:
Name of the array
Type of the array
Besides those, one more information is needed, compared to passing a variable:
Bounds of the array
To avoid referring to elements out of the bounds.
2018/11/12
Comp208 Computers in Engineering

13. Example passing array as argument
! Input a list of real number and calculate their sum.
PROGRAM Test
IMPLICIT NONE
INTEGER :: Data(1000)
INTEGER :: ActualSize, i
INTEGER :: SumFunc
READ(*,*) ActualSize
READ(*,*) (Data(i), i=1, ActualSize)
WRITE(*,*) "Sum = ", SumFunc(Data, ActualSize)
END PROGRAM Test
INTEGER FUNCTION SumFunc(x, n)
INTEGER :: n
INTEGER :: x(n)
INTEGER :: Total
INTEGER :: i
Total = 0.0
DO i = 1, n
Total = Total + x(i)
END DO
SumFunc = Total
END FUNCTION SumFunc
2018/11/12
Comp208 Computers in Engineering

14. Comp208 Computers in Engineering
How do we input the data?
The straightforward way to read data into an array uses a counted DO loop:
INTEGER :: data(100)
INTEGER :: n, i
READ(*,*) n
DO i = 1, n
READ(*,*) data(i)
END DO
In this example, if the input to n is 15, the READ(*,*) statement executes 15 times. Each time it is executed, it reads one line and takes the first integer value on that line.
Therefore, excluding the input for n, 15 input lines are required.
2018/11/12
Comp208 Computers in Engineering

15. 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.
2018/11/12
Comp208 Computers in Engineering

16. Output with Implied DO Loops
Implied DO Loops can also be used to output expressions. Note that any expressions can appear, not just array elements.
INTEGER :: data(10)
INTEGER :: n = 5, i
DO i = 1, 5
WRITE(*,*) i, data(i)
END DO
WRITE(*,*) (i, data(i), i=1, n)
The first WRITE(*,*) is executed five times. Thus, the output is on five different lines.
The second WRITE(*,*) is equivalent to
WRITE(*,*) 1, data(1), 2, data(2), 3, data(3), 4, & data(4), 5, data(5)
Therefore, the ten output values are on the same line.
2018/11/12
Comp208 Computers in Engineering

17. Example passing array as argument
! Input a list of real number and calculate their sum.
PROGRAM Test
IMPLICIT NONE
INTEGER :: Data(1000)
INTEGER :: ActualSize, i
INTEGER :: Sum
READ(*,*) ActualSize
READ(*,*) (Data(i), i=1, ActualSize)
WRITE(*,*) "Sum = ", Sum(Data, ActualSize)
END PROGRAM Test
INTEGER FUNCTION Sum(x, n)
INTEGER :: n, I, total
INTEGER :: x(n)
Total = 0.0
DO i = 1, n
Total = Total + x(i)
END DO
Sum = Total
END FUNCTION Sum 1 2 3 4 5 15 6 9 10
Data x
Sum=43
2018/11/12
Comp208 Computers in Engineering

18. Example passing array as argument
Sometimes if the function only processes part of the array, we need to pass the bounds (lower and/or upper)
PROGRAM Test ……
WRITE(*,*) “Sum from 2 to n-1:”, sumFunc(data, n, 2, n-1)
END PROGRAM Test
INTEGER FUNCTION SumFunc(x, n, startIndex, endIndex)
IMPLICIT NONE
INTEGER :: x(n)
INTEGER :: Total
INTEGER :: i
Total = 0
DO i = startIndex, endIndex
Total = Total + x(i)
END DO
SumFunc = Total
END FUNCTION SumFunc 1 2 3 4 5 15 6 9 10
Data x
Sum=18
2018/11/12
Comp208 Computers in Engineering

19. Example with array arguments
Problem: Given an array, reverse its elements. For example, given: 15 6 3 9 10 10 9 3 6 15
2018/11/12
Comp208 Computers in Engineering

20. Example: reversing an array
Recall that we have SUBROUTINE swap(x,y) before. We can reuse this SUBROUTINE.
Which two elements need to swap?
In general, to which position will a(i) be swapped?
i = (n+1)-j then a(i) a(j)
What happens if there are odd elements in the array?
The middle element swaps with itself.
a(1) a(5)
a(2) a(4)
a(3) a(3) 15 6 3 9 10
2018/11/12
Comp208 Computers in Engineering

21. Example: reverse an array
SUBROUTINE reverseArray(a, n)
IMPLICIT NONE
INTEGER :: n, I
INTEGER :: a(n)
DO i=1, n
CALL swap(a(i), a(n+1-i))
END DO
END SUBROUTINE
n/2
2018/11/12
Comp208 Computers in Engineering

22. Example: reverse an array
SUBROUTINE swap( a, b )
real :: a, b
real::temp
temp = a
a = b
b = temp
END SUBROUTINE swap
YES!
Changed to INTEGER
Note that SUBROUTINE reverseArray will use this SUBROUTINE to swap two integers. But this SWAP requires two real number as arguments. Will this be a problem?
2018/11/12
Comp208 Computers in Engineering

23. Example: reverse an array
PROGRAM test
IMPLICIT NONE
INTEGER::data(5)
INTEGER::i
Do i=1,5
data(i)=I
End Do
WRITE(*,*) (data(i), i=1,5)
CALL reverseArray(data, 5)
END PROGRAM test
2018/11/12
Comp208 Computers in Engineering

