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cohorts.adoc

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Cohorts

As stated in the library overview, one of the major differences between binary floating point and decimal floating point is the existence of cohorts. From IEEE 754-2008: "The set of representations a floating-point number maps to is called the floating-point number’s cohort; the members of a cohort are distinct representations of the same floating-point number."

For example decimal32_t has a precision of 7. That means all the following are valid ways to represent the number 300 using that type:

  1. 3e+2

  2. 30e+1

  3. 300e+0

  4. 3000e-1

  5. 30000e-2

  6. 300000e-3

  7. 3000000e-4

These values can be useful in certain applications, like preserving the precision of results. A value is said to be normalized when the effects of these cohorts is removed. This is done by appending zeros to the significand and reducing the exponent until the significand has the same number of significant digits as the type can represent. For example 1 - 6 above would all be represented the same as value 7. By default, most methods DO NOT observe cohorts. Below is an example of how cohorts can be preserved if one so wishes.

Cohort Preserving <charconv> Example

This example can be found in the examples/ folder as charconv_cohort_preservation.cpp.

link:example$charconv_cohort_preservation.cpp[role=include]

Expected Output:

Values are equal and bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Successful Roundtrip of value: 3e+02

Values are equal and NOT bitwise equal.
Values are equal and bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Successful Roundtrip of value: 3.0e+02

Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Successful Roundtrip of value: 3.00e+02

Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Successful Roundtrip of value: 3.000e+02

Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Successful Roundtrip of value: 3.0000e+02

Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and bitwise equal.
Values are equal and NOT bitwise equal.
Successful Roundtrip of value: 3.00000e+02

Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and NOT bitwise equal.
Values are equal and bitwise equal.
Successful Roundtrip of value: 3.000000e+02