Runge-Kutta 4 order method. Severe 161: Array bounds exceeded.

[Metalman9]

So here I am trying to solve Rayleigh's differential equation using Rungen-Kutta 4th order analytic method.

This is what the exercise gives me: X(0 to 40), Y1(0)=0.1, Y2(0)=0, H=0.1, μ=0.5

This is what I ve written and gives me this error:

	IMPLICIT REAL*8(A-K,O-Z)
	DIMENSION X(401)
	DIMENSION Y1(401)
	DIMENSION Y2(401)

	
	F2(X,Y1,Y2)=0.5D0*Y2-(0.5D0/3.0D0*(Y2**3))-Y1

	H=0.1D0
	X(0)=0
	Y1(0)=0.01D0
	Y2(0)=0


	DO I=1,400
	X(I)=X(I-1)+H
	END DO


	DO I=0,400

	K1=H*Y2(I)
	L1=H*F2(X(I),Y1(I),Y2(I))
	K2=H*(Y2(I)+L1/2.0D0)
	L2=H*F2(X(I)+H/2.0D0,Y1(I)+K1/2.0D0,Y2(I)+L1/2.0D0)
	K3=H*(Y2(I)+L2/2.0D0)
	L3=H*F2(X(I)+(H/2.0D0),Y1(I)+(K2/2.0D0),Y2(I)+(L2/2.0D0))
	K4=H*(Y2(I)+L3)
	L4=H*F2(X(I)+H,Y1(I)+K3,Y2(I)+L3)
	
	Y1(I+1)=Y1(I)+(1.0D0/6.0D0)*(K1+2.0D0*K2+2.0D0*K3+K4)
	Y2(I+1)=Y2(I)+(1.0D0/6.0D0)*(K1+2.0D0*K2+2.0D0*K3+K4)
	END DO
	
	WRITE(*,30)X(I),Y1(I),Y2(I)
30	FORMAT(2X,F8.6,2X,F8.6,2X,F8.6)
	

	STOP
	END

Any ideas why am I exceeding array bounds?

 

[D H]

Array indices in Fortran start at 1. You are setting and accessing x(0), y1(0), and y2(0). That index of 0 is out of bounds.

 

[jtbell]

You can easily make the array indices start at zero, which may be easier than fixing your index-manipulating code so that it doesn't use a zero index value.

	DIMENSION X(0:401)
	DIMENSION Y1(0:401)
	DIMENSION Y2(0:401)

 

[AlephZero]

Your next question will probably be "why doesn't the WRITE statement output the correct values"

To answer that yourself, print out the value of I after the DO loop. If I remember the FORTRAN standard correctly, it won't be equal to 400.

 

[LudusRex]

It is difficult to determine the exact cause of the error without seeing the full code and understanding the specific problem you are trying to solve. However, one potential issue could be related to how you are defining and using the arrays X, Y1, and Y2. It may be helpful to double check the size of these arrays and make sure they are properly initialized before the DO loops. It could also be helpful to print out the values of these arrays at each iteration to see if they are changing as expected. Additionally, it may be beneficial to use a debugger or step through the code to identify where the error is occurring. Overall, carefully checking your code and data inputs can help you identify and resolve the issue.