将FORTRAN循环转换为Java

Question

For an assignment I've been given some stuff to do involving FORTRAN code, but the only issue is that we haven't been taught it yet so I'm not entirely sure what's going on so I've attempted to convert it to Java to try get a grasp on it. The following is the FORTRAN code:

L1:        DO 20 I = 1, 512
L2:           SUM(I) = 0
L3:           DO 40 J = 1, I
L4: 40         SUM(I) = SUM(I) + 1
L5: 20      CONTINUE

The idea is that L2 and L4 both take a machine cycle and I have to work out how long it takes for the loop to complete. The following is my Java which I think is at least reasonably close to working out the value I want:

public static void main(String[] args) {
    int cycles = 0;
    for(int i = 1; i < 512; i++){
        cycles = cycles + 1;
        for(int j = 1; j < i; j++){
            cycles = cycles +1;
        }
    }
    System.out.println(cycles);
}

Does this seem correct? Any help is appreciated. I've thought through it mathematically and got a different answer (although both are close to each other) so I'm not sure which is better.

EDIT: I'd like to make clear that I'm not attempting to port the FORTRAN directly to Java, rather just calculate the cycle time mentioned above by using Java.

EDIT 2: I'm not trying to create the Array, only calculate the cycles taken during the loops. Because both lines L2 and L4 take a cycle, I've swapped it in the Java ONLY to figure out the cycles taken, not do what the FORTRAN loop does.

Answer 1

In the Fortran the statement

DO 20 I = 1, 512

starts a loop whose end is the line with label 20 . Similarly, the inner loop ends on the statement labelled 40 . In modern Fortran this might look like

    DO I = 1, 512
       SUM(I) = 0
       DO J = 1, I
          SUM(I) = SUM(I) + 1
       END DO
    END DO

or even, since Fortran (since Fortran 90) has array statements and, as Duffymo has observed, SUM is an array

       DO I = 1, 512
          SUM(I) = I
       END DO

or, as I would write it, using an array constructor with an implied do loop:

SUM = [(I,I=1,512)]

The Fortran sets element I of SUM to I .

So, to answer OP's question more directly, the original Fortran code executes line 2 512 times and line 4 1+2+3+4+...+512 times.

My opinion is that writing a Java (or indeed any language) program to compute this sum is exactly the sort of thing that students of computer science (or software engineering or ...) should be taught not to do; there is a well-known closed-form equation for the sum of the first N integers (italicised to clarify what the term you should be Googling for is) and any aspiring software developer ought to know this closed-form. Such an aspirant ought to know this closed-form precisely to be able to figure out how many operations are invoked in loops such as the ones shown without having to write a program to iterate pointlessly.

To conclude, OP's Java program would get an F- on any course I taught because it is not an implementation of what ought to have been implemented - a function to calculate the sum of the first N integers. That F- would be applied irrespective of the correctness or otherwise of the program. Since I'm not a teacher that's not much of a threat, but I simply wouldn't hire anyone pretending to be a software engineer without this knowledge.