SeExpr

Simple ASCII Grapher Tutorial

## Programmer Tutorial

Getting started with SeExpr is relatively easy. SeExpr gives you a way to evaluate one or many evaluations of an expression. What changes between different applications of expressions is mainly the particular variables that are accessible (sometimes also the set of functions). Each application of expressions generally has it's own subclass of Expression that gets instantiated. To get started we're going to go through a simple application that is an ascii graphing calculator. This is located in the src/demos/asciiGraph.cpp part of the source tree.

## Problem Overview

We are going to write a function grapher that displays in ASCII. In particular for a given f(x) we can evaluate it at all the x's in a window and draw the resulting y's. For example if the user ran our program

    ./asciiGraph "val=.5*PI*x;7*sin(val)/val"

we would get

                                  |
                                  |
                                 ###
                                # |#
                               ## |##
                               #  | #
                              ##  | ##
                              #   |  #
                             ##   |  ##
                             #    |   #
                             #    |   ##
                 ####       #     |    #       ####
    #######-----##--###-----#-----|----##-----##--###-----######
          ######      ##   #      |     #    #      ######
                       ## ##      |     ## ##
                        ###       |      ###
                                  |
                                  |
                                  |

or if we did

    ./asciiGraph "x-3"

we'd get

                                  |                         ####
                                  |                     ####
    ------------------------------|-----------------####--------
                                  |             ####
                                  |         #####
                                  |     #####
                                  | ####
                                ####
                            ####  |
                        ####      |
                    ####          |
                #####             |
            ####                  |
        ####                      |
    ####                          |
                                  |

## Implement the subclass

First we subclass Expression and give it a constructor, typically one that takes an expression string.

    class GrapherExpr:public SeExpr2::Expression
    {
    public:
        //! Constructor that takes the expression to parse
        GrapherExpr(const std::string& expr)
            :SeExpr2::Expression(expr)
        {}
    ...
    };

### A simple variable reference

This is not a very interesting subclass of expression until we add some additional variables. Variables on some applications may be very dynamic. In this case, however, we only need 'x', so we make a ExprVarRef subclass

    class GrapherExpr:public SeExpr2::Expression
    {
    ...
        //! Simple variable that just returns its internal value
        struct SimpleVar:public SeExpr2::ExprVarRef
        {
            SimpleVar() : SeExpr2::ExprVarRef(SeExpr2::ExprType().FP(1).Varying()), val(0.0) {}
            double val; // independent variable
            void eval(double* result) {result[0] = val;}
            void eval(const char** result) {}
        }
    ...
    };

Once we have this we need an instance to store our variable and provide a reference to that. We make it mutable, because resolveVar() is const. One does not need to store a variable reference in a given expression. In fact, it may be useful to use the same ExprVarRef from multiple expressions! For example, if you have 10 expressions that all have access to the same variables, this is an important optimization.

    class GrapherExpr:public SeExpr2::Expression
    {
    ...
        mutable SimpleVar x;
    ...
    };

### Binding our variable reference

If we now tried to use expressions, the variable would still not be found by our expressions. To make it bindable we need to override the resolveVar() function as follows:

        //! resolve function that only supports one external variable 'x'
        SeExpr2::ExprVarRef* resolveVar(const std::string& name) const
        {
            if(name == "x") return &x;
            return 0;
        }
    ...
    };

### Variable setting

Next we need to make a way of setting the variable. As the controlling code will use the expression evaluation, it will repeatedly alternate between setting the independent variables that are used and calling evaluate(). What it has to do depends very much on the application. In this case we only need to set the independent variable x as:

    class GrapherExpr:public SeExpr2::Expression
    {
    ...
        void setX(double x_input)
        {x.val=x_input;}
    ...
    };

## Evaluating expressions

Evaluating an expression is pretty easy. But before we can do that we need to make an instance.

        GrapherExpr expr("x+x^2");

However, there might be errors in the expression you must check with isValid() before you can evaluate. Then you can print a parse error as well

        if(!expr.isValid()){
            std::cerr"expression faield "<<expr.parseError()<<std::endl;
            exit(1);
        }

Finally, we can loop through all the x points in the graph and find out the y value as follows:

       // evaluate the graph
        const int samplesPerPixel=10;
        const double one_over_samples_per_pixel=1./samplesPerPixel;
        for(int i=0;i<w;i++){
            for(int sample=0;sample<samplesPerPixel;sample++){
                // transform from device to logical coordinatex
                double dx=double(sample)*one_over_samples_per_pixel;
                double x=double(dx+i)/double(w)*(xmax-xmin)+xmin;
                // prep the expression engine for evaluation
                expr.setX(x);
                // evaluate and pull scalar value
                Vec3d val=expr.evaluate();
                double y=val[0];
                // transform from logical to device coordinate
                int j=(y-ymin)/(ymax-ymin)*h;
                // store to the buffer
                if(j>=0 && j<h)
                    buffer[i+j*w]='#';
            }
        }    for(int i=0;i<w;i++){

## Running the program

Usage is

    ./asciiGraph

Be sure to put the expressions in quotation marks i.e. Some fun examples.

    ./asciiGraph 'x'
    ./asciiGraph 'x^2'
    ./asciiGraph '.1*x^3-2*x'
    ./asciiGraph 'fit(noise(.5*x),0,1,-10,10)'
    ./asciiGraph 'fit(smoothstep(x,-8,8),0,1,-9,9)'