I'm writing a derivative calculator in C++, and in order to properly perform the derivation operations, I need to parse the input equation at various character indexes along the equation's string.
I'm using isdigit() to parse out the numerical values from the equation and then store them into a separate string array, however now I need to parse out the mathematical symbols from the equation to identify which operation I need to perform.
Is there any way I can modify (overwrite?) isdigit() to recognize custom values from a string array? I'd like to avoid iteration to make my code a little less cluttered, since I'm already going to be using plenty of loops for the rest of this program and I want my code to be easy to follow. Does overwriting and inheritance in C++ work similarly to inheritance in Java (with the exception of multiple inheritance/interfaces)?
Please refrain from posting solutions that are irrelevant to the scope of this question, IE; different approaches to deriving equations in C++, as I've used this approach for some fairly specific reasons.
Thanks
You can use the new powerful C++11 regular expressions library that does almost what ever parsing you want. This way, you'll avoid iterations and code cluttering.
You can just use strchr. (Not everyone will like the macros here, but they do make combining character classes easy.)
#define OPERATOR "+-*/"
#define DIGIT "0123456789"
// Is c an operator
if (strchr(OPERATOR, c)) {
// Yes it is
}
or:
// Is c an operator or a digit?
if (strchr(OPERATOR DIGIT, c)) {
// Yup
}
Overriding and Inheritance works more or less the same as in Java.
You need to define a function as virtual and redefine it in derived class.
I know the "Please refrain from posting...", but I've written a library that does function parsing and derivation.
It is available at https://github.com/B3rn475/MathParseKit
I hope you can find some tips there.
Related
My question is nearly-identical to C equivalent to Fortran namelist
The key difference is that I'm using C++/17 and wanted to know if there is a more C++ idiomatic manner of solving the issue.
There is no equivalent to Fortran's Namelist in C++. And there is no portable way of achieving this.
If you want to achieve a similar construct, you'd need some general parsing function working with lines on an istream:
It would read lines and ignore anything following a "!";
It would read the name that follows the first "&" to see if it matches the expected name;
It would stop reading when encountering a sole "/" on a line;
It would parse every line in-between, finding the a string with the name of the variable, the "=", and a string with the value on the right of the equal operator, and store the two strings in a std::map<std::string,std::string>.
For ad hoc reading, you'd call the parsing function to transform istream lines into the map. You would then access the map to initialise the variables. An easy and powerful way would be to use a stringstream sst(my_map["variable name"]); sst>>my_variable;
If the Namelist would correspond more or less to a class or a struct X, the idiom would then be to overload a friend istream& operator>> (istream&, X&);. This overloaded extractor would then work as in the ad-hoc case to initialise the member variables.
Of course, I simplify somewhat the algorithm, since you'd also need to cope with errors in the input file: what do you do if an expected variable is missing ? what do you do if the value read is incompatible with the target variable ?
If you are migrating Fortran code to C++ and you must to use Namelist to avoid disruptions, this could be a portable way to go forward.
But if it's just an habit of yours, and the file format is not mandatory, then you may consider using json, with one of the many libraries existing to read and write this format (22 libraries are listed on the linked page). The advantage is that you don't have to reinvent the wheel, you'd be more interopearble with a lot of other languages, and json seems more future-proof.
I want to write a program that takes an string like x^2+1 and understand it.
I want to ask the user to enter her/his function and I want to be able to process and understand it. Any Ideas?
char s[100];
s <- "x*I+2"
x=5;
I=2;
res=calc(s);
I think it could be done by something like string analyses but I think Its so hard for me.
I have another Idea and that is using tcc in main program and doing a realtime compile and run and delete a seprated program (or maybe function) that has the string s in it.
and I will create a temp file every time and ask tcc to compile it and run it by exec or similar syntax.
/*tmp.cpp:*/
#include <math.h>
void main(/*input args*/){
return x*I+2;
}
the tmp.cpp will created dynamically.
thanks in advance.
I am not sure what do you expect. It's too complex to give the code as answer, but the general idea is not very complex. It's not out of reach to code, even for a normal hobbyist programmer.
You need to define grammar, tokenize string, recognize operators, constants and variables.
Probably put expression into a tree. Make up a method for substituting the variables... and you can evaluate!
You need to have some kind of a parser. The easiest way to have math operations parsable is to have them written in RPN. You can, however, write your own parser using parser libraries, like Spirit from boost or Yacc
I use with success , function parser
from www it looks like it supports also std::complex, but I never used it
As luck would have it, I recently wrote one!
Look for {,include/}lib/MathExpression/Term. It handles complex numbers but you can easily adapt it for plain old floats.
The licence is GPL 2.
The theory in brief, when you have an expression like
X*(X+2)
Your highest level parser can parse expressions of the form A + B + C... In this case A is the whole expression.
