Transcript Lecture IV More complicated Fortran programming

REVIEW of Lecture 4

Relational operators:
<, <=, >, >=, ==, /=
 return result of .TRUE. or .FALSE
 lower precedence than all arithmetic operators
2 + 7 >= 3 * 3  .TRUE.

There is no associativity
a < b < c  illegal

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Note that == means “is equal to” but = means
“assign the value on the right”
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REVIEW of lecture 4 (cont.)
Logical Operators
.NOT.
High
.AND.
.OR.
.EQV., .NEQV.
Low
They all associate from left to right, except .NOT.
INTEGER :: age=34, old=92, young=16
age==56 .or. .not.(old/=92)
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REVIEW of Lecture 4 (cont.)

Simplest form
IF (logical expression) single statement

Simpler form
IF (logical expression) THEN
block of statements
END IF

IF-THEN-ELSE-ENDIF statement
IF (logical-expression) THEN
first statement block, s_1
ELSE
second statement block, s_2
END IF
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REVIEW of Lecture 4 (cont.)

Complex form
IF (logical expression 1) THEN
block 1
ELSEIF (logical expression 2) THEN
block 2
...
ELSEIF (logical expression N) THEN
block N-1
[ELSE
block N]
ENDIF
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Area of a Triangle
Heron’s formula gives the area of a triangle in terms of the
lengths of its sides.
To use it, we must make sure that the sides form a triangle.
There are two necessary and sufficient conditions:
1.
2.
All side lengths must be positive
The sum of any two sides must be greater than the third
If these conditions are met, the formula is:
Area = SQRT(s*(s-a)*(s-b)*(s-c)),
where a, b and c are the lengths of the sides
and s is half the perimeter.
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Area of a Triangle
! -----------------------------------------------------! Compute the area of a triangle using Heron's formula
! -----------------------------------------------------PROGRAM HeronFormula
IMPLICIT NONE
REAL
:: a, b, c
! three sides
REAL
:: s
! half of perimeter
REAL
:: Area
! triangle area
LOGICAL :: Cond_1, Cond_2
! two logical conditions
READ(*,*)
a, b, c
Cond_1 = (a > 0.) .AND. (b > 0.) .AND. (c > 0.0)
Cond_2 = (a+b > c) .AND. (a+c > b) .AND. (b+c > a)]
IF (Cond_1 .AND. Cond_2) THEN
s
= (a + b + c) / 2.0
Area = SQRT(s*(s-a)*(s-b)*(s-c))
WRITE(*,*) "Triangle area = ", Area
ELSE
WRITE(*,*) "ERROR: this is not a triangle!“
END IF
END PROGRAM HeronFormula
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Lecture IV More
complicated Fortran
programming
Yi Lin
Sept 13, 2005
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IF statement example

Prints a season associated with a given
month in terms of value. If the value of the
integer month is not in the range 1-12, print
“not a month!”
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1, 2, 3: Winter
4, 5: Spring
6, 7, 8: Summer
9, 10: Fall
11, 12: Winter
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IF statement example (cont.)
INTEGER::month
READ(*,*) month !input an integer from keyboard
IF (month ==4 .OR. month==5) THEN
WRITE(*,*) “Spring”
ELSEIF (month >=6 .AND. month <=8) THEN
WRITE(*,*) “Summer”
ELSEIF (month==9 .OR. Month==10) THEN
WRITE(*,*) “FALL”
ELSEIF (month==11 .OR. month==12 .OR. (month>=1 .AND. month<=3)) THEN
WRITE(*,*) “WINTER”
ELSE
WRITE(*,*) “Not a month!”
ENDIF
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SELECT CASE statement
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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
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SELECT CASE statement
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Semantics
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Firstly evaluate expression. The result of expression may
be of type character, logical or integer. Value must be of the
same type as the result of expression
If the value of the expression is value I, execute block I.
If the value of the expression does not match any of values
listed, evaluate block default.
After one of block has been executed, continue to execute
statements following END SELECT
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SELECT CASE statement
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
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SELECT CASE statement
To
avoid redundancy, use
CASE (value1, value2, …, valuei)
Block of statements
Instead
of
CASE (value1)
Block of statements, S
CASE (value2)
Block of statements, S
….
CASE (valuei)
Block of statements, S
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They are the same statements
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SELECT CASE statement

More aggressively, if value1, value2, …, and
valuei are a consecutive sequence of integer,
e.g., 1, 2, 3, 4. Use this:
CASE (value1:valuei)
Block of statements

