### Background & Techniques

There is nothing very useful about the results produced here. It
is mainly an investigation into how to combine exponents and
output from our Big Integer arithmetic unit to apply **+**, **-**, **x** and (maybe) **÷ **operations
to floating point numbers of arbitrary size.

The program was prompted by a
viewer's question about limitations of the Windows Calculator
program. The Windows Calc.exe calculator limits input and output to 32
significant digits and input of exponents (using the Exp key) to 4
digits. Using the X^Y key we can enter exponents up to
43300. The largest intrinsic number type,
Extended, uses 80bit (10 byte) number with 64 reserved bits for significant
digits. This is equivalent to 19 or 20 decimal
digits, so some sort of large number handling routines must be used to
reach 32 digit accuracy. It is likely that
the limitations are be imposed by display space considerations and,
perhaps, by the math library Microsoft uses to calculate function
values. By the way, the PowerToy Calculator, available as
a free download from Microsoft, allows users to choose 32, 64, 128 or 512
digit accuracy - with a warning that 512 digit mode may be very
slow. I guess!

I allow up to 50 digits to be used, primarily. to keep the display
space to a reasonable size.

#### Programmer's Notes

I define a **TBigFloat** class which contains
fields Exponent, the exponent of the number, **DecPart**, a **TInteger** big
integer object containing the digits and an integer field reflecting the
number of significant digits to display.

The **DecPart** field represent a decimal
value between -1 and +1. **Exponent** is the power of ten that **DecPart**
must be multiplied by to get the real value of the number. So
negative exponents reflect the number of 0's that must be inserted between
the decimal point and the leftmost digit of **DecPart**.
Positive exponents represent the position of the decimal point moving
right from the leftmost digit of **DecPart**, adding 0's to the right
end of the number if required.

Procedure **GetNumber**
converts a string to its internal representation and **Shownumber**
builds a string version of the number in "Normal" or
"Scientific" format. **Split **is a method
that uses the number of digits and the exponent value to determine how
many digits are to the left and right of the implied decimal point.
This allows the appropriate operand to be be shifted left (multiplied by
10) as necessary to align the decimal points before
adding.

**Addendum March 23, 2003:** A viewer suggested an algorithm
for division, which I implemented today. The new version
divides by successive approximation. A initial range for the
quotients is set at 0 for the lowest value and 10^{(dividend exponent -
divisor exponent) }as the maximum. The algorithm loops
making quotient guesses by splitting the difference between the last guess
that was too high and the last guess that was too low. Quotients are
checked by multiplying the quotient guess by the divisor and comparing the
result to the dividend. during each iteration. This may not be
the optimum method, but it seem to work on the cases tested so
far. A "rounding" procedure was required to
trim results back the specified number of significant digits.
Otherwise exact divisors tended to produce quotients ending in
9999999999... The **ShowNumber** procedure was
modified to remove extra trailing 0's after a decimal point, regardless of
the significant digits specification.

**Addendum April 13, 2005:** The **TInteger** class used by
**TBigFloat** has been moved the DFF Library. in unit **UBigIntsV2**.
If you wish to recompile this program, a one-time download of the
library will be necessary.

**Addendum April 4, 2006:** **UBigFloatV2** was
added to the DFF Library file and removed from the source zip
file. BigFloatTest program has added then **Compare**
operation for testing.

**Addendum December 5, 2006:** Charles Doumar has contributed a
number of additions to the** UBigFloatV2** unit including **Power**,
**Log** (natural and base 10), and **Exp** functions.
The DFF Library file has been upgraded to **DFFLibV08** to include
the revised UBigFloatV2 unit and a few others.
**BigFloatTest** was upgraded to test the new big float functions.

**Addendum February 7, 2007: **A few changes/enhancements
posted today in **UBigFloatV3** contained in a new library release **
DFFLibV10**. Most were "clean-up" activirties except for the
change in the definition of the **Round** procedure.

**Addendum October 16, 2009:** **BigFloatTest** was
reposted today to incorporate two small changes to UBigFloatV3.
The **Add **procedure to add one **TBigFloat** number to another
could produce erroneous results when a number with a very large exponent
was added to another with a very large negative exponent (a very small
number). Also procedure **Reciprocal** could loop
and produce an "Out of Memory" error under certain conditions.
Division uses **Reciprocal** to divide by multiplying and a user
encountered the error when computing 1/99,999,999,999,999,999,999.
Thanks to Charles Doumar for the corrections posted yesterday in
UBigFloatV3 in library file **DFFLibV13**.

**August 29, 2012:** The "**Round**" procedure in **
UBigFloatV3** was rewritten today to correct erroneous results when
0 digits to the right of the decimal point was specified and and values were
between -1 and +1.

### Running/Exploring the Program