Its a very trivial question and related to coding Style and I am just asking to make my coding style more readable
Suppose I have a Collection like linkedList and an Array and I need to iterate over both simultaneously.
currently the best way I know is to get a iterator over list and define a index variable outside the iterator loop and increment the index variable simultaneously to access both next elements {list and array}. Please refer the example below
LinkedList<Integer> list = new LinkedList<Integer>();
Integer[] arr = new Array[25];
// lets suppose both have 25 elements.
// My Iteration method will be
int index =0;
for (Integer val : list) {
System.out.println(val);
System.out.println(arr[index++]);
}
so is it the only way or is there any other way I can perform this iteration in more readable and more relatable manner, where I don't have to take index variable separately.
I know it can be possible that array might have less or more elements than collection but I am only talking about the cases where they have equal and we need to iterate over Both of them.
PS : anybody can write a code that a computer can understand, actual challenge is to write code which humans can understand easily.
What you have is essentially fine: it's simple, and simple can be sufficient to make code readable.
The only thing I would caution about is the side effect of index++ inside arr[index++]: if, say, you want to use the same value multiple times in the loop body, you couldn't simply copy+paste.
Consider pulling out a variable as the first thing in the loop to store the "current" array element (which is essentially what the enhanced for loop does for the list element).
for (Integer val : list) {
Integer fromArr = arr[index++];
// ...
}
Just to point out an alternative without having a separate variable for the index, you can use ListIterator, which provides you with the index of the element.
// Assuming list and are have same number of elements.
for (ListIterator<Integer> it = list.listIterator();
it.hasNext();) {
// The ordering of these statements is important, because next() changes nextIndex().
Integer fromArr = arr[it.nextIndex()];
Integer val = it.next();
// ...
}
ListIterator is not an especially widely-used class, though; its use may in and of itself be confusing.
One of the downsides of the ListIterator approach is that you have to use the it correctly: you shouldn't touch it inside the loop (after getting the values), you have to put the statements in the right order, etc.
Another approach would be to create a library method analogous to Python's enumerate:
static <T> Iterable<Map.Entry<Integer, T>> enumerate(Iterable<? extends T> iterable) {
return () -> new Iterator<T>() {
int index = 0;
Iterator<? extends T> delegate = iterable.iterator();
#Override public boolean hasNext() { return delegate.hasNext(); }
#Override public Map.Entry<Integer, T> next() {
return new AbstractMap.SimpleEntry<>(index++, delegate.next());
}
};
}
This returns an iterable of map entries, where the key is the index and the value is the corresponding value.
You could then use this in an enhanced for loop:
for (Map.Entry<Integer, Integer> entry : enumerate(list)) {
Integer fromList = entry.getValue();
Integer fromArr = arr[entry.getKey()];
}
One option is to have 2 iterators, but I don't think it is any clearer:
for (Iterator<Integer> i1 = list.iterator(), i2 = Arrays.asList(arr).iterator();
i1.hasNext() && i2.hasNext();) {
System.out.println(i1.next());
System.out.println(i2.next());
}
But it is more robust in that it finishes at the shorter of the 2 collections.
I tried to simplify and handle size wise collections where both need not be of the same size. I believe this would work even if the sizes are not same and just one loop would suffice. Code snippet below:
LinkedList<Integer> list = new LinkedList<Integer>();
Integer[] arr = new Array[25];
int maxLength= Math.max(list.size(),arr.size());
//Looping over the lengthy collection( could be Linkedlist or arraylist)
for(int i=0;i<maxLength;i++){
if(list.size()>i)
System.out.println(list[i]);
if(arr.size()>i)
System.out.println(arr[i]);
}
Hope this helps! Thanks
Is it possible to add elements to a collection while iterating over it?
More specifically, I would like to iterate over a collection, and if an element satisfies a certain condition I want to add some other elements to the collection, and make sure that these added elements are iterated over as well. (I realise that this could lead to an unterminating loop, but I'm pretty sure it won't in my case.)
The Java Tutorial from Sun suggests this is not possible: "Note that Iterator.remove is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress."
So if I can't do what I want to do using iterators, what do you suggest I do?
