The ternary conditional operator in C

Question

What is the need for the conditional operator  Functionally it is redundant  since it implements an if-else construct  If the conditional operator is more efficient than the equivalent if-else assignment  why can t if-else be interpreted more efficiently by the compiler

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In C  the real utility of it is that it s an expression instead of a statement  that is  you can have it on the right-hand side  RHS  of a statement   So you can write certain things more concisely

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like dwn said  Performance was one of its benefits during the rise of complex processors  MSDN blog Non-classical processor behavior  How doing something can be faster than not doing it gives an example which clearly says the difference between ternary  conditional  operator and if else statement   give the following code    include  lt windows h gt   include  lt stdlib h gt   include  lt stdlib h gt   include  lt stdio h gt   int array 10000    int countthem int boundary     int count   0   for  int i   0  i  lt  10000  i        if  array i   lt  boundary  count        return count     int   cdecl wmain int  wchar t        for  int i   0  i  lt  10000  i    array i    rand     10    for  int boundary   0  boundary  lt   10  boundary        LARGE INTEGER liStart  liEnd    QueryPerformanceCounter  amp liStart      int count   0    for  int iterations   0  iterations  lt  100  iterations         count    countthem boundary          QueryPerformanceCounter  amp liEnd     printf  count  7d  time    I64d n            count  liEnd QuadPart - liStart QuadPart       return 0      the cost for different boundary are much different and wierd  see the original material   while if change    if  array i   lt  boundary  count      to   count     array i   lt  boundary    1   0    The execution time is now independent of the boundary value  since      the optimizer was able to remove the branch from the ternary expression    but on my desktop intel i5 cpu windows 10 vs2015  my test result is quite different with msdn blog   when using debug mode  if else cost   count       0  time   6434 count  100000  time   7652 count  200800  time   10124 count  300200  time   12820 count  403100  time   15566 count  497400  time   16911 count  602900  time   15999 count  700700  time   12997 count  797500  time   11465 count  902500  time   7619 count 1000000  time   6429   and ternary operator cost   count       0  time   7045 count  100000  time   10194 count  200800  time   12080 count  300200  time   15007 count  403100  time   18519 count  497400  time   20957 count  602900  time   17851 count  700700  time   14593 count  797500  time   12390 count  902500  time   9283 count 1000000  time   7020    when using release mode  if else cost   count       0  time   7 count  100000  time   9 count  200800  time   9 count  300200  time   9 count  403100  time   9 count  497400  time   8 count  602900  time   7 count  700700  time   7 count  797500  time   10 count  902500  time   7 count 1000000  time   7   and ternary operator cost   count       0  time   16 count  100000  time   17 count  200800  time   18 count  300200  time   16 count  403100  time   22 count  497400  time   16 count  602900  time   16 count  700700  time   15 count  797500  time   15 count  902500  time   16 count 1000000  time   16   the ternary operator is slower than if else statement on my machine   so according to different compiler optimization techniques  ternal operator and if else may behaves much different

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Some of the more obscure operators in C exist solely because they allow implementation of various function-like macros as a single expression that returns a result  I would say that this is the main purpose why the    and   operators are allowed to exist  even though their functionality is otherwise redundant   Lets say we wish to implement a function-like macro that returns the largest of two parameters  It would then be called as for example   int x   LARGEST 1 2     The only way to implement this as a function-like macro would be   define LARGEST x y    x   gt   y     x     y     It wouldn t be possible with an if     else statement  since it does not return a result value   Note  The other purpose of    is that it in some cases actually increases readability  Most often if   else is more readable  but not always  Take for example long  repetitive switch statements   switch something      case A       if x    A              array i    x            else             array i    y            break     case B       if x    B              array i    x            else             array i    y            break            This can be replaced with the far more readable  switch something      case A  array i     x    A    x   y  break    case B  array i     x    B    x   y  break           Please note that    does never result in faster code than if-else  That s some strange myth created by confused beginners  In case of optimized code     gives identical performance as if-else in the vast majority of the cases    If anything     can be slower than if-else  because it comes with mandatory implicit type promotions  even of the operand which is not going to be used  But    can never be faster than if-else      Note  Now of course someone will argue and wonder why not use a function  Indeed if you can use a function  it is always preferable over a function-like macro  But sometimes you can t use functions  Suppose for example that x in the example above is declared at file scope  The initializer must then be a constant expression  so it cannot contain a function call  Other practical examples of where you have to use function-like macros involve type safe programming with  Generic or  X macros

