Is there a built in swap function in C?

问题:

Is there any built in swap function in C which works without using a third variable?

回答1:

No.
C++ has but it works like c = a;a = b; b = c;
C++ builtin swap function: swap(first,second);
Check this: http://www.cplusplus.com/reference/algorithm/swap/

You can use this to swap two variable value without using third variable:

a=a^b;
b=a^b;
a=b^a;

You can also check this:

https://stackoverflow.com/questions/756750/swap-the-values-of-two-variables-without-using-third-variable

How to swap without a third variable?

回答2:

Why do you not want to use a third variable? It's the fastest way on the vast majority of architectures.

The XOR swap algorithm works without a third variable, but it is problematic in two ways:

1. The variables must be distinct i.e. swap(&a, &a) will not work.
2. It is slower in general.

It may sometimes be preferable to use the XOR swap if using a third variable would cause the stack to spill, but generally you aren't in such a position to make that call.

To answer your question directly, no there is no swap function in standard C, although it would be trivial to write.

回答3:

There is no such function in standard C.

(In C++ you have std::swap().)

---

Maybe a macro from this question can be useful for you.

回答4:

Assuming you want a C solotion, not a C++ one, you could make it a macro, at least using GCC extension to have it generic enough, something like

 #define SWAP(x,y) do {   \ 
   typeof(x) _x = x;      \
   typeof(y) _y = y;      \
   x = _y;                \
   y = _x;                \
 } while(0)

beware of tricks like invocations swap(t[i++],i); to avoid them, use the address operator &. And you'll better use a temporary (for integers, there is a famous and useless trick with exclusive-or).

^{PS: I'm using two local variables _x and _y (but I could have used one local variable only) for better readability, and perhaps also to enable more optimizations from the compiler.}

回答5:

There is no standard function in C to swap two variables.

A macro can be written this way:

#define SWAP(T, a, b) do { T tmp = a; a = b; b = tmp; } while (0)

and the macro can be called this way:

int a = 42;
int b = 2718;

SWAP(int, a, b);

Some solutions for a writing a SWAP macro should be avoided:

#define SWAP(a, b) do { a = b + a; b = a - b; a = a - b; } while (0)

when operands are of signed types an overflow can occur and signed overflow are undefined behavior.

Also a solution trying to optimize the XOR solution like this should be avoid:

#define SWAP(a, b) (a ^= b ^= a ^=b)

a is modified twice between the previous and the next sequence point, so it violates the sequence points rules and is undefined behavior.

回答6:

Since you may copy any object representation into an unsigned char array in C, the following macro allows you to swap any two objects:

#define SWAP(X,Y) \
    do { \
        unsigned char _buf[sizeof(*(X))]; \
        memmove(_buf, (X), sizeof(_buf)); \
        memmove((X),  (Y), sizeof(_buf)); \
        memmove((Y), _buf, sizeof(_buf)); \
    } while (0)

GCC will even generate optimal code for this in some cases. You might not keep your job though...

回答7:

There is is a C++ library function. It swaps the values of two integer variables. For example, swap(x, y); will swap the values of variables x and y. Similarly, swap(mat[i][j], mat[j][i]); will swap two values in matrix mat, namely the value in row i column j and the value in row j column i.

回答8:

There is no built-in swap function but you can try this

a = a ^ b;

b = a ^ b;

a = b ^ a;

回答9:

there is std::swap since in general it depends on your processor, whether it supports swaping. there is an instruction called "compare and swap", but it only works on types that fit into a register and is guaranteed to be atomic. There is a built-in implementation of compare and swap (CAS) from gcc it's used for synchronization of thread and mutex implementations and probably way out of scope for your purpose so it's best to stick with just using a temporary variable or if you are really stuck to C you can always use a macro like this:

#define swap(a,b) a=a^b; \
                  b=a^b; \
                  a=b^a;

回答10:

I believe I've come up with a type-agnostic function for swapping any two values in standard C, though since I'm fairly new to the language I may have overlooked something. It uses the XOR swap algorithm, and I'm sure it could be optimized more, but it works as long as the two values point to the same number of bytes, specified by the 3rd argument:

void swapn(void *a, void *b, size_t n) {
    if (a == b) {
        return;
    }

    size_t i;
    char *x = (char *)a,
        *y = (char *)b;

    for (i = 0; i < n; i++) {
        *x ^= *y;
        *y ^= *x;
        *x ^= *y;
        x++;
        y++;
    }
}

Example usage:

// swap two integers
int x = 5,
    y = 30;

printf("%d\t%d\n", x, y);

swapn(&x, &y, sizeof(int));

printf("%d\t%d\n\n", x, y);

// swap two floats
float a = 9.23f,
    b = 6.83f;

printf("%.2f\t%.2f\n", a, b);

swapn(&a, &b, sizeof(float));

printf("%.2f\t%.2f\n\n", a, b);

// swap two doubles
double p = 4.7539,
    q = 0.9841;

printf("%.4f\t%.4f\n", p, q);

swapn(&p, &q, sizeof(double));

printf("%.4f\t%.4f\n\n", p, q);

// swap two chars
char m = 'M',
    n = 'n';

printf("%c\t%c\n", m, n);

swapn(&m, &n, sizeof(char));

printf("%c\t%c\n\n", m, n);

// swap two strings of equivalent length
char s[] = "Hello",
    t[] = "World";

printf("%s\t%s\n", s, t);

swapn(s, t, sizeof(s));

printf("%s\t%s\n\n", s, t);

The output is:

5   30
30  5

9.23    6.83
6.83    9.23

4.7539  0.9841
0.9841  4.7539

M   n
n   M

Hello   World
World   Hello

回答11:

#define swap(T, x, y) \
    {                 \
        T tmp = x;    \
        x = y;        \
        y = tmp;      \
    }

int main()
{
    int a = 10;
    int b = 20;
    printf("a=%d b=%d\n", a, b);
    swap(int, a, b);
    printf("a=%d b=%d\n", a, b);

    return 0;
}