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Midterm Review Programming in Fortran

This review covers units of Fortran programs, variables, expressions, statements, control statements, functions, subroutines, and more.

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Midterm Review Programming in Fortran

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  1. Midterm ReviewProgramming in Fortran Yi Lin Feb 13, 2007 Comp208 Computers in Engineering

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

  4. Expression and Mixed mode calculation • 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” Comp208 Computers in Engineering

  5. 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 + 2.5  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) Comp208 Computers in Engineering

  6. Evaluate Complex ExpressionArithmetic operators: precedence 2+3*4 = ? = 5 * 4 = 20 Or = 2+12 = 14 3/3**4 = ? Comp208 Computers in Engineering

  7. Evaluation Complex ExpressionArithmetic operators: associativity 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 Comp208 Computers in Engineering

  8. 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 Comp208 Computers in Engineering

  9. 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 Comp208 Computers in Engineering

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

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

  12. Control statements • Logical IF IF (logical-exp) statement .FALSE. .TRUE. Log Exp One statement Stmt following END IF Comp208 Computers in Engineering

  13. Control statements .FALSE. Log Exp1 • 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 .FALSE. .TRUE. Log Exp2 .TRUE. 1st Block 2nd block …… Stmt following END IF Comp208 Computers in Engineering

  14. 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 Comp208 Computers in Engineering

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

  16. Can be INTEGER, CHARACTER, LOGICAL No REAL (min:max) i.e., 6, 7, 8 (value1, value2) (value1, value2, min:max) SELECT CASE statement example 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 Comp208 Computers in Engineering

  17. 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 block of statements END DO Count=start No Right step Yes Start exceeds stop Yes No Block of statements Count=count+step Next stmt after END DO Comp208 Computers in Engineering

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

  19. 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: DO i=3,3 write(*,*) I End do Example 5: i=1 DO WHILE(i<10) write(*,*) I i=i+1 END DO Comp208 Computers in Engineering

  20. Loop within a loop • Example Do i=1, 3 a = 1 Do j=1,3 a = a+1 End do a = a+1 End Do Write(*,*) a Comp208 Computers in Engineering

  21. 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+1 ! I increased by 1 at each step END DO Without IF(i>10) EXIT, the program will not be able to stop. Comp208 Computers in Engineering

  22. 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: element Comp208 Computers in Engineering

  23. 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 Comp208 Computers in Engineering

  24. 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]) Comp208 Computers in Engineering

  25. j=3 a(2,3) i=2 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) Comp208 Computers in Engineering

  26. ! 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 DO END PROGRAM DO j=1,4 a(1, j)=0 END DO DO j=1,4 a(2, j)=0 END DO DO j=1,4 a(3, j)=0 END DO Two dimensional array, example Comp208 Computers in Engineering

  27. 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) IMPLICIT NONE [declarations] [statements] [other subprograms] END SUBROUTINE subroutine-name Comp208 Computers in Engineering

  28. 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. Comp208 Computers in Engineering

  29. 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. Comp208 Computers in Engineering

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

  31. 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::Sum INTEGER :: ActualSize INTEGER :: i READ(*,*) ActualSize READ(*,*) (Data(i), i=1, ActualSize) WRITE(*,*) "Sum = ", Sum(Data, ActualSize) END PROGRAM Test INTEGER FUNCTION Sum(x, n) IMPLICIT NONE INTEGER, INTENT(IN):: n INTEGER, DIMENSION(n), INTENT(IN) :: x INTEGER :: Total INTEGER :: i Total = 0.0 DO i = 1, n Total = Total + x(i) END DO Sum = Total END FUNCTION Sum Comp208 Computers in Engineering

  32. 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 INTEGER :: data(100) INTEGER :: n, i 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. Comp208 Computers in Engineering

  33. FORMAT statement, F • Example REAL::x=1.0, y=1100.1003 write(*, 900) x, y 900 format (F3.1, F9.4) • (F3.1,F9.4): 1.01100.1003 • (F3.1,F10.4): 1.0#1100.1003 • (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 Comp208 Computers in Engineering

  34. A5 I6 ###a=##1000 A I4 a=1000 A I3 a=*** 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) Comp208 Computers in Engineering

  35. 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 Comp208 Computers in Engineering

  36. a=######51 a=#######6 … inputData.txt 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 Comp208 Computers in Engineering

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