asked on

How do I reverse the order of integer elements in an ArrayList?

How do I reverse the order of integer elements in an ArrayList? I have an ArrayList with integer elements. I am trying to print the elements so that the last element is the first printed. For the below code segment if I have an ArrayList like: [2, 7,3,1] and am printing them in a frame using a loop so that the ouput is:

Reference String Value 2: 2 -1 -1 -1
Reference String Value 7: 2 7 -1 -1
Reference String Value 3: 2 7 3 -1
Reference String Value 1: 2 7 3 1

I want it to print as:

Reference String Value 2: 2 -1 -1 -1
Reference String Value 7: 7 2 -1 -1
Reference String Value 3: 3 7 2 -1
Reference String Value 1: 1 3 7 2

referenceString is the ArrayList in the code.

case 4:
                    System.out.println();
                    int count4 = 0;
                    int frame[] = new int[PHYSICAL_FRAMES];
                    for(int i = 0; i < PHYSICAL_FRAMES; i++){
                        frame[i] = -1;
                    }

                    do{
                        int pg = 0;

                        while(pg < stringLength){
                            page = referenceString.get(pg);
                            pg++;

                            flag = true;
                            for(int j = 0; j < PHYSICAL_FRAMES; j++){
                                if(page == frame[j]){
                                    flag = false;
                                    break;
                                }
                            }
                            if(flag){
                                frame[count4] = page;
                                count4++;
                                if(count4 == PHYSICAL_FRAMES)
                                    count4 = 0;
                                    System.out.print("Reference String Value " + page + ": ");
                                    for(int j = 0; j < PHYSICAL_FRAMES; j++)
                                        System.out.print(frame[j] + "   ");
                                        System.out.println();
                                        pageFault++;
                            }
                            else{
                                System.out.print("frame :");
                                for(int j = 0; j < PHYSICAL_FRAMES; j++)
                                    System.out.print(frame[j]+"  ");
                                    System.out.println();
                            }
                            chn++;
                        }
                    }
                    while(chn != stringLength);
                    System.out.println("Page fault:" + pageFault);
                    System.out.println();
                    break;

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for_yan

for(int j=0; j<arralits.size(); j++){
System.out.printn(arraylist.get(arraylist.size() -1 -j));

}

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for_yan

It is hard to undersatnd your example, but if you want to print the ArrayList in the reverse order,
then the above code will do it for you

for_yan

if you want to reverse just the segement of your array list:

for(int j=0; j<num; j++){
System.out.printn(arraylist.get(num-1 -j));

}

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That would print in the way you wanted many times :

for(int i=0; i<arraylist.size(); i++){
for(int j=0; j<i; j++){
System.out.print(arraylist.get(i-1 -j) + "  ");

}
for(int j=i; i<arraylist.size(); j++){
System.out.print("-1 ");
}
System.out.println("");

}

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for_yan

corrected a few mistakes - this works:

        ArrayList<Integer> arraylist = new ArrayList<Integer>();

        arraylist.add(2);
         arraylist.add(7);
         arraylist.add(3);
         arraylist.add(1);


        for(int i=0; i<arraylist.size(); i++){
for(int j=0; j<i; j++){
System.out.print(arraylist.get(i-1 -j) + "  ");

}
for(int j=i; j<arraylist.size(); j++){
System.out.print("-1 ");
}
System.out.println("");

}

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for_yan

Output from the above

-1 -1 -1 -1 
2  -1 -1 -1 
7  2  -1 -1 
3  7  2  -1

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for_yan

Now it is whifted by one - exactly as you wanted (see output below):

        ArrayList<Integer> arraylist = new ArrayList<Integer>();

        arraylist.add(2);
         arraylist.add(7);
         arraylist.add(3);
         arraylist.add(1);


        for(int i=1; i<arraylist.size()+1; i++){
for(int j=0; j<i; j++){
System.out.print(arraylist.get(i-1 -j) + "  ");

}
for(int j=i; j<arraylist.size(); j++){
System.out.print("-1 ");
}
System.out.println("");

}

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2  -1 -1 -1 
7  2  -1 -1 
3  7  2  -1 
1  3  7  2

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ASKER CERTIFIED SOLUTION

for_yan

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See answer

nsing991

ASKER

Thank you. This actually fits more in the context of what I wanted it to do. I'm trying to simulate demand paging using this first in first out algorithm. I will have to tweek some things but this helps tremendously.

for_yan

In your code you are actually using not ArrayList
but array of ints,
so this is with array

        int [] frame = {2,7,3,1};


        for(int i=1; i<frame.length+1; i++){

            System.out.print("Reference string value " + frame[i-1] + ": ");
for(int j=0; j<i; j++){
System.out.print(frame[i-1 -j] + "  ");

}
for(int j=i; j<frame.length; j++){
System.out.print("-1 ");
}
System.out.println("");

}

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Reference string value 2: 2  -1 -1 -1 
Reference string value 7: 7  2  -1 -1 
Reference string value 3: 3  7  2  -1 
Reference string value 1: 1  3  7  2

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nsing991

ASKER

Ahhh. Thanks for pointing that out. I'm using an ArrayList elsewhere in my code but for this particular segment it is an array.

for_yan

yes, but as you see the difference is of course only in some formal syntax