How about building a Queue with the elements you want to iterate over; when you want to add elements, enqueue them at the end of the queue, and keep removing elements until the queue is empty. This is how a breadth-first search usually works.
There are two issues here:
The first issue is, adding to an Collection after an Iterator is returned. As mentioned, there is no defined behavior when the underlying Collection is modified, as noted in the documentation for Iterator.remove:
... The behavior of an iterator is
unspecified if the underlying
collection is modified while the
iteration is in progress in any way
other than by calling this method.
The second issue is, even if an Iterator could be obtained, and then return to the same element the Iterator was at, there is no guarantee about the order of the iteratation, as noted in the Collection.iterator method documentation:
... There are no guarantees concerning the
order in which the elements are
returned (unless this collection is an
instance of some class that provides a
guarantee).
For example, let's say we have the list [1, 2, 3, 4].
Let's say 5 was added when the Iterator was at 3, and somehow, we get an Iterator that can resume the iteration from 4. However, there is no guarentee that 5 will come after 4. The iteration order may be [5, 1, 2, 3, 4] -- then the iterator will still miss the element 5.
As there is no guarantee to the behavior, one cannot assume that things will happen in a certain way.
One alternative could be to have a separate Collection to which the newly created elements can be added to, and then iterating over those elements:
Collection<String> list = Arrays.asList(new String[]{"Hello", "World!"});
Collection<String> additionalList = new ArrayList<String>();
for (String s : list) {
// Found a need to add a new element to iterate over,
// so add it to another list that will be iterated later:
additionalList.add(s);
}
for (String s : additionalList) {
// Iterate over the elements that needs to be iterated over:
System.out.println(s);
}
Edit
Elaborating on Avi's answer, it is possible to queue up the elements that we want to iterate over into a queue, and remove the elements while the queue has elements. This will allow the "iteration" over the new elements in addition to the original elements.
Let's look at how it would work.
Conceptually, if we have the following elements in the queue:
[1, 2, 3, 4]
And, when we remove 1, we decide to add 42, the queue will be as the following:
[2, 3, 4, 42]
As the queue is a FIFO (first-in, first-out) data structure, this ordering is typical. (As noted in the documentation for the Queue interface, this is not a necessity of a Queue. Take the case of PriorityQueue which orders the elements by their natural ordering, so that's not FIFO.)
The following is an example using a LinkedList (which is a Queue) in order to go through all the elements along with additional elements added during the dequeing. Similar to the example above, the element 42 is added when the element 2 is removed:
Queue<Integer> queue = new LinkedList<Integer>();
queue.add(1);
queue.add(2);
queue.add(3);
queue.add(4);
while (!queue.isEmpty()) {
Integer i = queue.remove();
if (i == 2)
queue.add(42);
System.out.println(i);
}
The result is the following:
1
2
3
4
42
As hoped, the element 42 which was added when we hit 2 appeared.
You may also want to look at some of the more specialised types, like ListIterator, NavigableSet and (if you're interested in maps) NavigableMap.
Actually it is rather easy. Just think for the optimal way.
I beleive the optimal way is:
for (int i=0; i<list.size(); i++) {
Level obj = list.get(i);
//Here execute yr code that may add / or may not add new element(s)
//...
i=list.indexOf(obj);
}
The following example works perfectly in the most logical case - when you dont need to iterate the added new elements before the iteration element. About the added elements after the iteration element - there you might want not to iterate them either. In this case you should simply add/or extend yr object with a flag that will mark them not to iterate them.
Use ListIterator as follows:
List<String> l = new ArrayList<>();
l.add("Foo");
ListIterator<String> iter = l.listIterator(l.size());
while(iter.hasPrevious()){
String prev=iter.previous();
if(true /*You condition here*/){
iter.add("Bah");
iter.add("Etc");
}
}
The key is to iterate in reverse order - then the added elements appear on the next iteration.
I know its been quite old. But thought of its of any use to anyone else. Recently I came across this similar problem where I need a queue that is modifiable during iteration. I used listIterator to implement the same much in the same lines as of what Avi suggested -> Avi's Answer. See if this would suit for your need.
