C 中的三元（条件）运算符

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

It's crucial for code obfuscation, like this:

Look->       See?!

No
:(
Oh, well
);

Answer 5

紧凑性以及将if-then-else构造内联到表达式中的能力。

Answer 6

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:

1. while
2. for
3. functions
4. 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.

Answer 7

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 & 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.

Answer 8

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 > 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.

Answer 9

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 >= (a)->num) ? array_abort((a),1) : 0,\
  *((type *) ((a)->space + array_global_index * (a)->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.

Answer 10

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

Answer 11

三元运算符可能比常规的else子句具有更高的性能，这在嵌入式应用程序中可能很关键，但编译器优化可能会消除这种差异。

Answer 12

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 <windows.h>
#include <stdlib.h>
#include <stdlib.h>
#include <stdio.h>

int array[10000];

int countthem(int boundary)
{
 int count = 0;
 for (int i = 0; i < 10000; i++) {
  if (array[i] < boundary) count++;
 }
 return count;
}

int __cdecl wmain(int, wchar_t **)
{
 for (int i = 0; i < 10000; i++) array[i] = rand() % 10;

 for (int boundary = 0; boundary <= 10; boundary++) {
  LARGE_INTEGER liStart, liEnd;
  QueryPerformanceCounter(&liStart);

  int count = 0;
  for (int iterations = 0; iterations < 100; iterations++) {
   count += countthem(boundary);
  }

  QueryPerformanceCounter(&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] < boundary) count++;

to

 count += (array[i] < 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.

Answer 13

• 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) > (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".

Answer 14

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

Answer 15

The same as

if(0)
do();


if(0)
{
do();
}