Thinking in Programming

Basic Recipes and Examples

Jose María Alvarez-Rodríguez

Carlos III University of Madrid

josemaria.alvarez@uc3m.es

Programming in a nutshell

Algorithms

The FORTRAN programming language

Compiling and executing
Basic source code snippets
Datatypes and Variable declarations
Flow sentences
Arrays
User-defined datatypes
Functions and procedures

What's next and References

How to think in programs?

Analyze and understand the problem
Design a solution

Is there any formal/mathematical model?

Code the formal model in the programming language

Use the capabilities of the programming language

Test and refactor

All the examples can be executed in a Linux machine using the Gfortran compiler.

Recipe 1

Name: Install the compiler in Windows
Steps:

Download the compiler
Install the IDE+compiler
Open the IDE
Check with a simple program

Result:

Recipe 2

Name: Compile the first program (Ctrl+F9)

Steps:

Write a program
Run->Compile (or Ctrl+F9)
Check the results

Result:

Recipe 3

Name: Run the program

Steps:

Compile the program (Ctrl+F9)
Run the program (F9)

Known Problems:

The console appears but we cannot see the result

Configure the IDE to (not) minimize after running

Create an executable file and run in console

Create a bat file and run in console

Include a final read sentence
 Program Hello
  Print *, "Hello World!"
  read *,
 End Program Hello

Recipe 3 (Result)

Result:

Recipe 4

Name: Declare and use variables

Steps:

Select datatype
Declare variable
Read and print out

Result:

 Program variables
      integer i1,i2
      read *,i1,i2
      print *,i1,i2
      read *,
 end program variables

Tip 1

Name: Set datatypes FORTRAN
Datatypes

Intrinsic types

Numeric types

Boolean
Integer
Real
Comples

Non-numeric types

Logical
Character
Pointer

User-defined datatypes

Registers
...

Exercise 1

"Create a program and declare variables of different datatypes"

 Program simpleDatatypes
      integer :: i1 = 0
      real :: r1 = 1.0
      character :: char1 = 'c'
      complex :: c1
      logical :: l1 = .true.
      c1 = CMPLX(2, 4)
      print *,i1, r1, char1, c1, logical
      read *,
 end program simpleDatatypes

Recipe 4

Name: Use of arithmetic expressions

Steps:

Create an arithmetic expression
Print out result

Result:

 Program arithmeticOperators
      print *, (5+2)
      print *, (5-2)
      print *, (5*2)
      print *, (5/2)
      print *, (5**2)
      read *,
 end Program arithmeticOperators

Tip 2

Name: Priority of operators and module

Priority: * / + -

MOD(A,P) computes the remainder of the division of A by P.

Recipe 5

Name: Use of logical expressions and comparison/relational operators

Steps:

Create logical expressions and combine with operators
Print out result

Result:

 Program logicalOperators
      print *, (.NOT. .TRUE.)
      print *, (.TRUE. .AND. .TRUE.)
      print *, (.TRUE. .OR. .FALSE.)
      print *, (1 < 2)
      print *, (1 > 2)
      print *, (1 < 2)
      print *, (1 >= 2)
      print *, (1 <= 2)
      read *,
 end Program logicalOperators

Exercise 2

"Create a program and combine all type of operator: arithmetic, logical and relational. Assign values to different variables"

Recipe 6

Name: Use of the IF sentence
Steps:

When a restriction must be checked...include and IF sentence

If a number is greater than 5 then print "It is greater than 5"

Result:

Program basicIF
      integer :: top = 5
      integer :: number = 0
      read *, number
      IF (number > top) then
         print *, "It is greater than ", top
      END IF
      read *,
end Program basicIF

Recipe 7

Name: Use of the IF-ELSE sentence
Steps:

When a restriction must be checked...include and IF sentence

If a number is greater than 5 then print "It is greater than 5" otherwise print "It is less than 5"

Result:

      Program basicIFElse
      integer :: top = 5
      integer :: number = 0
      read *, number
      IF (number > top) then
         print *, "It is greater than ", top
      ELSE
        print *, "It is less than ", top
      END IF
      read *,
      end Program basicIFElse

Tip 3

Name: IF-ELSE syntax template

IF (logical-expression-1) THEN
   statements-1
ELSE IF (logical-expression-2) THEN
   statements-2
ELSE IF (logical-expression-3) THEN
   statement-3
ELSE IF (.....) THEN
   ...........
ELSE
   statements-ELSE
END IF

IF (a < b .AND. a < c) THEN
   Result = a
ELSE IF (b < a .AND. b < c) THEN
   Result = b
ELSE
   Result = c
END IF

Select Sentence?

 SELECT CASE (selector)
   CASE (label-list-1)
      statements-1
   CASE (label-list-2)
      statements-2
   CASE (label-list-3)
      statements-3
     .............
   CASE (label-list-n)
      statements-n
   CASE DEFAULT
      statements-DEFAULT
END SELECT
INTEGER :: year

SELECT CASE (year)
   CASE (1)
      WRITE(*,*)  'Freshman'
   CASE (2)
      WRITE(*,*)  'Sophomore'
   CASE (3)
      WRITE(*,*)  'Junior'
   CASE (4)
      WRITE(*,*)  'Senior'
   CASE DEFAULT
      WRITE(*,*)  "Unknown"
END SELECT

Recipe 8

Name: I need to repeat some sentence(s) n times
Steps:

Use a loop
Init all variables that are going to be used in the loop
Set the stop condition
Select the adequate loop

Write a program to show the first 20 natural numbers (descending and using different loops).