ModifyWhileIterateQueue.java
import java.util.ArrayList;
import java.util.List;
import java.util.ListIterator;
public class ModifyWhileIterateQueue<T> {
ListIterator<T> listIterator;
int frontIndex;
List<T> list;
public ModifyWhileIterateQueue() {
frontIndex = 0;
list = new ArrayList<T>();
listIterator = list.listIterator();
}
public boolean hasUnservicedItems () {
return frontIndex < list.size();
}
public T deQueue() {
if (frontIndex >= list.size()) {
return null;
}
return list.get(frontIndex++);
}
public void enQueue(T t) {
listIterator.add(t);
}
public List<T> getUnservicedItems() {
return list.subList(frontIndex, list.size());
}
public List<T> getAllItems() {
return list;
}
}
ModifyWhileIterateQueueTest.java
#Test
public final void testModifyWhileIterate() {
ModifyWhileIterateQueue<String> queue = new ModifyWhileIterateQueue<String>();
queue.enQueue("one");
queue.enQueue("two");
queue.enQueue("three");
for (int i=0; i< queue.getAllItems().size(); i++) {
if (i==1) {
queue.enQueue("four");
}
}
assertEquals(true, queue.hasUnservicedItems());
assertEquals ("[one, two, three, four]", ""+ queue.getUnservicedItems());
assertEquals ("[one, two, three, four]", ""+queue.getAllItems());
assertEquals("one", queue.deQueue());
}
Using iterators...no, I don't think so. You'll have to hack together something like this:
Collection< String > collection = new ArrayList< String >( Arrays.asList( "foo", "bar", "baz" ) );
int i = 0;
while ( i < collection.size() ) {
String curItem = collection.toArray( new String[ collection.size() ] )[ i ];
if ( curItem.equals( "foo" ) ) {
collection.add( "added-item-1" );
}
if ( curItem.equals( "added-item-1" ) ) {
collection.add( "added-item-2" );
}
i++;
}
System.out.println( collection );
Which yeilds:
[foo, bar, baz, added-item-1, added-item-2]
Besides the solution of using an additional list and calling addAll to insert the new items after the iteration (as e.g. the solution by user Nat), you can also use concurrent collections like the CopyOnWriteArrayList.
The "snapshot" style iterator method uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible and the iterator is guaranteed not to throw ConcurrentModificationException.
With this special collection (usually used for concurrent access) it is possible to manipulate the underlying list while iterating over it. However, the iterator will not reflect the changes.
Is this better than the other solution? Probably not, I don't know the overhead introduced by the Copy-On-Write approach.
public static void main(String[] args)
{
// This array list simulates source of your candidates for processing
ArrayList<String> source = new ArrayList<String>();
// This is the list where you actually keep all unprocessed candidates
LinkedList<String> list = new LinkedList<String>();
// Here we add few elements into our simulated source of candidates
// just to have something to work with
source.add("first element");
source.add("second element");
source.add("third element");
source.add("fourth element");
source.add("The Fifth Element"); // aka Milla Jovovich
// Add first candidate for processing into our main list
list.addLast(source.get(0));
// This is just here so we don't have to have helper index variable
// to go through source elements
source.remove(0);
// We will do this until there are no more candidates for processing
while(!list.isEmpty())
{
// This is how we get next element for processing from our list
// of candidates. Here our candidate is String, in your case it
// will be whatever you work with.
String element = list.pollFirst();
// This is where we process the element, just print it out in this case
System.out.println(element);
// This is simulation of process of adding new candidates for processing
// into our list during this iteration.
if(source.size() > 0) // When simulated source of candidates dries out, we stop
{
// Here you will somehow get your new candidate for processing
// In this case we just get it from our simulation source of candidates.
String newCandidate = source.get(0);
// This is the way to add new elements to your list of candidates for processing
list.addLast(newCandidate);
// In this example we add one candidate per while loop iteration and
// zero candidates when source list dries out. In real life you may happen
// to add more than one candidate here:
// list.addLast(newCandidate2);
// list.addLast(newCandidate3);
// etc.
