How to solve equations with one unknown at runtime in C++?

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So, the goal of the program is:
1) get an equation from the user with one unknown as a std::string (e.g. "62 = x * 1000")
2) solve the equation and print the value of the unknown (denoted as "x")

What I have tried:

i found TinyExpr, which is pretty amazing, but it cannot solve equations with unknown variables.
i also have installed Boost, but i almost never used it.

Posted 23-May-23 7:04am

Avtem

Updated 23-May-23 9:23am

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Rick York 23-May-23 13:14pm

Your program should solve them exactly as you would do it yourself. An expression handler would be a good front end to use.

Avtem 23-May-23 13:23pm

"Your program should solve them exactly as you would do it yourself." - but to implement such a functionality probably will require hundreds lines of code - parsing numbers, signs, operator precedence and of course testing and error handling.
TinyExpr is only capable to solve one part of equation and it's already pretty large.
i feel like someone already created such a library, that's why i asked for help, because i haven't found one yet.

Rick York 23-May-23 13:46pm

There probably is code around that can do this but the point of most homework is to do it yourself. A secondary point of it is to learn how to find what you need for yourself.

0x01AA 23-May-23 14:25pm

There are a lot of solver examples: Google for "solver c++". Solver in very easy words means for your example to solve the equation (x + 38)%4") - 30 = 0 by varying parameter x (also called root search).

Avtem 23-May-23 14:41pm

So far i haven't found any code that solves the problem i am trying to solve.
Unfortunately googling for "solver C++" gave me results that don't mention equations with one unknown variable

0x01AA 23-May-23 15:10pm

There are a lot, I'm sure.
But meanwhile I had a closer look to your equation and if '%' means Modulo then of course there will no solution. This because (x+38)%4 has no chance to give a reminder of 30 ;)

Avtem 24-May-23 10:46am

Sorry, my bad - i provided a bad example. i already fixed it.
Hopefully i'll find a library that can do equations

0x01AA 24-May-23 13:45pm

No problem. But you mentioned you tried TinyExpr. That means, you can use it and write the code to varying the variable in a loop 'in a structured way' and let TinyExpr evaluate. That comes nearly to a solver.
Sorry for my English...

Avtem 24-May-23 14:04pm

As i mentioned in other comment - i am looking for an existing library that solves the problem entirely.
i could spent several days on developing such library, but it's not worth the effort and time

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OriginalGriff · Answer 1 · 2023-05-23T09:09:00

Quote:
... to implement such a functionality probably will require hundreds lines of code - parsing numbers, signs, operator precedence and of course testing and error handling.

Yes, it would.

Unfortunately, that's because it's a complex project - to expand on what Rick has said, you need to do several things:
1) Verify the entered expression is valid.
2) Break the expression into operators and operands
3) Identify the unknown.
4) Reformat the expression so that the unknown is alone on one side of the equals sign evaluating "chunks" as you go:

30=2x+4
2x=30-4
2x=26
x=26/2
x=13

Exactly the same way you would if you were doing it manually.

This may help you get started: How to Write Code to Solve a Problem, A Beginner's Guide[^]

But ... do note that your example 30 = (x + 38)%4 fails the first step as there is no unique solution: you cannot "reverse" modulus operators to get back the original input as it discards info. So you can't get from 30 = (x + 38)%4 to (x + 38) = 30 <any operator> 4 as you can with the paired operators like add/subtract and mutiply/divide.