Eigen
By eigen
ExprTK
C++ Mathematical Expression Parsing And Evaluation Library https://www.partow.net/programming/exprtk/index.html (by ArashPartow)
The number of mentions indicates the total number of mentions that we've tracked plus the number of user suggested alternatives.
Stars  the number of stars that a project has on GitHub. Growth  month over month growth in stars.
Activity is a relative number indicating how actively a project is being developed. Recent commits have higher weight than older ones.
For example, an activity of 9.0 indicates that a project is amongst the top 10% of the most actively developed projects that we are tracking.
Stars  the number of stars that a project has on GitHub. Growth  month over month growth in stars.
Activity is a relative number indicating how actively a project is being developed. Recent commits have higher weight than older ones.
For example, an activity of 9.0 indicates that a project is amongst the top 10% of the most actively developed projects that we are tracking.
Eigen
Posts with mentions or reviews of Eigen.
We have used some of these posts to build our list of alternatives
and similar projects.
We haven't tracked posts mentioning Eigen yet.
Tracking mentions began in Dec 2020.
ExprTK
Posts with mentions or reviews of ExprTK.
We have used some of these posts to build our list of alternatives
and similar projects. The last one was on 20220422.

A Cpp library to evaluate mathematical literal expressions dynamically
I have found many libraries that can evaluate mathematical expressions, e.g. this one: https://www.partow.net/programming/exprtk/index.html

Better library for evaluating math expressions stored on a String
I have used https://github.com/ArashPartow/exprtk for this task, since it is cpp we also used on IOS, however in android we changed to exp4j since it requires less space and did the same job.

What do I do if a repo only has a make file and I'm on windows?
For the specific example repo you cited, the project website also offers some preconfigured example MSVC solutions. You may find this much easier to get set up and running if using a Makefile is not a hard requirement.
 What are some projects that taught you a lot?

How to calculate an expression that is stored in a string in C++
This ExprTk library looks promising: https://www.partow.net/programming/exprtk/index.html

A boolean logic simulator written in c++ & opengl 3.3. This is a toy project I'm currently developing to help me learn c++ & opengl. No libraries used, apart from stb_image and glad.
Now there are lots of libraries out there for expression processing as well. I would recommend exprtk: https://github.com/ArashPartow/exprtk
 library for solving equations from string?

Regular language input math operations
If your actual goal isn't to implement it yourself and you need it for something else, take a look at this library: https://github.com/ArashPartow/exprtk
What are some alternatives?
When comparing Eigen and ExprTK you can also consider the following projects:
GLM  OpenGL Mathematics (GLM)
TinyExpr  tiny recursive descent expression parser, compiler, and evaluation engine for math expressions
OpenBLAS  OpenBLAS is an optimized BLAS library based on GotoBLAS2 1.13 BSD version.
muparser  muparser is a fast math parser library for C/C++ with (optional) OpenMP support.
blaze
CGal  The public CGAL repository, see the README below
ceressolver  A large scale nonlinear optimization library
Boost.Multiprecision  Boost.Multiprecision