// This is here so we don't have to use helper index variable for iteration
// through source.
source.remove(0);
}
}
}
For examle we have two lists:
public static void main(String[] args) {
ArrayList a = new ArrayList(Arrays.asList(new String[]{"a1", "a2", "a3","a4", "a5"}));
ArrayList b = new ArrayList(Arrays.asList(new String[]{"b1", "b2", "b3","b4", "b5"}));
merge(a, b);
a.stream().map( x -> x + " ").forEach(System.out::print);
}
public static void merge(List a, List b){
for (Iterator itb = b.iterator(); itb.hasNext(); ){
for (ListIterator it = a.listIterator() ; it.hasNext() ; ){
it.next();
it.add(itb.next());
}
}
}
a1 b1 a2 b2 a3 b3 a4 b4 a5 b5
I prefer to process collections functionally rather than mutate them in place. That avoids this kind of problem altogether, as well as aliasing issues and other tricky sources of bugs.
So, I would implement it like:
List<Thing> expand(List<Thing> inputs) {
List<Thing> expanded = new ArrayList<Thing>();
for (Thing thing : inputs) {
expanded.add(thing);
if (needsSomeMoreThings(thing)) {
addMoreThingsTo(expanded);
}
}
return expanded;
}
IMHO the safer way would be to create a new collection, to iterate over your given collection, adding each element in the new collection, and adding extra elements as needed in the new collection as well, finally returning the new collection.
Given a list List<Object> which you want to iterate over, the easy-peasy way is:
while (!list.isEmpty()){
Object obj = list.get(0);
// do whatever you need to
// possibly list.add(new Object obj1);
list.remove(0);
}
So, you iterate through a list, always taking the first element and then removing it. This way you can append new elements to the list while iterating.
Forget about iterators, they don't work for adding, only for removing. My answer applies to lists only, so don't punish me for not solving the problem for collections. Stick to the basics:
List<ZeObj> myList = new ArrayList<ZeObj>();
// populate the list with whatever
........
int noItems = myList.size();
for (int i = 0; i < noItems; i++) {
ZeObj currItem = myList.get(i);
// when you want to add, simply add the new item at last and
// increment the stop condition
if (currItem.asksForMore()) {
myList.add(new ZeObj());
noItems++;
}
}
I tired ListIterator but it didn't help my case, where you have to use the list while adding to it. Here's what works for me:
Use LinkedList.
LinkedList<String> l = new LinkedList<String>();
l.addLast("A");
while(!l.isEmpty()){
String str = l.removeFirst();
if(/* Condition for adding new element*/)
l.addLast("<New Element>");
else
System.out.println(str);
}
This could give an exception or run into infinite loops. However, as you have mentioned
I'm pretty sure it won't in my case
checking corner cases in such code is your responsibility.
This is what I usually do, with collections like sets:
Set<T> adds = new HashSet<T>, dels = new HashSet<T>;
for ( T e: target )
if ( <has to be removed> ) dels.add ( e );
else if ( <has to be added> ) adds.add ( <new element> )
target.removeAll ( dels );
target.addAll ( adds );
This creates some extra-memory (the pointers for intermediate sets, but no duplicated elements happen) and extra-steps (iterating again over changes), however usually that's not a big deal and it might be better than working with an initial collection copy.
Even though we cannot add items to the same list during iteration, we can use Java 8's flatMap, to add new elements to a stream. This can be done on a condition. After this the added item can be processed.
Here is a Java example which shows how to add to the ongoing stream an object depending on a condition which is then processed with a condition:
List<Integer> intList = new ArrayList<>();
intList.add(1);
intList.add(2);
intList.add(3);
intList = intList.stream().flatMap(i -> {
if (i == 2) return Stream.of(i, i * 10); // condition for adding the extra items
return Stream.of(i);
}).map(i -> i + 1)
.collect(Collectors.toList());
System.out.println(intList);
The output of the toy example is:
[2, 3, 21, 4]
In general, it's not safe, though for some collections it may be. The obvious alternative is to use some kind of for loop. But you didn't say what collection you're using, so that may or may not be possible.
This question already has answers here:
Removing an element from an Array (Java) [duplicate]
(15 answers)
Closed 9 years ago.
