I stumbled into this code for swapping two integers without using a temporary variable or the use of bitwise operators.

    int main(){
    
        int a=2,b=3;
        printf(&quot;a=%d,b=%d&quot;,a,b);
        a=(a+b)-(b=a);
        printf(&quot;\na=%d,b=%d&quot;,a,b);
        return 0;
    }

But I think this code has undefined behavior in the swap statement `a = (a+b) - (b=a);` as it does not contain any **sequence points** to determine the order of evaluation. 

My question is: *Is this an acceptable solution to swap two integers?*

Why is a = (a+b) - (b=a) a bad choice for swapping two integers?

I am new to java. How to write the java equivalent of the following C code.

    void Swap(int *p, int *q)
    {
       int temp;
       temp = *p;
       *p = *q;
       *q = temp;
    } 

How to write a basic swap function in Java

Are there any special methods to make swapping elements in an NSMutableArray easier or more direct?

Swapping elements in an NSMutableArray

An implementation of `std::swap` might look like this:

&lt;!-- language: lang-cpp --&gt;

    template &lt;class T&gt; void swap (T&amp; a, T&amp; b)
    {
      T c(std::move(a)); a=std::move(b); b=std::move(c);
    }
    template &lt;class T, size_t N&gt; void swap (T (&amp;a)[N], T (&amp;b)[N])
    {
      for (size_t i = 0; i&lt;N; ++i) swap (a[i],b[i]);
    }

An implementation [`std::exchange` n3668][1] might look like this:

     template&lt; typename T, typename U = T &gt;
       T exchange( T &amp; obj, U &amp;&amp; new_val )
       {
         T old_val = std::move(obj);
         obj = std::forward&lt;U&gt;(new_val);
         return old_val;
       }

It says:

&gt; For primitive types, this is equivalent to the obvious implementation,
&gt; while for more complex types, this definition
&gt; 
&gt; - Avoids copying the old value when that type defines a move constructor
&gt; - Accepts any type as the new value, taking advantage of any converting assignment operator
&gt; - Avoids copying the new value if it&#39;s a temporary or moved.
&gt; 
&gt; I chose the name for symmetry with atomic_exchange, since they behave
&gt; the same except for this function not being atomic.

[n3746][2] also proposes a built-in swap operator that looks like this:

    inline C&amp; C::operator :=: (C&amp;&amp; y) &amp;  { see below; return *this; } 
    inline C&amp; C::operator :=: (C&amp; y)  &amp;  { return *this :=: std::move(y); }

From what I gather, the proposals would like all three of these options to live side by side, rather than replacing each other. Why is it necessary to have three different ways to swap objects?


  [1]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3668.html
  [2]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3746.pdf

std::swap vs std::exchange vs swap operator

Here is the question: write a method that swaps two variables. These two variables should be primitives. It doesn&#39;t need to be generic e.g. two `int` variables. Is there a way?!

Is it possible to write swap method in Java?

[MSDN says](http://msdn.microsoft.com/en-us/library/f1863z4b.aspx):

&gt; `swap` should be used in preference to `iter_swap`, which was included in the C++ Standard for backward compatibility.

But [comp.std.c++ says](https://groups.google.com/forum/?fromgroups=#!topic/comp.std.c++/AMH7TXg5EsI):

&gt; Most STL algorithms operate on iterator ranges. It therefore makes sense to
use `iter_swap` when swapping elements within those ranges, since that is its
intended purpose --- swapping the elements pointed to by two iterators. This
allows optimizations for node-based sequences such as `std::list`, whereby the
nodes are just relinked, rather than the data actually being swapped.

So which one is correct? Should I use `iter_swap`, or should I use `swap`? (Is `iter_swap` only for backwards compatibility?) Why?

iter_swap() versus swap() -- what&#39;s the difference?

how do I make my swap function in java if there is no method by which we can pass by reference?  Could somebody give me a code?

    swap(int a, int b)
    {
         int temp = a;
         a = b;
         b = temp;
    }

But the changes wont be reflected back since java passes parameters by value.


Java method to swap primitives

Under Linux, how do I find out which process is using the swap space more?

