2016 Update

Task/Call-a-function/Fortran/call-a-function-4.f

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+      REAL FUNCTION INTG8(F,A,B,DX)	!Integrate function F.
+       EXTERNAL F	!Some function of one parameter.
+       REAL A,B		!Bounds.
+       REAL DX		!Step.
+       INTEGER N	!A counter.
+        INTG8 = F(A) + F(B)	!Get the ends exactly.
+        N = (B - A)/DX		!Truncates. Ignore A + N*DX = B chances.
+        DO I = 1,N		!Step along the interior.
+          INTG8 = INTG8 + F(A + I*DX)	!Evaluate the function.
+        END DO			!On to the next.
+        INTG8 = INTG8/(N + 2)*(B - A)	!Average value times interval width.
+      END FUNCTION INTG8	!This is not a good calculation!
+
+      FUNCTION TRIAL(X)		!Some user-written function.
+       REAL X
+        TRIAL = 1 + X		!This will do.
+      END FUNCTION TRIAL	!Not the name of a library function.
+
+      PROGRAM POKE
+      INTRINSIC SIN	!Thus, not an (undeclared) ordinary variable.
+      EXTERNAL TRIAL	!Likewise, but also, not an intrinsic function.
+      REAL INTG8	!Don't look for the result in an integer place.
+       WRITE (6,*) "Result=",INTG8(SIN,  0.0,8*ATAN(1.0),0.01)
+       WRITE (6,*) "Linear=",INTG8(TRIAL,0.0,1.0,        0.01)
+      END