How to multiply a given number by 2 without using arithmetic operators in c?

Question

How to multiply a given number by 2 without using arithmetic operators in c language?

Answer 1

Use bit wise << operator:

x = x << 1;

This works for integer and long numbers (not floating point numbers).

It basically shifts the binary contents one position to the left, which is equivalent to multiplying by 2

Answer 2

Left shift.

But why would you want to do that? Leave that kind of optimization to the compiler.

List of operators and plenty of examples on wikipedia.

Answer 3

Just to extend on kgiannakakis post:

The shift operator << works because it shifts at the binary level - effectively in base 2. Just as moving a number by one place to the left in decimal (base 10) is the same as multiplying by 10. For example:

23 (move to left one digit) = 230 = 23 * 10

For the example (using the prefix 0b to represent binary numbers):

    3 * 2 = 6
0b11 << 1 = 0b110

Shifting by other numbers is equivalent to multiplying by 2 'n' times, or multiplying by 2 to the nth power (2^n)

   3 * 8 =    3 * 2^3    = 24
0b11 * 8 =   0b11 << 3   = 0b11000 (note the trailing zeros)

And an example in decimal to finish it off:

23 (move to left 3 places) = 23 * 1000 = 23 * 10^3 = 23000

Answer 4

Caution: Shifting might not work for signed variables,

int x;
int tmp;

if (x < 0) {

   tmp = -x;
   tmp = tmp << 1;
   x = -tmp;
}
else {

   x = x << 1;
}