Swap two variables without using a temporary variable

Question

I 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

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Yes  use this code   stopAngle   Convert ToDecimal 159 9   startAngle   Convert ToDecimal 355 87     The problem is harder for arbitrary values    -

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First of all  swapping without a temporary variable in a language as C  is a very bad idea   But for the sake of answer  you can use this code   startAngle   startAngle   stopAngle  stopAngle   startAngle - stopAngle  startAngle   startAngle - stopAngle    Problems can however occur with rounding off if the two numbers differ largely  This is due to the nature of floating point numbers   If you want to hide the temporary variable  you can use a utility method   public static class Foo        public static void Swap lt T gt   ref T lhs  ref T rhs            T temp   lhs          lhs   rhs          rhs   temp

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If you can change from using decimal to double you can use the Interlocked class  Presumably this will be a good way of swapping variables performance wise  Also slightly more readable than XOR   var startAngle   159 9d  var stopAngle   355 87d  stopAngle   Interlocked Exchange ref startAngle  stopAngle     Msdn  Interlocked Exchange Method  Double  Double

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lt deprecated gt   You can do it in 3 lines using basic math - in my example I used multiplication  but simple addition would work also   float startAngle   159 9F  float stopAngle   355 87F   startAngle   startAngle   stopAngle  stopAngle   startAngle   stopAngle  startAngle   startAngle   stopAngle    Edit  As noted in the comments  this wouldn t work if y   0 as it would generate a divide by zero error which I hadn t considered   So the   - solution alternatively presented would be the best way to go    lt  deprecated gt     To keep my code immediately comprehensible  I d be more likely to do something like this    Always think about the poor guy that s gonna have to maintain your code    static bool Swap lt T gt  ref T x  ref T y        try               T t   y          y   x          x   t          return true            catch               return false            And then you can do it in one line of code   float startAngle   159 9F float stopAngle   355 87F Swap lt float gt  ref startAngle  ref stopAngle     Or     MyObject obj1   new MyObject  object1    MyObject obj2   new MyObject  object2    Swap lt MyObject gt  ref obj1  ref obj2     Done like dinner   you can now pass in any type of object and switch them around

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Sometimes I wish it were possible to write a function in MSIL inline in C   similar to how you can write inline assembler in C  For the record  I once wrote a helper library for C  with various functions for things that were impossible to write in C  but can be written in MSIL  non-zero-based arrays for example   I had this function   method public hidebysig static void Swap lt T gt              T amp  a            T amp  b       cil managed         maxstack 4      ldarg 1         push a amp  reference     ldarg 2         push b amp  reference     ldobj   T       pop b amp   push b     ldarg 2         push b amp  reference     ldarg 1         push a amp  reference     ldobj   T       pop a amp   push a     stobj   T       store a in b amp      stobj   T       store b in a amp      ret    And no locals needed  Of course this was just me being silly

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You can try the following code  It is much more better than the other code   a   a   b  b   a - b  a   a - b

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Here is some different process to swap two variables     process one a b a  b a-b  a a-b  printf  a   d  b    d  a b      process two a 5  b 10  a a b- b a   printf   na   d  b    d  a b      process three a 5  b 10  a a b  b a b  a b a  printf   na   d  b    d  a b      process four a 5  b 10  a b- a-1  b a  b 1  a a  b 1  printf   na   d  b    d  a b

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int a   4  b   6  a    b    a    b    Works for all types including strings and floats

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In C  7    startAngle  stopAngle     stopAngle  startAngle

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we can do that by doing a simple trick  a   20  b   30  a   a b     add both the number now a has value 50 b   a-b     here we are extracting one number from the sum by sub a   a-b     the number so obtained in above help us to fetch the alternate number from sum System out print  swapped numbers are a      a  b      b

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If you want to swap 2 string variables   a    a b  Substring  b a  Length     An helper method accordingly   public static class Foo       public static void SwapString  ref string a  ref string b           a    a b  Substring  b a  Length             Usage would be then   string a  Test 1   string b  Test 2   Foo SwapString a  b

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C  7 introduced tuples which enables swapping two variables without a temporary one   int a   10  int b   2   a  b     b  a     This assigns b to a and a to b

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Not in C   In native code you might be able to use the triple-XOR swap trick  but not in a high level type-safe language   Anyway  I ve heard that the XOR trick actually ends up being slower than using a temporary variable in many common CPU architectures    You should just use a temporary variable  There s no reason you can t use one  it s not like there s a limited supply

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BenAlabaster showed a practical way of doing a variable switch  but the try-catch clause is not needed  This code is enough   static void Swap lt T gt  ref T x  ref T y         T t   y       y   x       x   t      The usage is the same as he shown   float startAngle   159 9F float stopAngle   355 87F Swap ref startAngle  ref stopAngle     You could also use an extension method   static class SwapExtension       public static T Swap lt T gt  this T x  ref T y                T t   y          y   x          return t            Use it like this   float startAngle   159 9F  float stopAngle   355 87F  startAngle   startAngle Swap ref stopAngle     Both ways uses a temporary variable in the method  but you don t need the temporary variable where you do the swapping

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With tuples  decimal startAngle   Convert ToDecimal 159 9   decimal stopAngle   Convert ToDecimal 355 87     startAngle  stopAngle     stopAngle  startAngle

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For binary types you can use this funky trick   a    b    a    b    As long as a and b are not the exact same variable  e g  aliases for the same memory  it works