24. Examples in Midterm 05 winter
PROGRAM QUESTION9
IMPLICIT NONE
INTEGER ARR(5), I, FUNC
DO I = 1 , 5
READ(*,*) ARR(I)
END DO
PRINT*, FUNC(ARR)
STOP
END PROGRAM QUESTION9
INTEGER FUNCTION FUNC(ARR)
INTEGER ARR(5), I, J
FUNC = 0
FUNC = FUNC + ARR(I)*10**(I-6)
END FUNCTION FUNC
Input : 1 2 3 4 5
Answer:
A. 0 B C D
E. None of the above
2018/11/12
Comp208 Computers in Engineering

25. Comp208 Computers in Engineering
Programming example
Given an array with n elements and an amount delta, please write a subroutine to increase all elements of an array by the amount.
Consider these hints:
What is the interface of your subroutine?
How many arguments does it takes?
What types?
How to implement the increasing of all elements?
2018/11/12
Comp208 Computers in Engineering

26. Multidimensional Arrays
The same rules apply to multidimensional arrays used as arguments
The extent of each dimension can be specified
If assumed shape arrays are used, only the lower bound is required
If the lower bound is 1, even that can be omitted
2018/11/12
Comp208 Computers in Engineering

27. Example with 2 dimensional array
PROGRAM Test
IMPLICIT NONE
INTEGER :: data(2,3), I, j
Do i=1, 2
Do j=1, 3
data(I,j)=i*j
end do
End do
WRITE(*,*) “Sum=“, SumFunc(data, 2, 3)
End program test
INTEGER FUNCTION SumFunc(a, n, m)
integer:: n, m, a(n,m)
INTEGER::I, j, Total
DO i=1, n
DO j=1, m
Total=total+a(I,j)
END DO
SumFunc = Total
END FUNCTION SumFunc
Bug:
Total is not initiated as 0
2018/11/12
Comp208 Computers in Engineering

28. A complete example: Is an M Prime?
Look for factors less than M
If M>2 it must be odd
We need a loop that checks goes through the potential factors
Potential factors must be odd numbers, 3, 5, 9, 11, …
For each one we check to see whether it divides M evenly
A clever observation: We only have to check for factors up to Sqrt(M)
2018/11/12
Comp208 Computers in Engineering

29. Comp208 Computers in Engineering
Testing for Primality
! This function receives an INTEGER formal argument Number. If it is a prime
! number, .TRUE. is returned; otherwise, this function returns .FALSE. !
LOGICAL FUNCTION Prime(Number)
IMPLICIT NONE
INTEGER :: Number
INTEGER :: Divisor
IF (Number < 2) THEN
Prime = .FALSE.
ELSE IF (Number == 2) THEN
Prime = .TRUE.
ELSE IF (MOD(Number,2) == 0) THEN
ELSE
Divisor = 3 DO
IF (Divisor*Divisor>Number .OR. MOD(Number,Divisor)==0) EXIT
Divisor = Divisor + 2
END DO
Prime = Divisor*Divisor > Number
END IF
END FUNCTION Prime
2018/11/12
Comp208 Computers in Engineering

30. Comp208 Computers in Engineering
Complete Program (1) !
! Find all prime numbers in the range of 2 and an input value !
PROGRAM Primes
IMPLICIT NONE
INTEGER :: Range, Number, Count
INTEGER::GetNumber
LOGICAL::prime
Range = GetNumber()
Count = 1
WRITE(*,*) 'Prime number #', Count, ': ', 2
DO Number = 3, Range, 2
IF (Prime(Number)) THEN
Count = Count + 1
WRITE(*,*) 'Prime number #', Count, ': ', Number
END IF
END DO
WRITE(*,*) 'There are ', Count, ' primes between 2 and ', Range
END PROGRAM Primes
2018/11/12
Comp208 Computers in Engineering

31. Comp208 Computers in Engineering
Complete Program (2) !
! This function does not require any formal argument.
! It prompts the user to enter an integer >= 2
INTEGER FUNCTION GetNumber()
IMPLICIT NONE
INTEGER :: N
WRITE(*,*) 'What is the range ? ' DO
READ(*,*) N
IF (N >= 2) EXIT
WRITE(*,*) 'The range value must be >= 2. Your input is ', N
WRITE(*,*) 'Please try again:'
END DO
GetNumber = N
END FUNCTION GetNumber
2018/11/12
Comp208 Computers in Engineering

32. Comp208 Computers in Engineering
Complete Program (3)
LOGICAL FUNCTION Prime(Number)
IMPLICIT NONE
INTEGER :: Number
INTEGER :: Divisor
IF (Number < 2) THEN
Prime = .FALSE.
ELSE IF (Number == 2) THEN
Prime = .TRUE.
ELSE IF (MOD(Number,2) == 0) THEN
ELSE
Divisor = 3 DO
IF (Divisor*Divisor>Number .OR. MOD(Number,Divisor)==0) EXIT
Divisor = Divisor + 2
END DO
Prime = Divisor*Divisor > Number
END IF
END FUNCTION Prime
2018/11/12
Comp208 Computers in Engineering