I have an array of Contact objects that has a MAX of 50 Contacts, but will have much less, so the array is initialized with a size of 50. But I need my method to remove the Contact and shift everything after it up. What I have seems to work at times, but not every time.
public Contact remove(String lstnm)
{
int contactIndex = findContactIndex(lstnm); // Gets the index of the Contact that needs to be removed
Contact contactToBeRemoved;
if(contactIndex == -1) // If the Contact is not in the Array
{
contactToBeRemoved = null;
}
else
{
contactToBeRemoved = Contact_List[contactIndex]; // Assigns the Contact that is going to be removed
for(int i = contactIndex; i < numContacts; i++) // From where the Contact was removed to the last Contact in the list
{
Contact_List[i] = Contact_List[i + 1]; // Shift all of the Contacts after the one removed down
}
numContacts -= 1; // One Contact is removed from the total number of Contacts
}
return contactToBeRemoved;
}
Arrays a fixed size you cannot resize them. ArrayList on the other hand auto resize each time you add a element.
So if I have a Array of 5 I can put 5 items in it, no more no less. One thing you can do is set objects in the Array to be null or 0.
Edit: With regards to your comment, just sort the Array. Look up a easy bubble sort algorithm in Java.
try
System.arraycopy(contactList, contactIndex + 1, contactList, contactIndex, contactList.length - contactIndex - 1);
Note that System.arraycopy is the most efficient way to copy / move array elements
your code would give exception at numContacts'th iteration since i+1 will go beyond size of array.
for(int i = contactIndex; i < numContacts-1; i++)
{
Contact_List[i] = Contact_List[i + 1];
}
Contact_List[Contact_List.length-1] = null;
Ps: its a very bad practice to use Array in such scenario, consider using ArrayList instead.
Why don't you convert your array into a List and use the remove(Object o) method that does exactly what you describe?
It would save you some time and some testing.
for such purpose use ArrayList
ArrayList<Contact> array = new ArrayList<Contact>(50);
creates a dynamic array with initial capacity of 50 (this can increase as more elements gets added to the ArrayList)
array.add(new Contact());
array.remove(contact); //assuming Contact class overrides equals()
ArrayList internally maintains an array and does re-sizing, restructuring as the elements are added or removed from it.
You can also use Vector<Contact> which is similar data-structure, but thread safe.
Array's become pretty useless when you know how to use arrayList, in my opinion. I suggest using arrayLists.
ArrayList tutorial
do like this when creating ht econtact arrayList:
import java.util.ArrayList;
public static void main(String args[]){
ArrayList<Contact> contacts = new ArrayList();
contacts.add(new Contact());
}
Use arrayLists, its the best way. Read tutorials, the are plenty of them.
I suggest it cause arralist are dynamic, that means you can add and remove items and it resized itself for you.
Hope I could help even if my answers isnt very complete
use collection rather than array so that you dont have to do all the shifting processes!
collection automatically shifts the elements and you dont have to worry about it!
you may do as follow,
ArrayList<Contact> list=new ArrayList<Contact>();
Contact c=new Contact();
Contact.Add(Contact);
Contact.remove(Contact);
and any more behaviours are available in ArrayList!
you may write you remove method as follows
public Contact remove(String lstnm)
{
Contact c=new Contact(1stnm);
Contact contactToBeRemoved=list.get(1);
List.remove(c);
return contactToBeRemoved;
}
but you have to override the equal() and compareTo() method of the object class in the Contact class!
otherwise nothing will work properly!
In PHP, you can dynamically add elements to arrays by the following:
$x = new Array();
$x[] = 1;
$x[] = 2;
After this, $x would be an array like this: {1,2}.
Is there a way to do something similar in Java?
Look at java.util.LinkedList or java.util.ArrayList
List<Integer> x = new ArrayList<Integer>();
x.add(1);
x.add(2);
Arrays in Java have a fixed size, so you can't "add something at the end" as you could do in PHP.
A bit similar to the PHP behaviour is this:
int[] addElement(int[] org, int added) {
int[] result = Arrays.copyOf(org, org.length +1);
result[org.length] = added;
return result;
}
Then you can write:
x = new int[0];
x = addElement(x, 1);
x = addElement(x, 2);
System.out.println(Arrays.toString(x));
But this scheme is horribly inefficient for larger arrays, as it makes a copy of the whole array each time. (And it is in fact not completely equivalent to PHP, since your old arrays stays the same).
The PHP arrays are in fact quite the same as a Java HashMap with an added "max key", so it would know which key to use next, and a strange iteration order (and a strange equivalence relation between Integer keys and some Strings). But for simple indexed collections, better use a List in Java, like the other answerers proposed.