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A binary XOR swap with a detailed example   XOR truth table   a b a b 0 0  0 0 1  1 1 0  1 1 1  0   Input   a   4  b   6    Step 1  a   a   b  a    0100 b    0110 a b  0010   2   a   Step 2  b   a   b  a    0010 b    0110 a b  0100   4   b   Step 3  a   a   b  a    0010 b    0100 a b  0110   6   a   Output   a   6  b   4

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For completeness  here is the binary XOR swap   int x   42  int y   51236  x    y  y    x  x    y    This works for all atomic objects references  as it deals directly with the bytes  but may require an unsafe context to work on decimals or  if you re feeling really twisted  pointers  And it may be slower than a temp variable in some circumstances as well

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The simple way to swap 2 numbers in just one line   a  a b - b a     eg  a 1  b 2  Step 1  a  1 2  -  b 1   Step 2  a 3-1      a 2 and  b 1    Efficient way is to use   C Programming       x    y    y    x    x    y    Java   x   x   y    y   x    Python  x  y   y  x   Note  Most common mistake people make    Swap using bitwise XOR  Wrong Solution in C C     x    y    x    y     Source  GeeksforGeek

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With C  7  you can use tuple deconstruction to achieve the desired swap in one line  and it s clear what s going on    decimal startAngle   Convert ToDecimal 159 9   decimal stopAngle   Convert ToDecimal 355 87     startAngle  stopAngle     stopAngle  startAngle

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I hope this might help     using System   public class Program       public static void Main                 int a   1234          int b   4321           Console WriteLine  Before  a  0  and b  1    a  b            b   b - a          a   a   b          b   a - b           Console WriteLine  After  a  0  and b  1    a  b

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startAngle    startAngle   stopAngle  -  stopAngle   startAngle

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Beware of your environment   For example  this doesn   t seem to work in ECMAscript  y    x    y    x    But this does  x    y    x  y    x    My advise  Assume as little as possible

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For the sake of future learners  and humanity  I submit this correction to the currently selected answer   If you want to avoid using temp variables  there are only two sensible options that take first performance and then readability into consideration    Use a temp variable in a generic Swap method   Absolute best performance  next to inline temp variable  Use Interlocked Exchange   5 9 times slower on my machine  but this is your only option if multiple threads will be swapping these variables simultaneously     Things you should never do    Never use floating point arithmetic   slow  rounding and overflow errors  hard to understand  Never use non-primitive arithmetic   slow  overflow errors  hard to understand  Decimal is not a CPU primitive and results in far more code than you realize  Never use arithmetic period  Or bit hacks   slow  hard to understand  That s the compiler s job  It can optimize for many different platforms     Because everyone loves hard numbers  here s a program that compares your options  Run it in release mode from outside Visual Studio so that Swap is inlined  Results on my machine  Windows 7 64-bit i5-3470    Inline       00 00 00 7351931 Call         00 00 00 7483503 Interlocked  00 00 04 4076651   Code   class Program       static void Swap lt T gt  ref T obj1  ref T obj2                var temp   obj1          obj1   obj2          obj2   temp             static void Main string   args                var a   new object            var b   new object             var s   new Stopwatch             Swap ref a  ref b      JIT the swap method outside the stopwatch          s Restart            for  var i   0  i  lt  500000000  i                          var temp   a              a   b              b   temp                    s Stop            Console WriteLine  Inline temp      s Elapsed             s Restart            for  var i   0  i  lt  500000000  i                          Swap ref a  ref b                     s Stop            Console WriteLine  Call             s Elapsed            s Restart            for  var i   0  i  lt  500000000  i                          b   Interlocked Exchange ref a  b                     s Stop            Console WriteLine  Interlocked      s Elapsed            Console ReadKey

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The right way to swap two variables is   decimal tempDecimal   startAngle  startAngle   stopAngle  stopAngle   tempDecimal    In other words  use a temporary variable   There you have it  No clever tricks  no maintainers of your code cursing you for decades to come  no entries to The Daily WTF  and no spending too much time trying to figure out why you needed it in one operation anyway since  at the lowest level  even the most complicated language feature is a series of simple operations   Just a very simple  readable  easy to understand  t   a  a   b  b   t  solution   In my opinion  developers who try to use tricks to  for example   swap variables without using a temp  or  Duff s device  are just trying to show how clever they are  and failing miserably    I liken them to those who read highbrow books solely for the purpose of seeming more interesting at parties  as opposed to expanding your horizons    Solutions where you add and subtract  or the XOR-based ones  are less readable and most likely slower than a simple  temp variable  solution  arithmetic boolean-ops instead of plain moves at an assembly level    Do yourself  and others  a service by writing good quality readable code   That s my rant  Thanks for listening  -   As an aside  I m quite aware this doesn t answer your specific question  and I ll apologise for that  but there s plenty of precedent on SO where people have asked how to do something and the correct answer is  Don t do it

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var a   15  var b   -214  a   b     b   a     This works great

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Very simple code for swapping two variables   static void Main string   args        Console WriteLine  Prof Owais ahmed        Console WriteLine  Swapping two variables         Console WriteLine  Enter your first number         int x   Convert ToInt32 Console ReadLine          Console WriteLine  Enter your first number         int y   Convert ToInt32 Console ReadLine          Console WriteLine  your vlaue of x is   x   nyour value of y is   y        int z   x      x   y      y   z       Console WriteLine  after Swapping value of x is   x   nyour value of y is   y       Console ReadLine

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a   a   b b   a - b a   a - b

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Here another approach in one line   decimal a   159 9m  decimal b   355 87m   a   b    b   a  - b

[c#] Swap two variables without using a temporary variable

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