Floating point
A floating-point number is a digital representation for a number in a certain subset of the rational numbers, and is often used to approximate an arbitrary real number on a computer. In particular, it represents an integer or fixed-point number (the mantissa) multiplied by a base (usually 2) to some integer power (the exponent); it is the binary analog of scientific notation (base 10). A floating point calculation is an arithmetic calculation done with floating point numbers, and often involves some approximation or rounding because the result of an operation may not be exactly representable,
In a floating-point number, the number of significant digits (the relative precision) is a constant, rather than the absolute precision as in fixed-point.
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2 Usage in computing 3 Problems with floating point 4 IEEE standard 5 Examples 6 References |
A floating-point number a is represented by two numbers m and e, such that a = m × be.
In any such system we pick a base b (called the base of numeration, also the radix)
and a precision p (how many digits to store).
m (which is called the mantissa, also the significand) is a p digit number of the form +-d.ddd...ddd (each digit being an integer between 0 and b-1 inclusive). If the leading digit of m is non-zero then the number is said to be normalized. Some descriptions use a separate sign bit (s, which represents -1 or +1) and require m to be positive.
e is called the exponent. (For more on the concept of "mantissa", see common logarithm.)
This scheme allows a large range of magnitudes to be represented within a limited precision field, which is not possible in a fixed point notation.
As an example, a floating point number with four decimal digits (b=10, p=4) could be used to represent 4321 or 0.00004321, but would not have enough precision to represent 432.123 and 43212.3 (which would have to be rounded to 432.1 and 43210). Of course, in practice, the number of digits is usually larger than four.
Floating point numbers usually behave very similarly to the real numbers they are used to approximate. However, this can easily lead programmers into over-confidently ignoring the need for numerical analysis. There are many cases where floating point numbers do not model real numbers well, even in simple cases such are representing the decimal fraction 0.1, which cannot be exactly represented in any binary floating-point format. For this reason, financial software tends not to use floating point number representation.
Errors in floating point computation can be :
The IEEE have standized the computer representation in IEEE 754. This standard is followed by almost all modern machines. The only exceptions are IBM Mainframes, which recently acquired an IEEE mode, and Cray vector machines, where the T90 series had an IEEE version, but the SV1 still uses Cray floating point format.
Representation
Hidden bit
When using binary (b=2), one bit can be saved if all numbers are required to be normalized. The leading digit of the mantissa of a normalised binary number is always non-zero, in particular it is always 1. This means that it does not need to be stored explicitly, for a normalised number it can be understood to be 1. The IEEE standard exploits this fact. Requiring all numbers to be normalised means that 0 cannot be represented; typically some special representation of zero is chosen.Usage in computing
While in the examples above the numbers are represented in the decimal system (that is the base of numeration, b = 10, computers usually do so in the binary system, which means that b = 2). In computers, floating-point numbers are sized by the number of bits used to store them. This size is usually 32 bits or 64 bits, often called "single-precision" and "double-precision". A few machines offer larger sizes; Intel FPUs such as 8087 (and its descendands integrated into the x86 architecture) offer 80 bit floating point numbers for intermediate results, and several systems offer 128 bit floating-point, generally implemented in software.Problems with floating point
IEEE standard
Examples
Note that though the examples in this article used a consistent system of floating-point notation, the notation is different from the IEEE standard. For example, in IEEE 754, the exponent is between the sign bit and the mantissa, not at the end of the number. Also the IEEE exponent uses a biased integer instead of a 2's-complement number. The readers need to understand that the examples serve the purpose of illustrating how floating-point numbers could be representated, but the actual bits shown in the article is different from what an IEEE 754-compliant number would look like. The placement of the bits in the IEEE standard enables two floating point numbers to be compared bitwise (sans sign-bit) to yield a result without interpreting the actual values. The arbituary system used in this article cannot do the same. Some good wikipedians with spare time can rewrite the examples using the IEEE standard if desired, though the current version is good enough as textbook examples for it highlighted all the major components of a floating-point notation. This also illustrated that a non-standard notation system also works as long as it is consistent.
References
See also: computable number, IEEE Floating Point Standard
