Get Absolute Value Without Using Abs Function Nor If Statement

get absolute value without using abs function nor if statement

From Bit Twiddling Hacks:

int v;           // we want to find the absolute value of v
unsigned int r;  // the result goes here 
int const mask = v >> sizeof(int) * CHAR_BIT - 1;

r = (v + mask) ^ mask;

How do I get the absolute value of an integer without using Math.abs?

You can use the conditional operator and the unary negation operator:

function absVal(integer) {
  return integer < 0 ? -integer : integer;
}

Finding absolute value of a number without using Math.abs()

If you look inside Math.abs you can probably find the best answer:

Eg, for floats:

    /*
     * Returns the absolute value of a {@code float} value.
     * If the argument is not negative, the argument is returned.
     * If the argument is negative, the negation of the argument is returned.
     * Special cases:
     * <ul><li>If the argument is positive zero or negative zero, the
     * result is positive zero.
     * <li>If the argument is infinite, the result is positive infinity.
     * <li>If the argument is NaN, the result is NaN.</ul>
     * In other words, the result is the same as the value of the expression:
     * <p>{@code Float.intBitsToFloat(0x7fffffff & Float.floatToIntBits(a))}
     *
     * @param   a   the argument whose absolute value is to be determined
     * @return  the absolute value of the argument.
     */
    public static float abs(float a) {
        return (a <= 0.0F) ? 0.0F - a : a;
    }

How to write absolute value in c

The shortest solution in your first piece of code is to change the printf statement as follows:

    printf("absValue = %u\n", (unsigned)((u<0)?-u:u));

This will print the absolute value of u. The type conversion (unsigned) ensures that the data type is as expected by printf. The statement (u<0)?-u:u uses the conditional operator to select the value -u if the condition (u<0) is true and u if the condition is false (i.e. u>=0).

The problem in your code is that u is a signed integer which means its value is stored using the Two's complement representation in 4 bytes(*) and printf is not intelligent. When you tell printf to display an unsigned integer, then printf will take the 4 bytes holding u and interpret them as an unsigned integer. Since negative numbers in Two's complement are stored as large positive integers, that is the result you see.

(*) The use of Two's complement and the int size of 4 is machine-dependent, but common.

Which is the fastest way to get the absolute value of a number

Conditionals are slower than plain arithmetic operations, but much, much faster than something as silly as calculating the square root.

Rules of thumb from my assembly days:

• Integer or bitwise op: 1 cycle
• Floating-point add/sub/mul: 4 cycles
• Floating-point div: ~30 cycles
• Floating-point exponentiation: ~200 cycles
• Floating-point sqrt: ~60 cycles depending on implementation
• Conditional branch: avg. 10 cycles, better if well-predicted, much worse if mispredicted

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