Result:

program desc20
  integer :: a

  do a = 20, 0, -1
   print*,a
  end do

  a = 20
  do while (a >= 0) 
   print*,a
   a = a - 1
  end do
end program desc20

Tip 4

Name: General Do-loop with exit
Template:

 DO
   statements-1
   IF (logical-expression)  EXIT
   statements-2
END DO


DO
   statements-1
   IF (logical-expression) THEN
      statements-THEN
      EXIT
   END IF
   statements-2
END DO

Remarks:

The use of the EXIT is not recommended

Exercise 3

What happened in the next DO loops?

 INTEGER :: i
DO
   IF (i <= 3)  EXIT
   WRITE(*,*)  i
   i = i - 1
END DO

INTEGER  :: i
i = 5
DO
   IF (i < -2)  EXIT
   WRITE(*,*)  i
END DO

 INTEGER :: i = 1, j = 5
DO
   IF (j < 0)  EXIT
   WRITE(*,*)  i
   i = i + 1
END DO

Tip 5

Name: Count DO-loop
Template:

 DO control-var = initial-value, final-value, [step-size]
   statements
END DO
INTEGER :: Counter, Init, Final, Step

READ(*,*)  Init, Final, Step
DO Counter = Init, Final, Step
   .....
END DO

Meaning of the count DO Loop

Before the DO-loop starts, the values of initial-value, final-value and step-size are computed exactly once.

step-size cannot be zero.

If the value of step-size is positive (i.e., counting up):

The control-var receives the value of initial-value
If the value of control-var is less than or equal to the value of final-value, the statements part is executed. Then, the value of step-size is added to the value of control-var. Go back and compare the values of control-var and final-value.
If the value of control-var is greater than the value of final-value, the DO-loop completes and the statement following END DO is executed.

If the value of step-size is negative (i.e., counting down):

The control-var receives the value of initial-value
If the value of control-var is greater than or equal to the value of final-value, the statements part is executed. Then, the value of step-size is added to the value of control-var. Go back and compare the values of control-var and final-value.
If the value of control-var is less than the value of final-value, the DO-loop completes and the statement following END DO is executed.

Example count DO loop

INTEGER  :: Count

DO Count = -3, 4, 2
   WRITE(*,*)  Count, Count*Count, Count*Count*Count
END DO
INTEGER, PARAMETER :: Init = 3, Final = 5
INTEGER            :: Iteration

DO Iteration = Init, Final
   WRITE(*,*)  'Iteration ', Iteration
END DO

INTEGER :: a, b, c
INTEGER :: List

READ(*,*)  a, b, c
DO List = MAX(a, b, c), MIN(a, b, c), -2
   WRITE(*,*)  List
END DO

Recipe 9

Name: I want to add up n numbers
Steps:

Use a DO loop
Configure the loop
Add the business logic

Result:

INTEGER :: Count, Number, Sum, Input

Sum = 0
DO Count = 1, Number
   READ(*,*)  Input
   Sum = Sum + Input
END DOINTEGER :: Count, Number, Sum, Input


REAL    :: Average

Sum = 0
DO Count = 1, Number
   READ(*,*)  Input
   Sum = Sum + Input
END DO
Average = REAL(Sum) / Number

Tip 6

Name:

The step size cannot be zero
It is not a good practice

INTEGER :: count

DO count = -3, 4, 0
   ...
END DO

Tip 7

Name:

Do not change the value of the control var

 INTEGER :: a, b, c

DO a = b, c, 3
   READ(*,*)  a ! the value of a is changed
   a = b-c      ! the value of a is changed
END DO

Tip 8

Name:

Do not change the value of any variable involved in initial-value, final-value and step-size.

 INTEGER :: a, b, c, d, e

DO a = b+c, c*d, (b+c)/e
   READ(*,*) b          ! initial-value is changed
   d = 5                ! final-value is changed
   e = -3               ! step-size is changed
END DO

Tip 9

Name:

When you have a count-down loop, make sure the step-size is negative.

 INTEGER :: i

DO i = 10, -10
   .....
END DO

Exercise 4

Given a set of integer input values, write a program to count the number of positive and negative values and compute their sums.

Exercise 5

Write a program to calculate the exponential function

See http://en.wikipedia.org/wiki/Exponential_function

The program reads in an initial value Begin, a final value End and a step size Step, and computes the exponential function value at Begin, Begin+Step, Begin+2*Step, ...

Exercise 6

Write a program to determine the number of input data items, excluding the negative one at the end, and compute the minimum and the maximum.

Exercise 7

Write a program to calculate the GCD using the Euclid's algorithm.