How to find out which processes are using swap space in Linux?

I have this two variables:

    var a = 1,
        b = 2;

My question is how to swap them? Only this variables, not any objects.

How to swap two variables in JavaScript

Can I easily swap two elements with jQuery?

I&#39;m looking to do this with one line if possible.

I have a select element and I have two buttons to move up or down the options, and I already have the selected and the destination selectors in place, I do it with an if, but I was wondering if there is an easier way.

Is there a native jQuery function to switch elements?

As we all know, the pythonic way to swap the values of two items ``a`` and ``b`` is 
```
a, b = b, a
```
and it should be equivalent to 
```
b, a = a, b
```
However, today when I was working on some code, I accidentally found that the following two swaps give different results:
```
nums = [1, 2, 4, 3]
i = 2
nums[i], nums[nums[i]-1] = nums[nums[i]-1], nums[i]
print(nums)
# [1, 2, 4, 3]

nums = [1, 2, 4, 3]
i = 2
nums[nums[i]-1], nums[i] = nums[i], nums[nums[i]-1]
print(nums)
# [1, 2, 3, 4]
```
This is mind-boggling to me. Can someone explain to me what happened here? I thought in a Python swap the two assignments happen simultaneously and independently.

Understand Python swapping: why is a, b = b, a not always equivalent to b, a = a, b?

Does the standard define precisely what I can do with an object once it has been moved from? I used to think that all you can do with a moved-from object is do destruct it, but that would not be sufficient.

For example, take the function template `swap` as defined in the standard library:

    template &lt;typename T&gt;
    void swap(T&amp; a, T&amp; b)
    {
        T c = std::move(a); // line 1
        a = std::move(b);   // line 2: assignment to moved-from object!
        b = std::move(c);   // line 3: assignment to moved-from object!
    }

Obviously, it must be possible to assign to moved-from objects, otherwise lines 2 and 3 would fail. So what else can I do with moved-from objects? Where exactly can I find these details in the standard?

(By the way, why is it `T c = std::move(a);` instead of `T c(std::move(a));` in line 1?)


What can I do with a moved-from object?

I need to write a function to convert big endian to little endian in C. I can not use any library function.


convert big endian to little endian in C [without using provided func]

Is there a LINQ way to swap the position of two items inside a `List&lt;T&gt;`?

Swap two items in List&lt;T&gt;

What is the proper way to enable my `swap` in STL algorithms?

1) Member `swap`. Does `std::swap` use SFINAE trick to use the member `swap`.

2) Free standing `swap` in the same namespace.

3) Partial specialization of `std::swap`.

4) All of the above.

Thank you.

EDIT: Looks like I didn&#39;t word my question clearly. Basically, I have a template class and I need STL algos to use the (efficient) swap method I wrote for that class.

how to provide a swap function for my class?

I&#39;d like to be able to swap two variables without the use of a temporary variable in C#. Can this be done?

    decimal startAngle = Convert.ToDecimal(159.9);
    decimal stopAngle = Convert.ToDecimal(355.87);

    // Swap each:
    //   startAngle becomes: 355.87
    //   stopAngle becomes: 159.9


Swap two variables without using a temporary variable

&lt;!-- language-all: lang-php --&gt;

Say for instance I have ...

    $var1 = &quot;ABC&quot;
    $var2 = 123

and under certain conditions I want to swap the two around like so...

    $var1 = 123
    $var2 = &quot;ABC&quot;

Is there a PHP function for doing this rather than having to create a 3rd variable to hold one of the values then redefining each, like so...

    $var3 = $var1
    $var1 = $var2
    $var2 = $var3


For such a simple task its probably quicker using a 3rd variable anyway and I could always create my own function if I really wanted to. Just wondered if something like that exists?

Update: Using a 3rd variable or wrapping it in a function is the best solution. It&#39;s clean and simple. I asked the question more out of curiosity and the answer chosen was kind of &#39;the next best alternative&#39;. Just use a 3rd variable. 

Is there a PHP function for swapping the values of two variables?

In Python, I&#39;ve seen two variable values swapped using this syntax:

    left, right = right, left

Is this considered the standard way to swap two variable values or is there some other means by which two variables are by convention most usually swapped?

