GiNaC | Special purpose C library for symbolic computations | Cryptography library

You are going to hate yourself/PARI for this (but love stack-overflow).

In the first line you have:

Given you need precise decimal places, all of the libraries you mentioned are either out of bounds or will require setting absurd levels of precision to guarantee your results are rounded accurately.

The problem is they're all based on binary floating point, so when you specify a precision, e.g. with mpf_set_default_prec in GMP/MPIR (or the equivalent call in MPFR, a fork of GMP that provides more guarantees on exact, portable precision), you're saying how many bits will be used for the significand (roughly, how many bits of integer precision exist before it is scaled by the exponent). While there is a rough correspondence to decimal precision (so using four bits of significand for every one decimal digit of precision needed is usually good enough for most purposes), the lack of decimal precision means explicit output rounding is always needed to avoid false precision, and there is always the risk of inconsistent rounding: a number that logically ends in 5 and should be subjected to rounding rules like round half-even, round half-up or round half-down might end up with false precision that makes it appear to not end exactly with 5, so the rounding rules aren't applied.

What you want is a decimal math library, like Java's BigDecimal, Python's decimal, etc. The C/C++ standard library for this is libmpdec, the core of the mpdecimal package (on Python 3.3+, this is the backend for the decimal module, so you can experiment with its behavior on Python before committing to using it in your C/C++ project). Since it's decimal based, you have guaranteed precision levels; if you set the precision to 4 and mode to ROUND_HALF_EVEN, then parsing "XX.X45" will consistently produce "XX.X4", while "XX.X75" will consistently produce "XX.X8" (the final 5 rounds the next digit to even numbers). You can also "quantize" to round to a specific precision after the decimal point (so your precision might be 100 to allow for 100 digits combined on left and right hand sides of the decimal point, but you can quantize relative to "0.0000" to force rounding of the right hand of the decimal to exactly four digits).

Point is, if you want to perform exact decimal rounding of strings representing numbers in the reals in C/C++, take a look at libmpdec.

For your particular use case, a Python version of the code (using the moneyfmt recipe, because formatting numbers like "1.1E-26" to avoid scientific notation is a pain) would look roughly like:

You need to provide an ex reference, not a symbol reference; Try this:

1. Clone the repository.

The link you gave is to the web interface to the repository. The actual repo is at git://www.ginac.de/ginac.git.

You need to compile the code and link it with your compiler. Suggested steps:

1. Run make in the top-most folder of the source code

2. Find the location of the library file by running find . -type f | grep "\.a$" in the terminal.

3. Add the path to the library file in your IDE or compiler.