Compute the remainder c of dividing a by b.
If the remainder c is zero, b is the greatest common divisor.
If c is not zero, replace a with b and b with the remainder c.
Go back to step (1).

Recipe 10

Name: I need to store and process n items of the same datatype
Steps:

Declare an array for n items
Init the values
Perform some operation

Remark:

The value of the integer-expression used as an index or subscript for an array element must be in the range of the extent used to declare the array.

Result:

type, DIMENSION( extent ) :: name-1, name-2, ..., name-nREAL, DIMENSION(-1:1)     :: a, Sum
INTEGER, DIMENSION(0:100) :: InputData

INTEGER, PARAMETER :: max = 100
LOGICAL, DIMENSION(1:max ) :: InputData

Exercise 8

Declare an array of 100 integers and load even positions with 0 and odd positions with the own value.

program array
 INTEGER, DIMENSION(1:100) :: x
 DO i = 1, 100
 IF (MOD(i,2) == 0) THEN
   x(i) = 0
 ELSE
   x(i) = i
 ENDIF
END DO
end program array

Exercise 9

Generate an identity matrix.

  program matrix
       INTEGER, DIMENSION(10, 10) :: x
       DO i = 1, 10
        DO j = 1, 10
          x(i,j) = 0
        END DO
        x(i,i)= 1
       END DO
 end program matrix

Exercise 10

Swapping the lower and upper diagonal parts (transpose of a matrix)

   program transpose
       INTEGER, DIMENSION(10, 10) :: x
       INTEGER :: t
        DO i = 1, 10
         DO j = i+1, 10
          t = x(i,j)
          x(i,j) = x(j,i)
          x(j,i) = t
        END DO
       END DO
  end program transpose!Another way...RESULT = TRANSPOSE(MATRIX)

Exercise 11

Multiply 2 matrix. Improve the source code...

m
c_i_k = ∑ a_i_k * b_k_i
k=1

program multiply INTEGER, DIMENSION(10, 10) :: a INTEGER, DIMENSION(10, 10) :: b INTEGER, DIMENSION(10, 10) :: c  DO i = 1, 10   DO j = 1, 10   c(i,j) = c(i,j) + a(i,j) * b(j,i)  END DO END DO end program multiply 
MATMUL(A,B)-->FORTRAN FUNCTION

Recipe 11

Name: I need to repeat some operation n times
Steps:

Create a function
Define the signature
Call the function
Use FORTRAN syntax (Functions and Subroutines)

Result:

 SUBROUTINE  subroutine-name (arg1, arg2, ..., argn)
   IMPLICIT  NONE
   [specification part]
   [execution part]
   [subprogram part]
END SUBROUTINE  subroutine-name

Exercise 12

Write a function to calculate the average of 3 numbers.

PROGRAM avgFunction REAL A,B,C REAL RESULT READ *, A,B,C RESULT = MyAVG(A,B,C) PRINT *, RESULT READ *, END PROGRAM avgFunction REAL FUNCTION MyAVG(X,Y,Z)  REAL X,Y,Z,SUM SUM = X + Y + Z  MyAVG = SUM / 3 RETURN END

What's is still missing...

More syntax in FORTRAN

Variable declaration...
Loops...
Array declaration...
Functions...

Functions and Subroutines
Modules
Input/Output
FORTRAN API
Pointers
Programming techniques

Recursion...

References

End of this seminar...

Thinking in Programming

By Jose María Alvarez

Thinking in Programming

This is a brief introduction to programming with the FORTRAN programming language

2,434

Thinking in Programming

Contents

How to think in programs?

Recipe 1

Recipe 2

Recipe 3

Recipe 3 (Result)

Recipe 4

Tip 1

Exercise 1

Recipe 4

Tip 2

Recipe 5

Exercise 2

"Create a program and combine all type of operator: arithmetic, logical and relational. Assign values to different variables"

Recipe 6

Recipe 7

Tip 3

Select Sentence?

Recipe 8

Tip 4

Exercise 3

Tip 5

Meaning of the count DO Loop

Example count DO loop

Recipe 9

Tip 6

Tip 7

Tip 8

Tip 9

Exercise 4

Exercise 5

Exercise 6

Exercise 7

Recipe 10

Exercise 8

Exercise 9

Exercise 10

Swapping the lower and upper diagonal parts (transpose of a matrix) program transpose INTEGER, DIMENSION(10, 10) :: x INTEGER :: t DO i = 1, 10 DO j = i+1, 10 t = x(i,j) x(i,j) = x(j,i) x(j,i) = t END DO END DO end program transpose!Another way...RESULT = TRANSPOSE(MATRIX)

Exercise 11

Recipe 11

Exercise 12

What's is still missing...

References

End of this seminar...

Thinking in Programming

More from Jose María Alvarez

Swapping the lower and upper diagonal parts (transpose of a matrix)

`program transpose INTEGER, DIMENSION(10, 10) :: x INTEGER :: t DO i = 1, 10 DO j = i+1, 10 t = x(i,j) x(i,j) = x(j,i) x(j,i) = t END DO END DO end program transpose!Another way...RESULT = TRANSPOSE(MATRIX)`