You recurse to parse an operator of higher precedence, A * B * C... In this case A is X and B is (X+2)
Keep recursing until you're parsing either basic tokens such as X or hit an opening parenthesis, in which case push some kind of stack to track where your are and recurse into the parentheses with the top-level low-precedence parser.
I recommend you use RAII and throw exceptions when there are parse errors.
use a Recursive descent parser
Sample: it's in german, but a small and powerfull solution
look here
here is exactly what You are searching for. Change the function read_varname to detect a variable like 'x' or 'I'.
I am considering the problem of validating real numbers of various formats, because this is very similar to a problem I am facing in design.
Real numbers may come in different combinations of formats, for example:
1. with/without sign at the front
2. with/without a decimal point (if no decimal point, then perhaps number of decimals can be agreed beforehand)
3. base 10 or base 16
We need to allow for each combination, so there are 2x2x2=8 combinations. You can see that the complexity increases exponentially with each new condition imposed.
In OO design, you would normally allocate a class for each number format (e.g. in this case, we have 8 classes), and each class would have a separate validation function. However, with each new condition, you have to double the number of classes required and it soon becomes a nightmare.
In procedural programming, you use 3 flags (i.e. has_sign, has_decimal_point and number_base) to identify the property of the real number you are validating. You have a single function for validation. In there, you would use the flags to control its behaviour.
// This is part of the validation function
if (has_sign)
check_sign();
for (int i = 0; i < len; i++)
{
if (has_decimal_point)
// Check if number[i] is '.' and do something if it is. If not, continue
if (number_base = BASE10)
// number[i] must be between 0-9
else if (number_base = BASE16)
// number[i] must be between 0-9, A-F
}
Again, the complexity soon gets out of hand as the function becomes cluttered with if statements and flags.
I am sure that you have come across design problems of this nature before - a number of independent differences which result in difference in behaviour. I would be very interested to hear how have you been able to implement a solution without making the code completely unmaintainable.
Would something like the bridge pattern have helped?
In OO design, you would normally
allocate a class for each number
format (e.g. in this case, we have 8
classes), and each class would have a
separate validation function.
No no no no no. At most, you'd have a type for representing Numeric Input (in case String doesn't make it); another one for Real Number (in most languages you'd pick a built-in type, but anyway); and a Parser class, which has the knowledge to take a Numeric Input and transform it into a Real Number.
To be more general, one difference of behaviour in and by itself doesn't automatically map to one class. It can just be a property inside a class. Most importantly, behaviours should be treated orthogonally.
If (imagining that you write your own parser) you may have a sign or not, a decimal point or not, and hex or not, you have three independent sources of complexity and it would be ok to find three pieces of code, somewhere, that treat one of these issues each; but it would not be ok to find, anywhere, 2^3 = 8 different pieces of code that treat the different combinations in an explicit way.
Imagine that add a new choice: suddenly, you remember that numbers might have an "e" (such as 2.34e10) and want to be able to support that. With the orthogonal strategy, you'll have one more independent source of complexity, the fourth one. With your strategy, the 8 cases would suddenly become 16! Clearly a no-no.
I don't know why you think that the OO solution would involve a class for each number pattern. My OO solution would be to use a regular expression class. And if I was being procedural, I would probably use the standard library strtod() function.
You're asking for a parser, use one:
http://www.pcre.org/
http://www.complang.org/ragel/
sscanf
boost::lexical_cast
and plenty of other alternatives...
Also: http://en.wikipedia.org/wiki/Parser_generator
Now how do I handle complexity for this kind of problems ? Well if I can, I reformulate.
In your case, using a parser generator (or regular expression) is using a DSL (Domain Specific Language), that is a language more suited to the problem you're dealing with.
Design pattern and OOP are useful, but definitely not the best solution to each and every problem.
Sorry but since i use vb, what i do is a base function then i combine a evaluator function
so ill fake code it out the way i have done it
function getrealnumber(number as int){ return getrealnumber(number.tostring) }
function getrealnumber(number as float){ return getrealnumber(number.tostring) }
function getrealnumber(number as double){ return getrealnumber(number.tostring) }
function getrealnumber(number as string){
if ishex(){ return evaluation()}
if issigned(){ return evaluation()}
if isdecimal(){ return evaluation()}
}
and so forth up to you to figure out how to do binary and octal
You don't kill a fly with a hammer.
I realy feel like using a Object-Oriented solution for your problem is an EXTREME overkill. Just because you can design Object-Oriented solution , doesn't mean you have to force such one to every problem you have.
From my experience , almost every time there is a difficulty in finding an OOD solution to a problem , It probably mean that OOD is not appropiate. OOD is just a tool , its not god itself. It should be used to solve large scale problems , and not problems such one you presented.
So to give you an actual answer (as someone mentioned above) : use regular expression , Every solution beyond that is just an overkill.
If you insist using an OOD solution.... Well , since all formats you presented are orthogonal to each other , I dont see any need to create a class for every possible combination. I would create a class for each format and pass my input through each , in that case the complexity will grow linearly.