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Compactness and the ability to inline an if-then-else construct into an expression

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It s syntatic sugar and a handy shorthand for brief if else blocks that only contain one statement  Functionally  both constructs should perform identically

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Since no one has mentioned this yet  about the only way to get smart printf statements is to use the ternary operator   printf   d item s   count  count  gt  1    s n      n      Caveat  There are some differences in operator precedence when you move from C to C   and may be surprised by the subtle bug s  that arise thereof

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Sometimes the ternary operator is the best way to get the job done   In particular when you want the result of the ternary to be an l-value   This is not a good example  but I m drawing a blank on somethign better   One thing is certian  it is not often when you really need to use the ternary  although I still use it quite a bit   const char  appTitle    amDebugging    DEBUG App 1 0     App v 1 0     One thing I would warn against though is stringing ternaries together   They become a real problem at maintennance time   int myVal   aIsTrue   aVal   bIsTrue   bVal   cIsTrue   cVal   dVal    EDIT  Here s a potentially better example   You can use the ternary operator to assign references  amp  const values where you would otherwise need to write a function to handle it   int getMyValue       if  myCondition       return 42    else     return 314     const int myValue   getMyValue         could become   const int myValue   myCondition   42   314    Which is better is a debatable question that I will choose not to debate

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There are a lot of things in C that aren t technically needed because they can be more or less easily implemented in terms of other things  Here is an incomplete list    while for functions structs   Imagine what your code would look like without these and you may find your answer  The ternary operator is a form of  syntactic sugar  that if used with care and skill makes writing and understanding code easier

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The same as   if 0  do      if 0    do

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The fact that the ternary operator is an expression  not a statement  allows it to be used in macro expansions for function-like macros that are used as part of an expression   Const may not have been part of original C  but the macro pre-processor goes way back   One place where I ve seen it used is in an array package that used macros for bound-checked array accesses   The syntax for a checked reference was something like aref arrayname  type  index   where arrayname was actually a pointer to a struct that included the array bounds and an unsigned char array for the data  type was the actual type of the data  and index was the index   The expansion of this was quite hairy  and I m not going to do it from memory   but it used some ternary operators to do the bound checking   You can t do this as a function call in C because of the need for polymorphism of the returned object   So a macro was needed to do the type casting in the expression  In C   you could do this as a templated overloaded function call  probably for operator     but C doesn t have such features   Edit  Here s the example I was talking about  from the Berkeley CAD array package  glu 1 4 edition    The documentation of the array fetch usage is   type array fetch type  array  position  typeof type  array t  array  int position       Fetch an element from an array   A   runtime error occurs on an    attempt to   reference outside the bounds of the   array   There is  no type-checking   that the value at the given position   is    actually of the type used when   dereferencing the array    and here is the macro defintion of array fetch  note the use of the ternary operator and the comma sequencing operator to execute all the subexpressions with the right values in the right order as part of a single expression     define array fetch type  a  i             array global index    i                     array global index  gt    a - gt num    array abort  a  1    0        type      a - gt space   array global index    a - gt obj size      The expansion for array insert   which grows the array if necessary  like a C   vector  is even hairier  involving multiple nested ternary operators

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The ternary operator is a syntactic and readability convenience  not a performance shortcut  People are split on the merits of it for conditionals of varying complexity  but for short conditions  it can be useful to have a one-line expression   Moreover  since it s an expression  as Charlie Martin wrote  that means it can appear on the right-hand side of a statement in C  This is valuable for being concise

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ternary   simple form of if-else  It is available mostly for readability

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It s crucial for code obfuscation  like this   Look- gt        See    No    Oh  well

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Some of the other answers given are great  But I am surprised that no one mentioned that it can be used to help enforce const correctness in a compact way   Something like this   const int n    x    0    10   20    so basically n is a const whose initial value is dependent on a condition statement  The easiest alternative is to make n not a const  this would allow an ordinary if to initialize it  But if you want it to be const  it cannot be done with an ordinary if  The best substitute you could make would be to use a helper function like this   int f int x        if x    0    return 10    else   return 20       const int n   f x     but the ternary if version is far more compact and arguably more readable

[c] The ternary (conditional) operator in C

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