If you want to avoid using List because of the overhead of wrapping every int in an Integer, consider using reimplementations of collections for primitive types, which use arrays internally, but will not do a copy on every change, only when the internal array is full (just like ArrayList). (One quickly googled example is this IntList class.)
Guava contains methods creating such wrappers in Ints.asList, Longs.asList, etc.
Apache Commons has an ArrayUtils implementation to add an element at the end of the new array:
/** Copies the given array and adds the given element at the end of the new array. */
public static <T> T[] add(T[] array, T element)
I have seen this question very often in the web and in my opinion, many people with high reputation did not answer these questions properly. So I would like to express my own answer here.
First we should consider there is a difference between array and arraylist.
The question asks for adding an element to an array, and not ArrayList
The answer is quite simple. It can be done in 3 steps.
Convert array to an arraylist
Add element to the arrayList
Convert back the new arrayList to the array
Here is the simple picture of it
And finally here is the code:
Step 1:
public List<String> convertArrayToList(String[] array){
List<String> stringList = new ArrayList<String>(Arrays.asList(array));
return stringList;
}
Step 2:
public List<String> addToList(String element,List<String> list){
list.add(element);
return list;
}
Step 3:
public String[] convertListToArray(List<String> list){
String[] ins = (String[])list.toArray(new String[list.size()]);
return ins;
}
Step 4
public String[] addNewItemToArray(String element,String [] array){
List<String> list = convertArrayToList(array);
list= addToList(element,list);
return convertListToArray(list);
}
You can use an ArrayList and then use the toArray() method. But depending on what you are doing, you might not even need an array at all. Look into seeing if Lists are more what you want.
See: Java List Tutorial
You probably want to use an ArrayList for this -- for a dynamically sized array like structure.
You can dynamically add elements to an array using Collection Frameworks in JAVA. collection Framework doesn't work on primitive data types.
This Collection framework will be available in "java.util.*" package
For example if you use ArrayList,
Create an object to it and then add number of elements (any type like String, Integer ...etc)
ArrayList a = new ArrayList();
a.add("suman");
a.add(new Integer(3));
a.add("gurram");
Now you were added 3 elements to an array.
if you want to remove any of added elements
a.remove("suman");
again if you want to add any element
a.add("Gurram");
So the array size is incresing / decreasing dynamically..
Use an ArrayList or juggle to arrays to auto increment the array size.
keep a count of where you are in the primitive array
class recordStuff extends Thread
{
double[] aListOfDoubles;
int i = 0;
void run()
{
double newData;
newData = getNewData(); // gets data from somewhere
aListofDoubles[i] = newData; // adds it to the primitive array of doubles
i++ // increments the counter for the next pass
System.out.println("mode: " + doStuff());
}
void doStuff()
{
// Calculate the mode of the double[] array
for (int i = 0; i < aListOfDoubles.length; i++)
{
int count = 0;
for (int j = 0; j < aListOfDoubles.length; j++)
{
if (a[j] == a[i]) count++;
}
if (count > maxCount)
{
maxCount = count;
maxValue = aListOfDoubles[i];
}
}
return maxValue;
}
}
This is a simple way to add to an array in java. I used a second array to store my original array, and then added one more element to it. After that I passed that array back to the original one.
int [] test = {12,22,33};
int [] test2= new int[test.length+1];
int m=5;int mz=0;
for ( int test3: test)
{
test2[mz]=test3; mz++;
}
test2[mz++]=m;
test=test2;
for ( int test3: test)
{
System.out.println(test3);
}
In Java size of array is fixed , but you can add elements dynamically to a fixed sized array using its index and for loop. Please find example below.
package simplejava;
import java.util.Arrays;
/**
*
* #author sashant
*/
public class SimpleJava {
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
// TODO code application logic here
try{
String[] transactions;
transactions = new String[10];
for(int i = 0; i < transactions.length; i++){
transactions[i] = "transaction - "+Integer.toString(i);
}
System.out.println(Arrays.toString(transactions));
}catch(Exception exc){
System.out.println(exc.getMessage());
System.out.println(Arrays.toString(exc.getStackTrace()));
}
}
}