I'm writing some excel-like C++ console app for homework.
My app should be able to accept formulas for it's cells, for example it should evaluate something like this:
Sum(tablename\fieldname[recordnumber], fieldname[recordnumber], ...)
tablename\fieldname[recordnumber] points to a cell in another table,
fieldname[recordnumber] points to a cell in current table
or
Sin(fieldname[recordnumber])
or
anotherfieldname[recordnumber]
or
"10" // (simply a number)
something like that.
functions are Sum, Ave, Sin, Cos, Tan, Cot, Mul, Div, Pow, Log (10), Ln, Mod
It's pathetic, I know, but it's my homework :'(
So does anyone know a trick to evaluate something like this?
Ok, nice homework question by the way.
It really depends on how heavy you want this to be. You can create a full expression parser (which is fun but also time consuming).
In order to do that, you need to describe the full grammar and write a frontend (have a look at lex and yacc or flexx and bison.
But as I see your question you can limit yourself to three subcases:
a simple value
a lookup (possibly to an other table)
a function which inputs are lookups
I think a little OO design can helps you out here.
I'm not sure if you have to deal with real time refresh and circular dependency checks. Else they can be tricky too.
For the parsing, I'd look at Recursive descent parsing. Then have a table that maps all possible function names to function pointers:
struct FunctionTableEntry {
string name;
double (*f)(double);
};
You should write a parser. Parser should take the expression i.e., each line and should identify the command and construct the parse tree. This is the first phase. In the second phase you can evaluate the tree by substituting the data for each elements of the command.
Previous responders have hit it on the head: you need to parse the cell contents, and interpret them.
StackOverflow already has a whole slew of questions on building compilers and interperters where you can find pointers to resources. Some of them are:
Learning to write a compiler (#1669 people!)
Learning Resources on Parsers, Interpreters, and Compilers
What are good resources on compilation?
References Needed for Implementing an Interpreter in C/C++
...
and so on.
Aside: I never have the energy to link them all together, or even try to build a comprehensive list.
I guess you cannot use yacc/lex (or the like) so you have to parse "manually":
Iterate over the string and divide it into its parts. What a part is depends on you grammar (syntax). That way you can find the function names and the parameters. The difficulty of this depends on the complexity of your syntax.
Maybe you should read a bit about lexical analysis.
We would like to have user defined formulas in our c++ program.
e.g. The value v = x + ( y - (z - 2)) / 2. Later in the program the user would define x,y and z -> the program should return the result of the calculation. Somewhen later the formula may get changed, so the next time the program should parse the formula and add the new values. Any ideas / hints how to do something like this ? So far I just came to the solution to write a parser to calculate these formulas - maybe any ideas about that ?
If it will be used frequently and if it will be extended in the future, I would almost recommend adding either Python or Lua into your code. Lua is a very lightweight scripting language which you can hook into and provide new functions, operators etc. If you want to do more robust and complicated things, use Python instead.
You can represent your formula as a tree of operations and sub-expressions. You may want to define types or constants for Operation types and Variables.
You can then easily enough write a method that recurses through the tree, applying the appropriate operations to whatever values you pass in.
Building your own parser for this should be a straight-forward operation:
) convert the equation from infix to postfix notation (a typical compsci assignment) (I'd use a stack)
) wait to get the values you want
) pop the stack of infix items, dropping the value for the variable in where needed
) display results
Using Spirit (for example) to parse (and the 'semantic actions' it provides to construct an expression tree that you can then manipulate, e.g., evaluate) seems like quite a simple solution. You can find a grammar for arithmetic expressions there for example, if needed... (it's quite simple to come up with your own).
Note: Spirit is very simple to learn, and quite adapted for such tasks.
There's generally two ways of doing it, with three possible implementations:
as you've touched on yourself, a library to evaluate formulas
compiling the formula into code
The second option here is usually done either by compiling something that can be loaded in as a kind of plugin, or it can be compiled into a separate program that is then invoked and produces the necessary output.
For C++ I would guess that a library for evaluation would probably exist somewhere so that's where I would start.
If you want to write your own, search for "formal automata" and/or "finite state machine grammar"
In general what you will do is parse the string, pushing characters on a stack as you go. Then start popping the characters off and perform tasks based on what is popped. It's easier to code if you force equations to reverse-polish notation.
To make your life easier, I think getting this kind of input is best done through a GUI where users are restricted in what they can type in.
If you plan on doing it from the command line (that is the impression I get from your post), then you should probably define a strict set of allowable inputs (e.g. only single letter variables, no whitespace, and only certain mathematical symbols: ()+-*/ etc.).
Then, you will need to:
Read in the input char array
Parse it in order to build up a list of variables and actions
Carry out those actions - in BOMDAS order
With ANTLR you can create a parser/compiler that will interpret the user input, then execute the calculations using the Visitor pattern. A good example is here, but it is in C#. You should be able to adapt it quickly to your needs and remain using C++ as your development platform.