How Does This Fortran Code Generate and Output Its Array Values?

[suezxc6]

Homework Statement

Can someone please explain how this is done?

Relevant Equations

.

      integer n, ar(12)
      ar(1) = 1
      ar(2) = 2
      do n=1, 10
            ar(n+2) = ar(n+1) + ar(n)
      end do
      do n=0,3
            print*, ar(12-3*n)
      end do
      print*, ar(ar(ar(4))-1)
      end

 

[FactChecker]

What do you mean "how it is done"? Do you mean to ask what it is doing? Since it is a homework problem, I am only allowed to give guiding hints. And you have to show some work.

 

[suezxc6]

Then can you please introduce me to materials explaining tracing and how to do it? I really am stuck.

 

[Dr Transport]

if you are using an IDE, run the code in debug mode. The old fashioned way to trace is to put print statements and either print to a file or the screen.

 

[suezxc6]

When i run it into terminal I get the values 233, 55, 13, 3, 21. I mainly want to know how i would get those numbers if i wrote it by hand. The material that i have does not properly explain how i would be able to achieve it.

 

[Dr Transport]

write it out methodically.

ar(1) = 1
ar(2) = 2

write out the for loop
n = 1 yields ar(3) = ar(2) + ar(1) which is 2 + 1 = 3
n = 2 yields ar(4) = ar(3) + ar(2) which is 3 + 2 = 5
etc...

the write statement is as little more complicated, but doable given what I showed above

 

[Mark44]

      integer n, ar(12)
      ar(1) = 1
      ar(2) = 2
      do n=1, 10
            ar(n+2) = ar(n+1) + ar(n)
      end do
      do n=0,3
            print*, ar(12-3*n)
      end do
      print*, ar(ar(ar(4))-1)
      end

Here's how you can hand-trace what the program does for the first half, up through the first do loop. Take a piece of paper and a pencil and write on it a box for n and a series of 12 connected boxes for the array ar.
The first two assignment statements store 1 in ar(1) and ar(2), respectively, so fill in the first two boxes of the array with those values.
In the first do loop, n starts off with the value 1. c
The first end do statement cause n to be incremented to 2. Since 2 is less than or equal to 10, the loop body executes again. What are the values of the indexes n + 2, n + 1, and n this time? Which array element gets set?
Continue with this process until the first do loop finishes, and proceed the same way with the second do loop.

 

[suezxc6]

Thanks i got it!