1. Control Structures

2. Control structures
Control structures are statements that determine the flow of information through your program.
There are not very many of them but they are VERY IMPORTANT
They are the logical building blocks of every program you will ever write.

3. Outline of control structures
Sequential
Decision (selection)
Loops (repetition)

4. Sequential control structures
These are the simplest of all
A program with sequential structure
Does each statement in order
It skips no statements
It repeats no statements

5. Sequential control example

6. Program example: INTEGER score PRINT*, “Please enter test score”
READ*, score
PRINT*, “The score entered was”, score
END

7. Flow chart
START
Prompt for score
Read score
Print score
START

8. Decision structures
A decision, or selection, structure is one that allows you to choose from among several options.
Decision structures allow you to skip some statements.

9. Types of decision/selection structures
A. Logical IF
B. Block IF
1. Single alternative IF
2. Double alternative IF
3. Multiple alternative IF

10. Program example: the LOGICAL IF
INTEGER score
PRINT*, “Please enter test score”
READ*, score
IF (score .ge. 60) PRINT*, “PASS”
END

11. Characteristics of the logical IF
It is all on one line
It handles only one alternative
It performs only one task
Unlike other forms of the IF statement it does not use the keyword THEN or the keyword ENDIF

12. Algorithm example 1. Declare variables 2. Prompt for score
3. Read score
4. If (score >= 60) then
4.1 print ‘Pass’
5. End

13. Flowchart Prompt for score Read score true Score >= 60 Print ‘PASS’
false

14. The single alternative IF
Handles only one alternative (like the logical IF
May include many tasks
Has block structure (IF…THEN, ENDIF)

15. Program example: The single alternative block IF
INTEGER score
PRINT*, “Please enter test score”
READ*, score
IF (score .ge. 60) THEN
PRINT*, “PASS”
ENDIF
END
BLOCK IF

16. Single alternative IF with multiple tasks
INTEGER score
PRINT*, “Please enter test score”
READ*, score
IF (score .ge. 60) THEN
PRINT*, “Congratulations!”
PRINT*, “You earned a passing grade.”
PRINT*, “Time to mom about”
PRINT*, “how well you are doing in”
PRINT*, “college this semester.”
ENDIF
END

17. Double alternative IF 1. Declare variables 2. Prompt for score
3. Read score
4. Process score
4.1 If (score >= 60) then
print ‘Pass’
4.2 else
print ‘Fail’
5. End

18. alternatives Algorithm numbering
3.1 (first alternative)
3.1.x (associated tasks)
3.2 (second alternative)
3.2.x (associated tasks)
The idea is that the program will execute the first alternative that has a true condition.
Only 1 alternative is selected

19. Flowchart Prompt for score Read score true false Score >= 60
Print ‘PASS’
Print ‘FAIL’

20. Program example INTEGER score PRINT*, “Please enter test score”
READ*, score
IF (score .ge. 60) THEN
PRINT*, “PASS”
ELSE
PRINT*, “FAIL”
ENDIF
END
1st alternative
2nd alternative

21. Parking problem example
INTEGER people, days
REAL permit, cost, total
PRINT*, “Please enter cost of permit”
READ*, permit
PRINT*, “How many days”
READ*, days
PRINT*, “Number of people in the car”
READ*, people
IF ((days .le. 0).or.(people .le. 0)) THEN
PRINT*, “ERROR - negative number”
ELSE
! Process good data in here
ENDIF
END

22. Nested loops
Nested IF’s are situations where one IF statement is inside of another one.
This is very common
Make sure the inner IF is ended before the outer IF is
you should not have overlapping structures!