Is there a standardized method to swap two variables in Python?

I found this code to swap two numbers without using a third variable, using the XOR `^` operator.

**Code:**  

    int i = 25;
    int j = 36;
    j ^= i;       
    i ^= j;
    j ^= i;
            
    Console.WriteLine(&quot;i:&quot; + i + &quot; j:&quot; + j);
    
    //numbers Swapped correctly
    //Output: i:36 j:25

Now I changed the above code to this equivalent code.

**My Code:** 

    int i = 25;
    int j = 36;

    j ^= i ^= j ^= i;   // I have changed to this equivalent (???).

    Console.WriteLine(&quot;i:&quot; + i + &quot; j:&quot; + j);
    
    //Not Swapped correctly            
    //Output: i:36 j:0

Now, I want to know, **Why does my code give incorrect output?**

Why does swapping values with XOR fail when using this compound form?

For example : A list

A B C D E

Given C , Switch to 

C A B D E

Notice that the array size will change, some items may removed in run times

    Collections.swap(url, url.indexOf(itemToMove), 0);

This statement is not working because it output C B A D E not C A B D E , how to fix it?

Thanks. 

How to move specific item in array list to the first item

How is the swap function implemented in the STL? Is it as simple as this:

    template&lt;typename T&gt; void swap(T&amp; t1, T&amp; t2) {
        T tmp(t1);
        t1=t2;
        t2=tmp;
    }

In other posts, they talk about specializing this function for your own class. Why would I need to do this? Why can&#39;t I use the `std::swap` function?



How does the standard library implement std::swap?

From page 291 of OCP Java SE 6 Programmer Practice Exams, question 25:

    public class Stone implements Runnable {
        static int id = 1;
    
        public void run() {
            id = 1 - id;
            if (id == 0) 
                pick(); 
            else 
                release();
        }
    
        private static synchronized void pick() {
            System.out.print(&quot;P &quot;);
            System.out.print(&quot;Q &quot;);
        }
    
        private synchronized void release() {
            System.out.print(&quot;R &quot;);
            System.out.print(&quot;S &quot;);
        }
    
        public static void main(String[] args) {
            Stone st = new Stone();
            new Thread(st).start();
            new Thread(st).start();
        }
    }

One of the answers is:

&gt; The output could be `P Q P Q`

I marked this answer as correct. My reasoning:

1. We are starting two threads. 
2. First one enters `run()`.
3. According to [JLS 15.26.1](https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.26.1), it firstly evaluates `1 - id`. Result is `0`. It is stored on the thread&#39;s stack. We are just about to save that `0` to static `id`, but...
4. Boom, scheduler chooses the second thread to run.
5. So, the second thread enters `run()`. Static `id` is still `1`, so he executes method `pick()`. `P Q ` is printed.
6. Scheduler chooses first thread to run. It takes `0` from its stack and saves to static `id`. So, the first thread also executes `pick()` and prints `P Q `.

However, in the book it&#39;s written that this answer is incorrect:

&gt; It is incorrect because the line `id = 1 - id` swaps the value of `id` between `0` and `1`. There is no chance for the same method to be executed twice.

I don&#39;t agree. I think there is some chance for the scenario I presented above. Such swap is not atomic. Am I wrong?

Is id = 1 - id atomic?

This is my swap function:

    template &lt;typename t&gt;
    void swap (t&amp; x, t&amp; y)
    {
        t temp = x;
        x = y;
        y = temp;
        return;
    }

And this is my function (on a side note v stores strings) call to swap values but whenever I try to call using values in a vector I get an error. I&#39;m not sure what I&#39;m doing wrong.

    swap(v[position], v[nextposition]); //creates errors



C++ trying to swap values in a vector

I am wondering if there is a more efficient way of swapping two elements in an Array, than doing something like this:

    String temp = arr[1];
    arr[1] = arr[2];
    arr[2] = temp;

Well, this is obviously not bad or even wrong, but I need to swap very often so I am interested if there are any Libs or something that provide a more efficient way to do this?

All Swap Solutions on Gang of Coders

Total of 24 Swap Solutions