In which, value1 is the minimum and valuei is
the maximum value.
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SELECT CASE statement
example
Can be INTEGER, CHARACTER, LOGICAL
No REAL
INTEGER::month
READ(*,*) month !input an integer from keyboard
SELECT CASE(month)
(value1, value2)
CASE (4,5)
WRITE(*,*) “Spring”
CASE (6:8)
(min:max) i.e., 6, 7, 8
WRITE(*,*) “Summer”
CASE (9,10)
WRITE(*,*) “FALL”
(value1, value2,
CASE (11, 12, 1:3) THEN
WRITE(*,*) “WINTER”
CASE DEFALUT
WRITE(*,*) “Not a month!”
END SELECT
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min:max)
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Repetition, DO statement


A count controlled loop uses a control clause to repeat a block of
statements a predefined number of times:
Syntax:
DO count = start, stop [,step]
block of statements
END DO

The control clause is made up of the following:
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count is an integer variable and is used as the 'control'.
start is an integer value (or expression) indicating the initial value of count.
stop is an integer value (or expression) indicating the final value of count.
step is an integer value (or expression) indicating the increment value of
count. The step is optional and has a default value of 1 if omitted.
Count variable is not allowed to change within the loop
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DO statement
Semantics
1.
2.
3.
1.
2.
3.
4.
5.
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First assign the variable, count, an integer value as
start
Compare variables count and stop.
If start does not exceed stop,
execute the statements in the block.
After finishing statements, count = count + step (if
step is not specified, it is 1 by default) .
Go back to step 2.
Else, continue to execute statements following END
DO
End IF
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DO statement examples
Example 1:
DO i=1, 10, 1
WRITE(*,*) i !write numbers 1, 2, …, 10
END DO
Example 2:
DO i=1, 10 ! Default step = 1
WRITE(*,*) i !write numbers 1, 2, …, 10
END DO
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DO statement examples (cont.)
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
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At the last step,
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i=9, write to the screen “9”
i=i+step=9+2=11
11 > 10, terminate the loop
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DO statement examples (cont.)

Step is negative. “Exceed” means “smaller than”.
Example 4:
DO j=10,2,-2 ! j decreased by 2
WRITE(*,*) j !write even numbers 10,8,6,4,2
END DO
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DO statement examples (cont.)

What if start > stop and step is positive? Or
What if start < stop and step is negative?
Example 4:
DO j=1, 10, -1 ! Start < stop and Step = -1
WRITE(*,*) j
END DO

Do nothing
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DO statement example
start

Given two integer A
and B, calculate the
sum of integers from
A to B inclusively.

Sum=0, i=A
.TRUE.
(i.e., B >= X >=A)
i>B
.FALSE.
Sum=sum+i
End
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i=i+1
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Accumulation example
Program Accumulation
IMPLICIT NONE
INTEGER::A,B,i, sum=0
write(*,*) "Input A, B:"
READ(*,*) A,B
DO i=A,B,1
sum=sum+i
END DO
write(*,*) "sum[A,B]=",sum
END program
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DO statement, CONTINUE

CONTINUE

The execution of the current step will be
stopped and continue to the next loop
DO i=1,10
if(i==8) CONTINUE
next step
! Step 8 stopped here and & go to
WRITE(*,*) i !write numbers 1 to 10 except 8
END DO
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Accumulation example
revisited
Program Accumulation
IMPLICIT NONE
INTEGER::A,B,i,sum=0
write(*,*) "Input A, B:"
READ(*,*) A,B
Calculate the sum of integers
between A and B, and
dividable by 2 and 3.
DO i=A,B,1
if(MOD(i,2)!=0 .OR. MOD(i,3)!=0) CONTINUE;
sum=sum+i
END DO
write(*,*) "sum[A, B]=",sum
END program
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DO statement, EXIT

EXIT

The loop will be terminated and exit.
DO i=1,10
if(i == 8) EXIT !Loop terminated when i==8
WRITE(*,*) i !write numbers 1 to 7 only
END DO
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Infinite DO loop
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In the absence of a control clause the block of
statements is repeated indefinitely.
DO
block
END DO