23. Good nesting IF ((days .le. 0).or.(people .le. 0)) THEN
PRINT*, “ERROR - negative number”
ELSE
calculate cost of daily pass
calculate total cost of pay lot
IF (total .gt. Permit) THEN
PRINT*, “Buy permit”
PRINT*, “Pay as you go”
ENDIF
END

24. Invalid nesting Intersecting control structures
IF ((days .le. 0).or.(people .le. 0)) THEN
PRINT*, “ERROR - negative number”
ELSE
calculate cost of daily pass
calculate total cost of pay lot
IF (total .gt. Permit) THEN
PRINT*, “Buy permit”
PRINT*, “Pay as you go”
ENDIF
END
Intersecting control structures

25. Multiple alternative IF
1. Declare variables
2. Prompt for score
3. Read score
4. Process score
4.1 If (score >= 90) then
print ‘A’
4.2 else if (score >= 80) then
print ‘B’
4.3 else if (score >= 70) then
print ‘C’
4.4 else if (score >= 60) then
print ‘D’
4.5 else
print ‘F’
5. End

26. Read score
true
Score
>= 90
false
Print ‘A’
true
Score
>= 80
false
Print ‘B’
true
false
Score
>= 70
Print ‘C’
true
false
Score
>= 60
Print ‘D’
Print ‘F’

27. Multiple alternative IF
IF (score .ge. 90) THEN
PRINT*, “A”
ELSE IF (score .ge. 80) THEN
PRINT*, “B”
ELSE IF (score .ge. 70) THEN
PRINT*, “C”
ELSE IF (score .ge. 60) THEN
PRINT*, “D”
ELSE
PRINT*, “F”
ENDIF
END

28. Algorithm, program correspondence
4.1 If (score >= 90) then
print ‘A’
4.2 else if (score >= 80) then
print ‘B’
4.3 else if (score >= 70) then
print ‘C’
4.4 else if (score >= 60) then
print ‘D’
4.5 else
print ‘F’
IF (score .ge. 90) THEN
PRINT*, “A”
ELSE IF (score .ge. 80) THEN
PRINT*, “B”
ELSE IF (score .ge. 70) THEN
PRINT*, “C”
ELSE IF (score .ge. 60) THEN
PRINT*, “D”
ELSE
PRINT*, “F”
ENDIF

29. Parking problem: Processing good data
IF (people .gt. 2) THEN
cost = 0.25
ELSE IF (people .eq. 2) THEN
cost = 0.50
ELSE
cost = 0.75
ENDIF
total = days * cost
IF (total .gt. permit) THEN
PRINT*, “Buy the permit”
PRINT*, “Pay as you go”

30. Parking problem example
IF ((days .le. 0).or.(people .le. 0)) THEN
PRINT*, “ERROR - negative number”
ELSE
IF (people .gt. 2) THEN
cost = 0.25
ELSE IF (people .eq. 2) THEN
cost = 0.50
cost = 0.75
ENDIF
total = days * cost
IF (total .gt. permit) THEN
PRINT*, “Buy the permit”
PRINT*, “Pay as you go”

31. Parking problem example
IF ((days .le. 0).or.(people .le. 0)) THEN
PRINT*, “ERROR - negative number”
ELSE
IF (people .gt. 2) THEN
cost = 0.25
ELSE IF (people .eq. 2) THEN
cost = 0.50
cost = 0.75
ENDIF
total = days * cost
IF (total .gt. permit) THEN
PRINT*, “Buy the permit”
PRINT*, “Pay as you go”

32. Parking problem example
IF ((days .le. 0).or.(people .le. 0)) THEN
PRINT*, “ERROR - negative number”
ELSE
IF (people .gt. 2) THEN
cost = 0.25
ELSE IF (people .eq. 2) THEN
cost = 0.50
cost = 0.75
ENDIF
total = days * cost
IF (total .gt. permit) THEN
PRINT*, “Buy the permit”
PRINT*, “Pay as you go”

33. Read permit, people, days
true
false
data > 0 t f
ERROR message
people
> 2
Cost = .25 t f
people
= 2
Cost = .50
Cost = .75
Total = days * cost t
Total
> permit f
Permit
Paylot

