# Decimal data type

Decimal data type

Some programming languages provide a built-in (primitive) or library decimal data type to represent non-repeating decimal fractions like 0.3 and -1.17 without rounding, and to do arithmetic on them. Examples are the `decimal.Decimal` type of Python, and analogous types provided by other languages.

## Rationale

Fractional numbers are supported on most programming languages as floating-point numbers or fixed-point numbers. However, such representations typically restrict the denominator to a power of two. Most decimal fractions (or most fractions in general) cannot be represented exactly as a fraction with a denominator that is a power of two. For example, the simple decimal fraction 0.3 (3/10) might be represented as 5404319552844595/18014398509481984 (0.299999999999999988897769...). This inexactness causes many problems that are familiar to experienced programmers. For example, the expression `0.1 * 7 == 0.7` might counterintuitively evaluate to false in some systems, due to the inexactness of the representation of decimals.

Although all decimal fractions are fractions, and thus it is possible to use a rational data type to represent it exactly, it may be more convenient in many situations to consider only non-repeating decimal fractions (fractions whose denominator is a power of ten). For example, fractional units of currency worldwide are mostly based on a denominator that is a power of ten. Also, most fractional measurements in science are reported as decimal fractions, as opposed to fractions with any other system of denominators.

A decimal data type could be implemented as either a floating-point number or as a fixed-point number. In the fixed-point case, the denominator would be set to a fixed power of ten. In the floating-point case, a variable exponent would represent the power of ten to which the mantissa of the number is multiplied.

Languages that support a rational data type usually allow the construction of such a value from a string, instead of a base-2 floating-point number, due to the loss of exactness the latter would cause. Usually the basic arithmetic operations ('+', '−', '×', '/', integer powers) and comparisons ('=', '<', '>', '≤') would be extended to act on them — either natively or through operator overloading facilities provided by the language. These operations may be translated by the compiler into a sequence of integer machine instructions, or into library calls. Support may also extend to other operations, such as formatting, rounding to an integer or floating point value, etc..

## Language support

• Starting with Python 2.4, Python's standard library includes a Decimal class in the module decimal.[1]
• Ruby's standard library includes a BigDecimal class in the module bigdecimal
• Java's standard library includes a Class java.math.BigDecimal class
• In Objective-C, the Cocoa and GNUstep APIs provide an NSDecimalNumber class and an NSDecimal C datatype for representing decimals whose mantissa is up to 38 digits long, and exponent is from -128 to 127.

## References

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