Division by two

In mathematics, division by two or halving has also been called mediation or dimidiation.^[1] The treatment of this as a different operation from multiplication and division by other numbers goes back to the ancient Egyptians, whose multiplication algorithm used division by two as one of its fundamental steps.^[2] Some mathematicians as late as the sixteenth century continued to view halving as a separate operation,^[3][4] and it often continues to be treated separately in modern computer programming.^[5] Performing this operation is simple in decimal arithmetic, in the binary numeral system used in computer programming, and in other even-numbered bases.

Binary

In binary arithmetic, division by two can be performed by a bit shift operation that shifts the number one place to the right. This is a form of strength reduction optimization. For example, 1101001 in binary (the decimal number 105), shifted one place to the right, is 110100 (the decimal number 52): the lowest order bit, a 1, is removed. Similarly, division by any power of two 2^k may be performed by right-shifting k positions. Because bit shifts are often much faster operations than division, replacing a division by a shift in this way can be a helpful step in program optimization.^[5] However, for the sake of software portability and readability, it is often best to write programs using the division operation and trust in the compiler to perform this replacement.^[6] An example from Common Lisp:

 (setq number #b1101001)   ; #b1101001  —  105
 (ash number -1)           ; #b0110100  —  105 >> 1 ⇒ 52
 (ash number -4)           ; #b0000110  —  105 >> 4 ≡ 105 / 2⁴ ⇒ 6

The above statements, however, are not always true when dealing with dividing signed binary numbers. Shifting right by 1 bit will divide by two, always rounding down. However, in some languages, division of signed binary numbers round towards 0 (which, if the result is negative, means it rounds up). For example, Java is one such language: in Java, -3 / 2 evaluates to -1, whereas -3 >> 1 evaluates to -2. So in this case, the compiler cannot optimize division by two by replacing it by a bit shift, when the dividend could possibly be negative.

Binary floating point

In binary floating-point arithmetic, division by two can be performed by decreasing the exponent by one (as long as the result is not a subnormal number). Many programming languages provide functions that can be used to divide a floating point number by a power of two. For example, the Java programming language provides the method java.lang.Math.scalb for scaling by a power of two,^[7] and the C programming language provides the function ldexp for the same purpose.^[8]

Decimal

The following algorithm is for decimal. However, it can be used as a model to construct an algorithm for taking half of any number N in any even base.

• Write out N, putting a zero to its left.
• Go through the digits of N in overlapping pairs, writing down digits of the result from the following table.

If first digit is	Even	Even	Even	Even	Even	Odd	Odd	Odd	Odd	Odd
And second digit is	0 or 1	2 or 3	4 or 5	6 or 7	8 or 9	0 or 1	2 or 3	4 or 5	6 or 7	8 or 9
Write	0	1	2	3	4	5	6	7	8	9

Example: 1738/2=?

Write 01738. We will now work on finding the result.

• 01: even digit followed by 1, write 0.
• 17: odd digit followed by 7, write 8.
• 73: odd digit followed by 3, write 6.
• 38: odd digit followed by 8, write 9.

Result: 0869.

From the example one can see that 0 is even.

If the last digit of N is odd digit one should add 0.5 to the result.

See also

• One half
• Median, a value that splits a set of data values into two equal subsets
• Bisection, the partition of a geometric object into two equal halves
• Dimidiation, a heraldic method of joining two coats of arms by splitting their designs into halves

References

1. ^ Steele, Robert (1922), The Earliest arithmetics in English, Early English Text Society, 118, Oxford University Press, p. 82 .
2. ^ Chabert, Jean-Luc; Barbin, Évelyne (1999), A history of algorithms: from the pebble to the microchip, Springer-Verlag, p. 16, ISBN 9783540633693 .
3. ^ Jackson, Lambert Lincoln (1906), The educational significance of sixteenth century arithmetic from the point of view of the present time, Contributions to education, 8, Columbia University, p. 76 .
4. ^ Waters, E. G. R. (1929), "A Fifteenth Century French Algorism from Liége", Isis 12 (2): 194–236, JSTOR 224785 .
5. ^ ^{a b} Wadleigh, Kevin R.; Crawford, Isom L. (2000), Software optimization for high-performance computing, Prentice Hall, p. 92, ISBN 9780130170088 .
6. ^ Hook, Brian (2005), Write portable code: an introduction to developing software for multiple platforms, No Starch Press, p. 133, ISBN 9781593270568 .
7. ^ "Math.scalb". Java Platform Standard Ed. 6. http://java.sun.com/javase/6/docs/api/java/lang/Math.html#scalb(double,%20int). Retrieved 2009-10-11. 
8. ^ Programming languages — C, International Standard ISO/IEC 9899:1999 , Section 7.12.6.6.