34. A good test question: What is printed?
INTEGER num
num = 5
IF (num .lt. 10) THEN
PRINT*, “num is less than 10”
ELSE IF (num .eq. 5) THEN
PRINT*, “num equals 5”
ELSE IF (num .gt. 0) THEN
PRINT*, “num is greater than 0”
ELSE
PRINT *, “all of the above”
ENDIF

35. Multiple choice A. num is less than 10 B. num equals 5
C. num is greater than 0
D. All of the above
E. num is less than 10
num equals 5
num is greater than 0

36. Correct answer A. num is less than 10 B. num equals 5
C. num is greater than 0
D. All of the above
E. num is less than 10
num equals 5
num is greater than 0

37. Why?
Remember that with multiple alternative structures the first true choice is the one that gets done.
All others get skipped.
This makes more sense if you see a diagram of the process.

38. num = 5
true
num
< 10
false
Print < 10
true
num
= 5
false
Print = 5
true
false
num
> 0
Print > 0
Print all

39. num = 5 true num < 10 false Print < 10 true num = 5 false
The only way you can get
here is if the first condition
if false!
true
num
< 10
false
Print < 10
true
num
= 5
false
Print = 5
true
false
num
> 0
Print > 0
Print all

40. num = 5
true
num
< 10
false
Print < 10
true
num
= 5
false
Print = 5
true
false
num
> 0
Print > 0
Print all

41. How selection decisions are made
Selection decisions are based around comparisons of the relationships between the data values.
Comparisons are made using relational operators.

42. Relational operators OPERATOR f77 f90 Greater than .gt. >
Greater than or equal to ge >=
Less than lt <
Less than or equal to le <=
Equal to eq ==
Not equal to ne /=

43. Logical operators
OPERATOR f77 and f90
and and.
or or.
not not.

44. Conditions and relational operators
All IF statements work by evaluating a condition.
Example:
IF (num .gt. 0) THEN
The condition is ‘num .gt. 0’
Every IF statement must have one set of parentheses around the entire condition.
The condition evaluates to either true or false.

45. Conditional evaluations
INTEGER num1, num2
num1 = 10
num2 = 20
IF (num1 .lt. num2) THEN
(10 < 20)
(true)

46. Evaluation results INTEGER num1, num2 num1 = 10 num2 = 20
IF (num1 .lt. num2) THEN
PRINT*, “num1 is larger”
ENDIF
PRINT*, “program over”
END
If the condition is true
the task is performed.
If the condition is false
the task is not performed.
Output if condition is true
num1 is larger
program over
Output if condition is false
program over

47. Evaluation results INTEGER num1, num2 num1 = 10 num2 = 20
IF (num1 .lt. num2) THEN
PRINT*, “num1 is larger”
ELSE
PRINT*, “num1 is not larger”
ENDIF
PRINT*, “program over”
END
If the condition is true
the task is performed.
If the condition is false
the false task is performed.
Output if condition is true
num1 is larger
program over
Output if condition is false
num1 is not larger
program over

48. Complex expressions
Sometimes it is necessary to mix both relational and logical operators in expressions.
In this case, it can be very difficult to decide what the computer is going to do.

49. Evaluation results INTEGER length, height length = 10 height = 20
IF ((length .lt. 0) .or. (height .lt. 0)) THEN
PRINT*, “ERROR - negative numbers”
ELSE
PRINT*, “The area is: “, length * height, “square feet”
ENDIF
END
Output if condition is true
ERROR - negative numbers
Output if condition is false
The area is: 200 square feet

50. Evaluation results INTEGER length, height length = 10 height = 20
IF ((length .lt. 0) .or. (height .lt. 0)) THEN
((false) .or. (false))
.or. truth table
(false)
true
true
true
true
false
true
true
false
true
false
false
false

