IF Statements

We have encountered an IF statement in A Simple Program in Section 3 in the FORTRAN95 Manual. Section 6.1 has summarized all the forms of the IF construct. Details on the logical expression used in the IF constructs can be found in the section 4.7.

(i) Single statement.

IF ( logical expression ) statement

(ii) Single block of statements.

IF ( logical expression ) THEN

things to be done if true

END IF

(iii) Alternative actions.

IF ( logical expression ) THEN

things to be done if true

ELSE

things to be done if false

END IF

(iv) Several alternatives (there may be several ELSE Ifs, and there may or may not be an ELSE).

IF ( logical expression-1 ) THEN

.........

ELSE IF ( logical expression-2 ) THEN

.........

[ELSE

.........

]

END IF

Keyword: Go To line_number or GOTO line_number

Statements in a program are normally executed line by line in sequence. A GOTO statement will interrupt the sequence and re-direct the program to a new place. After a GOTO statement, the computer will execute the statement labelled by the line_number in the GOTO statement, instead of the statement immediate after the GOTO statement.

Exercises

Remember to add your name, group, course and date at the beginning of each program in comment lines.

1) (optional) Copy the quadratic equation solver program Prog9 into U: drive, save it as Prog9.f95 and execute it.

Program prog9

!=======

! program to solve a quadratic

! user types in the coefficients

!program works out if roots are complex, real or coincident

!=======

! ax*x + b*x + c =0

! calculates b*b - 4(a*c)

!

Complex croot1,croot2

Print *,'program to find roots of a quadratic'

Print *,'solves a*x*x + b*x + c =0 '

Print *, 'type in three coefficients'

read (*,*) a,b,c

d=b*b - 4*a*c

if (ABS (d) < 0.00000001) go to 101

if (d < 0.0) go to 100

!------------------

! discriminant is positive so square root ok

!------------------

root1= (-b + SQRT (d))/(2.0*a)

root2= (-b -SQRT (d))/(2.0*a)

print *," root1 = ",root1

print *," root2 = ",root2

stop

!------------------

! complex roots - print out message

!-----------------

100 print *, ' roots are complex'

croot1= CMPLX((-b/(2.0*a)), SQRT(ABS(d)))

croot2=CONJG (croot1)

Print *, croot1,croot2

stop

!------------------

! roots are coincident - floating point arithmetic never

! gives exactly zero

!--------------------

101 root1= (-b)/(2*a)

print *, " both roots are ",root1

stop

end

2) What happens if we type 0.0 for the first coefficient? Change the program to correct this.

3) List all the variables in Prog9 with their types.

4) List all the keywords used in Prog9.

5) List all the intrinsic functions used in Prog9.

6) How many single IF statements are there in Prog9?

7) Draw the flowchart of Prog9 (optional).

8) Read the following code, what will be the output if input a = 0, 1 or -1?
read(*,*) a
if (a. lt. 0) a = -a
write(*,*) a
9) The following program segment is for trapping unwanted input to variable a. What is the valid data range of a?

100 read(*,*) a
if (a.le.0) goto 100
write(*,*) a

10) What does this program segment do, which uses the IF construct with a single block of statements? What will be the output if a = 10, b = 100 or a = 100, b = 10?
Read(*,*) a, b
if (a .gt. b) then
temp = a
a = b
b = temp
end if
write(*,*) ‘numbers are in ascending order’
write(*,*) a, b
11) The following program converts a mark into a grade. What are the output of the program for mark = 85, 73, 101, 60, 33, 55?

Read(*,*) mark
if (mark.le.100.and.mark.ge.80) then
Write(*,*) ‘mark = ’, mark, ‘ Grade A’

else if (mark.lt.80.and.mark.ge.60) then
Write(*,*)‘mark = ’, mark, ‘ Grade B’

else if (mark.lt.60.and.mark.ge.40) then
Write(*,*)‘mark = ’, mark, ‘ Grade C’

else if (mark.lt.40.and.mark.ge.0) then
Write(*,*)‘mark = ’, mark, ‘ Fail’

else

Write(*,*) ‘ incorrect mark’

end if

12) (optional) Write a program (if_Q12.f95), which uses the IF construct with alternative actions to calculate the value of the following function for x between -1 and 1. The number of steps for x is entered via keyboard at the run-time of the program.

13) (optional) Write a program (stress.f95), which uses the IF construct with several alternatives to provide advice for a student who gives his present stress level.

Input a stress level (0-100)

Output an advice according to the stress level:

0-40: impossible.

41-80: you are fine.

81-100: see your personal tutor.

Other numbers: input error

14) Write a program (checkout.f95) for supermarket checkout according to the flowchart.