stacks

**bd0652** · September 9th, 2012, 10:01 PM

I apologize but I have no idea exactly what I'm doing and I apologize now in advance if it his hard to read since I'm posting this via cellphone since I do not have access to the internetfrom my pc. I am using Netbeans attempting to write a code with the list of these colors. yellow red white red yellow white white green red white. I need to store the colors Ina stack or perhaps multiple stacks. I cannot print them prior to storing them in the stack.
2. print the colors in reverse input sequence.
3. print the colors in input sequence
I cannot use arrays or files for temporary storage. and to use multiple ArrayStacks<T>objects.
this is what I had which I know is not nearly right

import java.util.*;
public class colorStack {
* * public static void main(String[] args) {

* * * * Stack<String> stack = new Stack<String>();
* * * * stack.push("white");
stack.push("red");
* stack.push("green");
stack.push("white");
}
}
I understand I need to push the strings to stack just no idea how. any help would be greatly appreciated thanks

**chronoz13** · September 9th, 2012, 11:10 PM

you are actually pushing it inside the stack, if you know the data structure of stacks(last in, first out), there are 2 major ways to handle data inside of it, the "push" and "pop", you can read its API(for java.util.Stack) for further information, or any basic data structures about Stacks.

Push - pushes the data into the stack, starts from the bottom most
Pop - pops(takes) the current data on the top of stack

**bd0652** · September 9th, 2012, 11:30 PM

Originally Posted by chronoz13

you are actually pushing it inside the stack, if you know the data structure of stacks(last in, first out), there are 2 major ways to handle data inside of it, the "push" and "pop", you can read its API(for java.util.Stack) for further information, or any basic data structures about Stacks.

Push - pushes the data into the stack, starts from the bottom most
Pop - pops(takes) the current data on the top of stack

so I am doing it backwards then right? then when I want to do different sequences I would start to pop. I sorry I'm very new to java and taking a course recommended by an advisor that required java pre requisites and it's too late to drop.

**chronoz13** · September 9th, 2012, 11:46 PM

for number 3.) it would be easy for you, yes, just pop everything you pushed on the stack so you can print it in sequence
for number 2.) i never tried it but i think you need another stack, or you can use Queue(first in first out structure) so you can print it in reverse sequence..

Edit for number 2.) Everytime you pop a data from stack 1 , you might push it in another stack (e.g stack 2), and see the order of elements

**chronoz13** · September 10th, 2012, 12:10 AM

let me correct my statement, sorry, i wasnt reading it carefully..

For you to print it in "INPUT" sequence which is for number 3.) you have to see the bottom most, then you need another stack
For you to print it in "REVERSED INPUT" sequence which is for number 2.) just pop everything from that stack

**bd0652** · September 10th, 2012, 06:57 AM

Originally Posted by chronoz13

let me correct my statement, sorry, i wasnt reading it carefully..

For you to print it in "INPUT" sequence which is for number 3.) you have to see the bottom most, then you need another stack
For you to print it in "REVERSED INPUT" sequence which is for number 2.) just pop everything from that stack

what dO you mean you need to see the bottom most?

**jps** · September 10th, 2012, 09:13 AM

He means, when you push some things on to the stack, and you want them back in the same order, the 'bottom most' is the first one you pushed.

For example, an array of integers. myInts = [0,1,2,3,4,5].
push myInts[0]
push myInts[1]
push myInts[2]
push myInts[3]
push myInts[4]
push myInts[5]

The 'bottom most' is the value 0. The first one pushed to the stack. The 'top most' is the value 5, or the last thing pushed to the stack. So if you called pop you would get the 5 back first.

**Zaphod_b** · September 10th, 2012, 11:48 AM

Reference Stack (abstract data type) - wikipedia

Next-to-bottom line:
If you push some objects onto a stack, the last one that you pushed is the first one that will be popped.
Therefore, if you push stuff on a stack and then pop the objects and print each one as it is popped, you will print them in the reverse order.

Now... A big intuitive leap to get to the
Bottom line:
Create a second stack. When you print each object that you pop off of the first stack, push it onto the second stack.

Now, what happens when you pop the objects off of the second stack and print each one as it is popped?

Try it! Use pencil and paper; no computer needed just yet.

Next: How to do it in Java? Reference Java Stack Class

I mean, your code for instantiating a Stack <String> object and pushing some Strings looks pretty good for starters (assuming your cell phone put '*' where you intended spaces).

So...

For the first part of the program, to print the Strings in reverse order:

Declare an object of type Stack<String>
Push some Strings onto the stack, using the Stack.push() method
Make a loop using the Stack.empty() method as loop control:
    While the stack is not empty
    BEGIN LOOP
        Use the Stack.pop() method to pop the top value from the stack:
           Set a String equal to the value popped from the stack.
           Print the popped String's value and do whatever else you need to do
    END LOOP

Now, implement that and take a look at what it prints. The Strings in reverse order, right?

So, now do the second part.

Cheers!

Z

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