51. Evaluation results INTEGER length, height length = 10 height = 20
IF ((length .lt. 0) .or. (height .lt. 0)) THEN
PRINT*, “ERROR - negative numbers”
ELSE
PRINT*, “The area is: “, length * height, “square feet”
ENDIF
END
Output if condition is false
The area is: 200 square feet

52. Evaluation results INTEGER length, height length = -10 height = 20
IF ((length .lt. 0) .or. (height .lt. 0)) THEN
((true) .or. (false))
.or. truth table
(true)
true
true
true
true
false
true
true
false
true
false
false
false

53. Evaluation results INTEGER length, height length = -10 height = 20
IF ((length .lt. 0) .or. (height .lt. 0)) THEN
((true) .or. (false))
.or. truth table
(true)
true
true
true
true
false
true
true
false
true
false
false
false

54. Evaluation results INTEGER length, height length = -10 height = -20
IF ((length .lt. 0) .or. (height .lt. 0)) THEN
((true) .or. (true))
.or. truth table
(true)
true
true
true
true
false
true
true
false
true
false
false
false

55. Evaluation results INTEGER length, height length = -10 height = -20
IF ((length .lt. 0) .or. (height .lt. 0)) THEN
((true) .or. (true))
.or. truth table
(true)
true
true
true
true
false
true
true
false
true
false
false
false

56. Summary of .or.
The only way you can get false is if both operands are false.
If either one, or both are true, then the whole thing is true.
.or. truth table
true
true
true
true
false
true
true
false
true
false
false
false

57. Evaluation results of .and.
INTEGER length, height
length = 10
height = 20
IF ((length .lt. 0) .and. (height .lt. 0)) THEN
PRINT*, “ERROR - negative numbers”
ELSE
PRINT*, “The area is: “, length * height, “square feet”
ENDIF
END
Output if condition is true
ERROR - negative numbers
Output if condition is false
The area is: 200 square feet

58. Evaluation results INTEGER length, height length = 10 height = 20
IF ((length .lt. 0) .and. (height .lt. 0)) THEN
((false) .and. (false))
.and. truth table
(false)
true
true
true
true
false
false
false
false
true
false
false
false

59. Evaluation results INTEGER length, height length = 10 height = 20
IF ((length .lt. 0) .and. (height .lt. 0)) THEN
PRINT*, “ERROR - negative numbers”
ELSE
PRINT*, “The area is: “, length * height, “square feet”
ENDIF
END
Output if condition is false
The area is: 200 square feet

60. Evaluation results INTEGER length, height length = -10 height = 20
IF ((length .lt. 0) .and. (height .lt. 0)) THEN
((true) .and. (false))
.and. truth table
(false)
true
true
true
true
false
false
false
false
true
false
false
false

61. Evaluation results INTEGER length, height length = -10 height = 20
IF ((length .lt. 0) .and. (height .lt. 0)) THEN
((true) .and. (false))
.and. truth table
(false)
true
true
true
true
false
false
false
false
true
false
false
false

62. Now we have got a problem
INTEGER length, height
length = -10
height = 20
IF ((length .lt. 0) .and. (height .lt. 0)) THEN
PRINT*, “ERROR - negative numbers”
ELSE
PRINT*, “The area is: “, length * height, “square feet”
ENDIF
END
Output if condition is false
The area is: -200 square feet

63. Evaluation results INTEGER length, height length = -10 height = -20
IF ((length .lt. 0) .and. (height .lt. 0)) THEN
((true) .and. (true))
.and. truth table
(true)
true
true
true
true
false
false
false
false
true
false
false
false

64. Evaluation results INTEGER length, height length = -10 height = -20
IF ((length .lt. 0) .and. (height .lt. 0)) THEN
((true) .and. (true))
.and. truth table
(true)
true
true
true
true
false
false
false
false
true
false
false
false

65. Summary of .and.
The only way you can get true is if both operands are true.
If either one, or both are false, then the whole thing is false.
.and. truth table
true
true
true
true
false
false
false
false
true
false
false
false