Thread: Java Stacks question

Hi everone, I have been doing java for around a year now
I'm not looking for somebody to give me an answer but for guidance and help.

This is the question:

I have a stack called 's'.
push(s,m) -> pushes element n onto the stack .
pop(s) -> pops element from the stack s.
peek(s) -> return value of top element on the stack.

(Other info: may use other variables, a loop and/or second stack, s2)

Question = Set j to the third element of a stack leaving the stack unchanged.

This is my attempt:

I'm assuming there are two elements already on a stack as it doesn't say there should or shouldn't be!

Stack 1 : {m,n} -> (m being top element on stack)

push (s2, pop(s)) //This is supposed to pop the element on stack 1 sending it to stack 2 (s2)

Stack 1 : { n }
Stack 2 : { m }

push (s2, pop(s)) //pop element on stack 1, send to stack 2 again

stack 1 : { }
stack 2: { n,m}

push (s,j) //This pushes element j onto stack 1

stack 1 : { j }
stack 2: { n,m}

push (s, pop(s2)) // pops element on stack 2, pushing it back onto stack 1

stack 1 : { n,j }
stack 2: { m}

push (s, pop(s2)) // pops element on stack 2, pushing it again back onto stack 1

stack 1 : { m,n,j }
stack 2: { }

Stack 1 is now unchanged but has j set to the third element. Would you say this is correct?
Also is 'push (s, pop(s2))' the right format ??

Thanks for any help I would appreciate it so much, just trying to make sure i'm on the right lines

Yes this is a different question. How would I go about making a program that does what I want?

Creating my stacks (s) and (s2) - Stack s = new Stack();
Stack s2 = new Stack();

Would i then push n & m on stack to begin with?

It created two stacks s & s2.

I then added s.push("n"); and s.push("m"); giving n & m in the stack1.... is this about right so far?

thanks

Thanks very much for the replies

I have added print statements and I can see exactly what is going on!

If I was going to take this one step further, (given integer 'n') and set j to the nth element of the stack, leaving the stack without its top 'n' elements.

All i have trouble with is getting my head around the wording and once that's done like you said I can start doing the code to check its right.

set j to the nth element of the stack... would I chose what the nth element is, for example I would set j to the 4th element in the stack, and get rid of the top 3 elements, or am i reading this wrong?

{1, 2, 3, 4, 5}

n = 3

nth element = 3rd element in stack, which is 3.

So you would pop() three times leaving {4,5}.

If it was j it would be exactly the same then? i.e

{l,m,j,n,o} n = 3

nth element = 3rd element in stack, which is j.
So you would pop() three times leaving {n,o}.

OR, would 'j' not be there to start with, because it says set j to the nth (3rd) element of the stack. {l,m,n,o}

Therefore I would have to do what I did before and use stack 2 to pop the first two elements, push 'j' into the stack, push the others from stack 2, then pop removing top n elements (3)

Thanks again for the help

Thanks very much for your help