The block of statements will be repeated forever, or
at least until somebody stops the program. In order
to terminate this type of loop the programmer must
explicitly transfer control to a statement outside the
loop.
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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.
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Examples in previous
midterms and/or finals
PROGRAM P4
IMPLICIT NONE
INTEGER :: I ,K
K=1
DO I = 12, 0, -1
WRITE (*,*) “Inside DO loop”
K = K +1
END DO
How many times will the
above program print
“Inside DO loop”?
a) 0
b) 1
c) 12
d) 13
e) None of the above
END PROGRAM P4
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Example in previous exams
What does the following program output
when presented with the following input
1. 4 5
2. 5 5
3. 10 20
Select the best answer:
PROGRAM QUESTION2
IMPLICIT NONE
INTEGER:: A,X,Y,N
a=0
READ (*,*) X,Y
DO N = 1,5,2
1)
IF (X==Y) THEN
A=X
A=A+A+X
ELSEIF(X<Y) THEN
A = 3*X + A
ELSE
A=X
END IF
Y=Y-X
END DO
WRITE (*,*) A,X,Y
END PROGRAM QUESTION2
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4
4
4
-7
10
10
2) 4
4
5
7
10
10
3) 4
4
5
5
10
10
4) 5
5
9
9
10
10
5) None of the above
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-10
7
-11
10
-7
-10
-10
-9
-9
-10
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Character processing
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Character constants
 E.g. “Hello world!”
Declaration
 CHARACTER::varName
! One single character
 CHARACTER(len=int):: varName ! String with length=int
 E.g., CHARACTER::c=“A” ! C=“A”
 E.g., CHARACTER(len=5):: str1=“Hello world!”
 E.g., CHARACTER(len=20)::str2=“Hello world!”
 E.g., CHARACTER(20)::str3=“Hello world!”
If length too short  the rightmost characters are truncated


E.g., str1 = “Hello”
if length too long  the rightmost characters are filled with spaces.

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E.g., str2= “Hello world!#########”
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Character processing (cont.)

Concatenation “//”
CHARACTER (len=24) :: name
CHARACTER (len=6) :: surname
surname = “Fredman”
name = “Prof.#”// “Nathan#”//surname
Name=“Prof.#Nathan#Friedman###”
# represents white space here.
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Character comparison
Dictionary index

Certain rules are applied when comparing
characters or strings (is “A” greater than “B”?)
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String comparison requires strings of the same length.
When comparing strings of different lengths, the shorter
is padded with blanks (right side).
The comparison is from left to right.
Comparison is character by character.
The comparison ends either when a difference is found
or the end of the string is reached.
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Character comparison (cont.)

Character comparison follows a collating
sequence, typically:
0 < 1 < 2 < ... < 9
A < B < ... < Z
a < b < ... < z
ANSI standard:
blank < numerals < upper case < lower case.
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Character comparison (cont.)

Example 1:



test = “abz” > “aBc”
test=.TRUE. Comparison stops at the second character “b”
> “B”. It doesn’t matter the order of the pair of the third
characters.
Example 2

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
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test = “Alexis” > “Alex”
The right string becomes “Alex##” (# : white space)
The first four characters are the same...
character “i” is greater than blank, test=.TRUE.
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Character intrinsic functions
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LEN(string) returns the length of a character string
INDEX(string,sustring) finds the location of a
substring in another string, returns 0 if not found
TRIM(string) returns the string with the trailing
blanks removed.
CHAR(int) converts an integer into a character
ICHAR(c) converts a character into an integer
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Character intrinsic functions
(cont.)

Examples
CHARACTER(len=12) :: surname, firstname
INTEGER :: length, pos
...
length = LEN(surname) ! len=12
firstname = “Walter”
pos = INDEX(firstname, “al”) ! pos=2
firstname = “Fred”
pos = INDEX(firstname, “al”) ! pos=0
length = LEN(TRIM(firstname)) ! len=4
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Character intrinsic functions
(cont.)


It is possible to convert character to integer or vice
versus. However, the conversion between
characters and integers is machine dependent.
Example:
INTEGER :: i
CHARACTER :: ch=“a”
...
i=ICHAR(CHAR(i))
ch=CHAR(ICHAR(ch))
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! i=? In WinXP
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SELECT CASE example

Given a grade in terms of “A,B,C,D,F”, output the corresponding
marks in terms of 100%
CHARACTER::grade
READ(*,*) grade
SELECT CASE(grade)
CASE (“A”)
WRITE(*,*) “90-100”
CASE (“B”)
WRITE(*,*) “80-90”
CASE (“C”)
WRITE(*,*) “70-80”
CASE (“D”)
WRITE(*,*) “60-70”
CASE (“F”)
WRITE(*,*) “<60”
CASE DEFAULT
WRITE(*,*) “unrecognized grade!”
END SELECT
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