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

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...

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)`