Monte Carlo integration Fortran 90 (#363)

* first lines

* added further code

* Figured the use of functions out

* Figured the use of functions out

* Running program. 1 Warning left. Pi estimate cruel"

* 1M samples

* formating style

* Applied the notes to the PR. Added correct precisions to constants.

* Added INTERFACE section to resolve raise of IMPLICIT error

* Change code according to review comments

* white spaces and spacing

* indexed?

Author: depate

CONTRIBUTORS.md

@@ -62,4 +62,8 @@ mukundan314
 <br>
 Trashtalk
 <br>
-Cyrus Burt
+Cyrus Burt
+<br>
+Patrik Tesarik
+<br>
+

book.json

@@ -135,6 +135,10 @@
         {
           "lang": "nim",
           "name": "Nim"
+        },
+        {
+          "lang": "f90",
+          "name": "Fortran"
         }
       ]
     }

contents/monte_carlo_integration/code/fortran/monte_carlo.f90

@@ -0,0 +1,51 @@
+FUNCTION in_circle(pos_x, pos_y, r)
+    IMPLICIT NONE
+    REAL(16), INTENT(IN) :: pos_x, pos_y, r
+    LOGICAL              :: in_circle
+
+    in_circle = (pos_x ** 2 + pos_y ** 2) < r ** 2
+
+END FUNCTION in_circle 
+
+PROGRAM monte_carlo
+    
+    IMPLICIT NONE
+    
+    INTERFACE
+        FUNCTION in_circle(pos_x, pos_y, r) 
+            IMPLICIT NONE
+            REAL(16), INTENT(IN) :: pos_x, pos_y, r
+            LOGICAL              :: in_circle
+        END FUNCTION in_circle 
+    END INTERFACE
+    
+    INTEGER  :: i,n
+    REAL(16) :: pos_x,pos_y, r, pi_est, pi_count, pi_error, pi
+    
+    ! Calculate Pi from trigonometric functions as reference
+    pi       = DACOS(-1.d0)
+    n        = 1000000
+    r        = 1d0
+    pos_x    = 0d0
+    pos_y    = 0d0
+    pi_count = 0d0
+    
+    DO i=0,n
+    
+        CALL RANDOM_NUMBER(pos_x)
+        CALL RANDOM_NUMBER(pos_y)
+    
+        IF (in_circle(pos_x, pos_y, r) .EQV. .TRUE.) THEN 
+    
+            pi_count = pi_count + 1d0
+    
+        END IF
+    END DO
+    
+    pi_est   = 4d0 * pi_count / n
+    pi_error = 100d0 * (abs(pi_est - pi)/pi)
+    
+    WRITE(*,'(A, F12.4)') 'The pi estimate is: ', pi_est
+    WRITE(*,'(A, F12.4, A)') 'Percent error is: ', pi_error, ' %'
+    
+END PROGRAM monte_carlo

contents/monte_carlo_integration/monte_carlo_integration.md

@@ -69,6 +69,8 @@ each point is tested to see whether it's in the circle or not:
 [import:6-7, lang:"nim"](code/nim/monte_carlo.nim)
 {% sample lang="ruby" %}
 [import:1-4, lang:"ruby"](code/ruby/monte_carlo.rb)
+{% sample lang="f90" %}
+[import:1-8, lang:"fortran"](code/fortran/monte_carlo.f90)
 {% endmethod %}
 
 If it's in the circle, we increase an internal count by one, and in the end,
@@ -137,6 +139,8 @@ Feel free to submit your version via pull request, and thanks for reading!
 [import, lang:"nim"](code/nim/monte_carlo.nim)
 {% sample lang="ruby" %}
 [import, lang:"ruby"](code/ruby/monte_carlo.rb)
+{% sample lang="f90" %}
+[import, lang:"fortran"](code/fortran/monte_carlo.f90)
 {% endmethod %}