How to find a random entered number in a stack

Question

So what I'm doing is creating a stack and then find a way to search for a random number in the stack. So basically if the code is:

    Stack thisstack = new Stack();  
    Scanner userinput = new Scanner(System.in);

    a = userinput.nextInt();  
    b = userinput.nextInt();  
    c = userinput.nextInt();

    showpush(thisstack, a);  
    showpush(thisstack, b);  
    showpush(thisstack, c);

My question is how would i find a number the user inputs, and be able to throw an exception if we look for a number that isn't there. so if the stack the user inputs is [42, 543, 12] and we look for 42 it shows 42 but if we look for 43 it says something like "number not in stack"

This is highly dependent on the language and nature of the stack. Many stack implementations (such as Java's Stack object) support search/find/indexOf. Short of that, you'd have to pop each object onto a new stack, and then pop them back onto this stack. If you're implementing your own Stack class, then you could implement a trivial linear scan of items in the stack. — Jeff Jirsa, Oct 30 '15 at 21:23
I am using java language and putting this code in eclipse. but how would popping and re-pushing find the number the user inputs — RStone, Oct 30 '15 at 21:59

score 1 · Answer 1 · edited May 23 '17 at 11:52

Since Stack class inherits from Vector class, you have access to its contains() method.

contains() returns true if the object is in the stack; false if not, so one trivial implementation of the showpush() method would be:

void showpush(Stack stack, int num){
    if (stack.contains(num)){
        System.out.println(num);
    } else {
        System.out.println("number not in stack");
    }
}

where num is your search target, and stack... well your stack.

One thing to note here is that when declaring a Stack (since it is a generic type) you also have to declare its type arguments (to ensure type safety), which bears the following change in thisstack's declaration:

Stack<Integer> thisstack = new Stack<Integer>();

You can also check Mureinik's answer here.

Edit/Appendix

Just for completeness, I have added a small demo with two different types of user input (int and float) being pushed into the stack. This is possible when declaring the stack with the more general <Object> type argument, as follows:

import java.util.Stack;
import java.util.Scanner;

public class Jst {
    void showpush(Stack stack, Object obj){
        if (stack.contains(obj)){
            System.out.println("Found "+obj+" in stack");
        } else {
            System.out.println(obj+" not in stack");
        }
    }

    public static void main(String[] Arguments){
        Jst j = new Jst(); //either this way or declare showpush as static
        Stack<Object> thisstack = new Stack<Object>(); 
        Scanner userinput = new Scanner(System.in);
        int a,pb;
        float b,pa;

        // get user input
        System.out.print("Enter two numbers (int, float):");
        try {
            a = userinput.nextInt();  
            b = userinput.nextFloat();
        } catch (Exception e) {
            System.out.println("Exception : "+e.toString());
            return;
        }

        // push user input into stack
        System.out.print("\nPushing user input into stack...");
        thisstack.push(a);
        thisstack.push(b);
        // check stack
        System.out.println("Checking stack...");
        j.showpush(thisstack, a);  
        j.showpush(thisstack, b);

        // pop stack contents
        System.out.print("\nPopping stack contents...");
        pa = (float)thisstack.pop();
        pb = (int)thisstack.pop();
        System.out.println("Popped contents : [ "+pa+", "+pb+" ] (Notice that Stack is LIFO)");
        // check stack
        System.out.println("Checking stack...");
        j.showpush(thisstack, a);  
        j.showpush(thisstack, b);  

        // push b back in stack
        System.out.print("\nPushing "+b+" back into stack...");
        thisstack.push(b);
        // check stack
        System.out.println("Checking stack...");
        j.showpush(thisstack, a);  
        j.showpush(thisstack, b);
    }
}

which yields:

C:\>java Jst
Enter two numbers (int, float):1 23.4

Pushing user input into stack...
Checking stack...
Found 1 in stack
Found 23.4 in stack

Popping stack contents...
Popped contents : [ 23.4, 1 ] (Notice that Stack is LIFO)
Checking stack...
1 not in stack
23.4 not in stack

Pushing 23.4 back into stack...
Checking stack...
1 not in stack
Found 23.4 in stack

How to find a random entered number in a stack

1 Answers1