diff options
Diffstat (limited to '')
| -rw-r--r-- | examples/shellmath/README.md | 166 |
1 files changed, 166 insertions, 0 deletions
diff --git a/examples/shellmath/README.md b/examples/shellmath/README.md new file mode 100644 index 0000000..1b47256 --- /dev/null +++ b/examples/shellmath/README.md @@ -0,0 +1,166 @@ +# Shellmath +Introducing decimal arithmetic libraries for the Bash shell, because +they said it couldn't be done... and because: + +. + + + +## Quick-start guide +Download this project and source the file `shellmath.sh` into your shell script, +then fire away at the shellmath API! + +The ___basic___ API looks like this: +``` + _shellmath_add arg1 arg2 [...] argN + _shellmath_subtract arg1 arg2 # means arg1 - arg2 + _shellmath_multiply arg1 arg2 [...] argN + _shellmath_divide arg1 arg2 # means arg1 / arg2 +``` + +The ___extended___ API introduces one more function: +``` + _shellmath_getReturnValue arg +``` + +This function optimizes away the need for ___$(___ subshelling ___)___ in order to capture `shellmath`'s output. +To use this feature, just be sure to set `__shellmath_isOptimized=1` at the top +of your script. (You can find an example in `faster_e_demo.sh`.) + +Operands to the _shellmath_ functions can be integers or decimal +numbers presented in either standard or scientific notation: +``` + _shellmath_add 1.009 4.223e-2 + _shellmath_getReturnValue sum + echo "The sum is $sum" +``` +Addition and multiplication are of arbitrary arity; try this on for size: +``` + _shellmath_multiply 1 2 3 4 5 6 + _shellmath_getReturnValue sixFactorial + echo "6 factorial is $sixFactorial" +``` +Subtraction and division, OTOH, are exclusively binary operations. + +## The demos +For a gentle introduction to `shellmath` run the demo `slower_e_demo.sh` +with a small whole-number argument, say 15: +``` +$ slower_e_demo.sh 15 +e = 2.7182818284589936 +``` + +This script uses a few `shellmath` API calls to calculate *e*, the mathematical +constant also known as [Euler's number](https://oeis.org/A001113). The argument +*15* tells the script to evaluate the *15th-degree* Maclaurin polynomial for *e*. +(That's the Taylor polynomial centered at 0.) Take a look inside the script to +see how it uses the `shellmath` APIs. + +There is another demo script very much like this one but *different*, and the +sensitive user can *feel* the difference. Try the following, but don't blink +or you'll miss it ;) +``` +$ faster_e_demo.sh 15 +e = 2.7182818284589936 +``` + +Did you feel the difference? Try the `-t` option with both scripts; this will produce +timing statistics. Here are my results +when running from my minGW64 command prompt on Windows 10 with an Intel i3 Core CPU: +``` +$ for n in {1..5}; do faster_e_demo.sh -t 15 2>&1; done | awk '/^real/ {print $2}' +0m0.055s +0m0.051s +0m0.056s +0m0.054s +0m0.054s + +$ for n in {1..5}; do slower_e_demo.sh -t 15 2>&1; done | awk '/^real/ {print $2}' +0m0.498s +0m0.594s +0m0.536s +0m0.511s +0m0.580s +``` + +(When sizing up these timings, do keep in mind that ___we are timing the +calculation of e from its Maclaurin polynomial. Every invocation of either +script is exercising the shellmath arithmetic subroutines 31 times.___) + +The comment header in `faster_e_demo.sh` explains the optimization and shows +how to put this faster version to work for you. + +## Runtime efficiency competitive with awk and bc +The file `timingData.txt` captures the results of some timing experiments that compare +`shellmath` against the GNU versions of the calculators `awk` and `bc`. The experiments +exercised each of the arithmetic operations and captured the results in a shell variable. +The result summary below shows that `shellmath` is competitive with `awk` and runs faster +than `bc` in these experiments. (One commenter noted that the differences in execution speed +can be partially explained by the fact that `shellmath` and `awk` use finite precision +whereas `bc` uses arbitrary precision. Another factor in these measurements is the need to +subshell 'awk' and 'bc' to capture their results, whereas 'shellmath' writes directly to +the shell's global memory.) + +Here are the run times of `shellmath` as a percentage of the `awk` and `bc` equivalents: +``` + versus awk versus bc + Addition: 82.2% 40.6% + Subtraction: 95.9% 50.5% + Multiplication: 135.9% 73.3% + Division: 80.3% 43.2% +``` + +Astute observers will note the experiments provide approximations to the sum, difference, +product, and quotient of *pi* and *e*. Unfortunately I did not gain insight as to which +of these values, if any, are +[transcendental](https://en.wikipedia.org/wiki/Transcendental_number#Possible_transcendental_numbers). + +You can find a deeper discussion of shellmath's runtime efficiency +[here](https://github.com/clarity20/shellmath/wiki/Shellmath-and-runtime-efficiency). + +## Background +The Bash shell does not have built-in operators for decimal arithmetic, making it +something of an oddity among well-known, widely-used programming languages. For the most part, +practitioners in need of powerful computational building blocks have naturally opted +for *other* languages and tools. Their widespread availability has diverted attention +from the possibility of *implementing* decimal arithmetic in Bash and it's easy to assume +that this ***cannot*** be done: + ++ From the indispensable _Bash FAQ_ (on _Greg's Wiki_): [How can I calculate with floating point numbers?](http://mywiki.wooledge.org/BashFAQ/022) + *"For most operations... an external program must be used."* ++ From Mendel Cooper's wonderful and encyclopedic _Advanced Bash Scripting Guide_: + [Bash does not understand floating point arithmetic. Use bc instead.](https://tldp.org/LDP/abs/html/ops.html#NOFLOATINGPOINT) ++ From a community discussion on Stack Overflow, _How do I use floating point division in bash?_ + The user's [preferred answer](https://stackoverflow.com/questions/12722095/how-do-i-use-floating-point-division-in-bash#12722107) + is a good example of _prevailing thought_ on this subject. + +Meanwhile, + ++ Bash maintainer (BDFL?) Chet Ramey sounds a (brighter?) note in [The Bash Reference Guide, Section 6.5](https://tiswww.case.edu/php/chet/bash/bashref.html#Shell-Arithmetic) + by emphasizing what the built-in arithmetic operators ***can*** do. + +But finally, a glimmer of hope: + ++ A [diamond-in-the-rough](http://stackoverflow.com/a/24431665/3776858) buried elsewhere + on Stack Overflow. + This down-and-dirty milestone computes the decimal quotient of two integer arguments. At a casual + glance, it seems to have drawn inspiration from the [Euclidean algorithm](https://mathworld.wolfram.com/EuclideanAlgorithm.html) + for computing GCDs, an entirely different approach than `shellmath`'s. + +Please try `shellmath` on for size and draw your own conclusions! + +## How it works +`shellmath` splits decimal numbers into their integer and fractional parts, +performs the appropriate integer operations on the parts, and recombines the results. +(In the spirit of Bash, numerical overflow is silently ignored.) + +Because if we can get carrying, borrowing, place value, and the distributive +law right, then the sky's the limit! As they say--erm, as they ___said___ in Rome, + + Ad astra per aspera. + +## And now... +You can run your floating-point calculations directly in Bash! + +## Please see also: +[A short discussion on arbitrary precision and shellmath](https://github.com/clarity20/shellmath/wiki/Shellmath-and-arbitrary-precision-arithmetic) |