How do you write a c function to swap two variables without using a temporary variable?

Answer 1

You can swap values a and b without a temporary as follows:

a=a^b

b=a^b

a=a^b

(^ is the bitwise XOR operator)

To show how this works, let's consider the values a=180 and b=204. In binary, 180 is 10110100 while 204 is 11001100. Thus:

a = 10110100

b = 11001100

a^b means that we look at the corresponding bits in each input value and output a 1 if one and only one of the two input bits is set. Thus the output (o) is:

a = 10110100

b = 11001100

o = 01111000

In the expression a=a^b, the output of a^b is assigned to a, thus the values of a and b now look like this:

a = 01111000

b = 11001100

We then repeat the operation, a^b, this time assigning the output to b:

a = 01111000

b = 11001100

o = 10110100

So now we have:

a = 01111000

b = 10110100

We repeat the operation one more time, assigning the output value to a:

a = 01111000

b = 10110100

0 = 11001100

So now we have:

a = 11001100

b = 10110100

The two values have now been swapped.

To write this in C, we can use shorthand expressions using the compound XOR-ASSIGN operator (^=):

a^=b

b^=a

a^=b

These individual expressions can then be combined into a single expression:

a^=b^=a^=b

Finally, to use this expression in a C function, we need to pass a and b by reference (using pointer variables):

void swap (int* a, int* b) {

(*a)^=(*b)^=(*a)^=(*b);

}

Note that a and b must be of an integral type (char, short, long, int, unsigned, etc). The example above only works on type int. If we wish to swap other types, or more complex types, we must treat those values as if they were an integral type. One way to achieve this is to treat those types as if they were an array of type char, supplying the length of the arrays through a separate argument:

void swap (char* a, char* b, size_t size) {

for (int i=0; i<size; ++i) {

a[i]^=b[i]^=a[i]^=b[i];

}

}

For example, to swap two doubles, we can use the following call:

int main (void) {

double a, b;

a = 3.14;

b = 1.1;

swap ((char*) &a, (char*) &b, sizeof (double));

assert (a==1.1);

assert (b==3.14);

return 0;

}

Note that a and b must of the same type.

Answer 2

char a,b; //suppose these two are character variable

//assigning different values to each
a='d';
b='f';

char temp; //temp for holding the value of one of the character

temp=a; //a's value stored in temp
a=b; //a's new value is the value in b
b=temp; //b takes the a's old value which is stored in temp

//operation done