How to subtract values of two lists/arrays in Java?

Question

I am working on a Java project and I am having a problem. I want to have the subtract of two lists or arrays a and b in list/array c but I don't know how to do that. I want "a[i] -b[i]" should be in next list c where the value should be c[i] similarly for 5-2=3 any suggestion and help would be appreciated.

Code:

public static void länge() {
    {
        BufferedReader br = null;
        try {
            br = new BufferedReader(new FileReader(new File("C:\\Users/Voodoothechild/Desktop/Pidata/Anfang.txt")));
            String line = null;
            while ((line = br.readLine()) != null) {         
                {
                    BufferedReader bro = null;
                    try {
                        bro = new BufferedReader(new FileReader(new File("C:\\Users/Voodoothechild/Desktop/Pidata/Ende.txt")));

                        String lines = null;
                        while ((lines = br.readLine()) != null) {

                            String[] anfang = line.split("\n");
                            String[] ende = lines.split("\n");

                            List<Integer> an = new ArrayList<Integer>();
                            List<Integer> en = new ArrayList<Integer>();

                            for (int index = 0 ; index<ende.length ; index++) {

                                an.add(Integer.parseInt(anfang[index]));
                                en.add(Integer.parseInt(ende[index]));
                                Integer[] anf = new Integer[an.size()];

                                int[] result = new int[anf.length]; 

                                for (int i = 0; i < result.length; i++) { 
                                    result[i] = anf[i] - end[i]; 
                                } 

                            } 
                        }    
                    } catch (FileNotFoundException e) {
                        e.printStackTrace();
                    } catch (IOException e) {
                        e.printStackTrace();
                    } finally {
                        if (bro != null) {
                            try {
                                bro.close();
                            } catch (IOException e) {
                                e.printStackTrace();
                            }
                        }
                    }
                }
            }
        } catch(FileNotFoundException e) {
            e.printStackTrace();
        } catch(IOException e) {
            e.printStackTrace();
        } finally {
            if (br != null) {
                try {
                    br.close();
                } catch(IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

Please fix the formatting of your code, which will only help you to solve your problem. By the way, looping over each pair or elements and subtracting is how I would do this. — Tim Biegeleisen, Nov 17 '16 at 00:57
Using either should fine. I might lean towards an array if you expect the two sets of data to always have the same size. — Tim Biegeleisen, Nov 17 '16 at 01:11

score 4 · Answer 1 · answered Nov 19 '16 at 17:58

It can be done easily using Stream API:

int[] list1 = {4,2,1};
int[] list2 = {2,2,-1};

IntStream.range(0, list1.length)
  .mapToObj(i -> list1[i] - list2[i])
  .collect(Collectors.toList());

or even easier using Javaslang:

Stream.range(0, list1.length)
  map(i -> list1[i] - list2[i]);

or by zipping two lists together:

List.ofAll(list1)
  .zip(List.ofAll(list2))
  .map(t -> t._1 - t._2);

score 1 · Answer 2 · edited Jun 20 '20 at 09:12

1

Its pretty straight forward. Before tackling a programming problem. Always do it in steps. Take your problem for example. You need to subtract values from 2 arrays with values.

int[] list1 = {4,2,1};

int[] list2 = {2,2,-1};

Now you have 2 list. Ok next step, write a loop to go through the list. But first make sure that both list have the same length or else you will get an index out of bounce.

for(int i =0; i< list1.length; i++)
    {
      list3[i] = list1[i] - list2[i];
    }

Anyway here is the full answer. Be sure to understand it

public class subtract {

  public static void main(String[] args)
  {
    int[] list1 = {4,2,1};
    int[] list2 = {2,2,-1};
    int[] list3 = new int[list1.length];
    
    //Looping the list
    for(int i =0; i< list1.length; i++)
    {
      list3[i] = list1[i] - list2[i];
    }
    
    //Print Statement
    for(int j =0; j< list3.length; j++)
    {
      System.out.println(list3[j]);
    }
  }

}

edited Jun 20 '20 at 09:12

Community

1
1

answered Nov 17 '16 at 01:12

Aurora_Titanium

472
1
8
23

You should add checks to make sure that both list1 and list2 are the same size. But you get the idea. – Aurora_Titanium Nov 17 '16 at 01:13
ich habe es davor schon mal so gehabt jedoch hat es mir "java.lang.NullPointerException" ausgespuckt: int[] results = null; for (int i = 0; i < anf.length; i++) { results[i] = anf[i] - end[i]; – Voodoothechild Nov 17 '16 at 01:19
1

dont write null. Write something like int[] results = new int[anf.length]; – Aurora_Titanium Nov 17 '16 at 01:20
still end up with the "java.lang.NullPointerException" – Voodoothechild Nov 17 '16 at 01:25
1

Basically you want to subtract it from array or arrayList? – Aurora_Titanium Nov 17 '16 at 01:27
ok ich habe mein problem gelöst: ich habe meine txt files in einem anderem public void in ein int[] gelesen und dann wieder in einer eigenen zusammen geführt. vielen dank für deine hilfe – Voodoothechild Nov 17 '16 at 03:56

score 0 · Answer 3 · answered Nov 17 '16 at 01:15

Here's a very simple answer you can apply to your program.

public static void main(String[] args) {
    int[] a = new int[4];
    int[] b = new int[4];
    int[] c = new int[4];

    a[0] = 5;
    a[1] = 12;
    a[2] = 20;
    a[3] = 50;
    b[0] = 1;
    b[1] = 2;
    b[2] = 3;
    b[3] = 4;

    for(int i=0; i<a.length; i++){
       c[i] = a[i] - b[i];
        System.out.println(c[i]);
    }
}

score 0 · Answer 4 · answered Jul 07 '19 at 01:37

The answer is the method removeAll of class Collection

ArrayList<Integer> s1 = new ArrayList<Integer>(Arrays.asList(a1));
ArrayList<Integer> s2 = new ArrayList<Integer>(Arrays.asList(a2));
s1.removeAll(s2);
Integer[] result = s1.toArray(new Integer[s1.size()]);

How to subtract values of two lists/arrays in Java?

4 Answers4

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