Notes for Exercise #6 for Computer Tools for Engineers          KJC (10/24/99)
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Learning Objectives:

  1) Use of simple IF, IF-THEN, and IF-THEN-ELSE
      - Comparing integer and real values
      - Working with logical variables
  2) Simple use of counter, pre-test, and post-test loops
  3) Introduction to functions

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Concept #1: Simple IF statement for comparing integer values

Example #1: (try this... what happens if 10.0 is entered?)

C *** Example #1
      INTEGER N, M
      PARAMETER (M = 10)
      READ *, N
      IF (N.EQ.M) PRINT *, 'ZAP'
      END

Exercise #1: Change the example program to output ZAP if N is 10 or if N is
             greater than 100.  Hint: magic numbers in the execution are bad.

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Concept #2: Comparing real values (remember precision issue)

Example #2: (try this... enter integer values, enter 10.0 and 10.0000001)

C *** Example #2
      REAL X, Y, EPS
      PARAMETER (EPS = 0.0001, Y = 10.0)
      READ *, X
      IF (ABS(X-Y).LT.EPS) PRINT *, 'ZAP'
      END

Exercise #2: Change the example to output ZAP if X is 10 or if X is 123.112.

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Concept #3: Working with logical values and IF-THEN-ELSE

Example #3: (try this... inputs of (100, 10), (99, 9), and (99, 999))

C *** Example #3
      LOGICAL FLAG1, FLAG2
      INTEGER X, Y
      READ *, X, Y
      FLAG1 = X.GT.Y
      FLAG2 = X.NE.100
      IF (FLAG1.AND.FLAG2) THEN
        PRINT *, '(1)'
      ELSE IF (FLAG2) THEN
        PRINT *, '(2)'
      ENDIF
      END

Exercise #3:  Write a program to input two values, SPEED and DIREC.  Output
              "OK" if SPEED is less than 100 and if DIREC is less than 45.
              Output "Warning" if SPEED is more than 100 or if DIREC is greater
              than 45.  Output "Watch Out" if SPEED is more than 100 and if
              DIREC is greater than 45.

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Concept #4: Counter loop

Example #4:

C *** Example #4
      INTEGER J
      DO 10 J = 1, 10, 2
        PRINT *, J
10    CONTINUE
      END

Exercise #4:  Write a program to do a count-down, "10, 9, 8, ... 0, blastoff"

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Concept #5: Pre-test loop

C *** Example #5
      INTEGER J
      J = 10
10    IF (J.GE.0) THEN
        PRINT *, J
        J = J - 1
        GOTO 10
      ENDIF
      END

Exercise #5:  Write a program that inputs a value, outputs it, and then repeats
              these steps until a negative value is entered.  The input of a
              negative value terminates than program.

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Concept #6: Post-test loop

C *** Example #6
      INTEGER J
      J = 10
10    CONTINUE
        PRINT *, J
        J = J - 1
      IF (J.GE.0) GOTO 10
      END

Exercise #6:  Write a program that inputs a value, outputs it, and then repeats
              these steps until a negative value is entered.  The input of a
              negative value terminates than program.

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Concept #7: Functions

C *** Example #7
      INTEGER J, DECR
      J = 10
10    CONTINUE
        PRINT *, J
        J = DECR(J)
      IF (J.GE.0) GOTO 10
      END

      INTEGER FUNCTION DECR(J)
        DECR = J - 1
      RETURN
      END

Exercise #7:  Write a program that inputs a value, calls a function that
              doubles the value, and then outputs the (returned) doubled
              value.

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Check-out Exercise:

  Write a program that outputs the first 20 values of the following
  sequence:  0, 1, 3, 6, 10, 15, 21, 28, etc.  Your program starts the
  output with 0 (i.e., it outputs that above values and then the next
  12 values.

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