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Thread: Java Stacks question

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    Wink 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


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    Default Re: Java Stacks question

    Is this a different question than the one you posted here: Can somebody help me with stacks?

    Anyway, the best way to test this is to throw together a little example program that does exactly what you described.
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    Default Re: Java Stacks question

    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?

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    Default Re: Java Stacks question

    Quote Originally Posted by sim18 View Post
    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?
    What happened when you tried exactly that?
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    Default Re: Java Stacks question

    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

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    Default Re: Java Stacks question

    Quote Originally Posted by sim18 View Post
    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
    Seems reasonable to me, but really, the best way to test is to simply let the code do the talking. It doesn't matter what we think, just what the program does. Try stepping through it with a debugger or at least adding some print statements to figure out exactly what's going on, if you aren't sure.
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    Default Re: Java Stacks question

    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?

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    Default Re: Java Stacks question

    Quote Originally Posted by sim18 View Post
    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?
    Sometimes the best way to figure that part out is to draw some examples. So say we have this as a stack:

    {1, 2, 3, 4, 5}

    ...and n = 3. What should happen?

    Well, first off, what is the 3rd (nth) element of the stack? In this example it's easy because it's 3. So what does it mean to set j equal to 3 and leave the stack without its 3 top values?
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    Default Re: Java Stacks question

    {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

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    Default Re: Java Stacks question

    That's really a question for your instructor, but the way I take it, the j in the question is just a temp variable name and has nothing to do with any 'j' values that happen to be in the stack. But I suppose I could be reading it wrong, so I'd recommend asking your instructor for clarification or some more examples.
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    Default Re: Java Stacks question

    Thanks very much for your help

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